RESPONSE OF APPLE (MALU'S SYLVESYRES} TO
VARIOUS LEVELS OF SOIL MOISTURE

Thesis for the Define of Pk. 13.

MICHIGAN STATE UNIVERSITY

Stanley Joseph Gamble
1963

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RESPONSE OF APPLE (MALUS SYLVESTRIS)

 

TO VARIOUS LEVELS OF SOIL MOISTURE

presented by

Stanley Joseph Gamble

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RESPONSE OF APPLE (MALUS SYLVESTRIS) TO VARIOUS

 

LEVELS OF SOIL MOISTURE

B y

Stanley joseph Gamble

AN ABSTRACT OF A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Horticulture

I963

ABSTRACT

RESPONSE OF APPLE (MALUS SYLVESTRIS) TO VARIOUS

 

LEVELS OF SOIL MOISTURE

by Stanley Joseph Gamble

Response of 18-year-old apple trees to soil moisture level was studied
for five years. Trees included three standard varieties on seedling root-
stocks, spaced at 40 feet. The soil was described as Bellefontaine sandy
loam, and was found to contain 8. 9 percent water at field capacity and l. 0
percent water at the permanent wilting point. The soil had a density of l. 8
in the surface 18 inches and an available water storing capacity of l. 7 inches
per foot of soil depth. Climatological conditions showed monthly averages
during the growing season of 67"“ F. temperature, 6. 3 inches of pan evapora—
tion; and 3. 0 inches of precipitation. Soil moisture treatments consisted of
(treatment 70) allowing available soil moisture to fall to 70 percent (0. 45
atmospheres tension) before being brought to field capacity: (treatment 40)
allowing available soil moisture to be depleted to 40 percent (approximately
I. 5 atmospheres tension) before being brought to field capacity: and (control)
receiving only rainfall. Soil moisture was measured by an electrical resis-
tance method at the major root zone depth of 18 inches, and was regulated
by sprinkler irrigation. Effects of soil moisture level were studied by

measuring yield and size of fruit, tree growth, leaf composition, and fruit

Stanley joseph Gamble - - 2
quality and composition. Available moisture in the control plots was re-
duced to near 20 percent for the first two years, to near 30 percent in
the third year, and to 50 percent in the. fourth year, and approached the
permanent wilting percentage in 1962.

Total yield of McIntosh and Red Delicious fruit for the period of the
experiment was not significantly increased by either irrigation practice.
Total yield of Northern Spy was increased by either irrigation treatment,
due to yield increases in the irrigated plots in the 1961 season. Irrigated
McIntosh fruit was larger in 1962, but significantly smaller Northern Spy
fruit was harvested from the high soil moisture plots in 1961. Fruit size
was more closely related to total yield than to irrigation treatment. Ter-
minal shoot growth was unaffected by soil moisture level, but trunk cir-
cum ference increase was significantly correlated to soil moisture increase.
There were no consistent effects of soil moisture level on leaf N, K, P,

Ca, Mg, Mn, Fe, Cu, B, Zn, M0, or Al.

Storage scald and ground color measurements on the fruit indicated
no relationship to soil moisture. Brown core was sigmificantly related to
high soil moisture levels, but was reduced under CA storage conditions.
Development of bitter pit in Northern Spy was not influenced by soil moisture.

In general, very slight response to the various soil moisture
treatments was achieved. It is suggested that deep rooting habits of
the apple tree, in addition to a relatively high soil moisture level in

the control plots plus relatively low evapotranspirat'ion rates, are

Stanley joseph Gamble - - 3
probably responsible for the general ineffectiveness of the supplemental
irrigation. It is further suggested that these results are applicable to other
situations if certain data concerning soil characteristics, tree type and
planting distance, and climatic conditions are known. Apparently, in com-
parable orchards on similar soils and under similar climatic conditions,
soil moisture may not be a limiting factor in apple tree growth and pro—

ductivity.

RESPONSE OF APPLE (MALUS SYLVESTRIS) TO VARIOUS

 

LEVELS OF SOIL MOISTURE

By

Stanley Joseph Gamble

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
Department of Horticulture

1963

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ACKNOWLEDGEMENTS

Most sincere appreciation is expressed to Dr. A. L. Kenworthy
for his assistance and guidance throughout this investigation. Appre-
ciation is also expressed to Drs. R. L. Carolus, A. E. Erickson,

E. H. Kidder, and R. P. Larsen for their suggestions in writing and
editing the manuscript.

Acknowledgement is made to the author's wife, Mary, for her
encouragement and sacrifice throughout the work, and to Mrs. M. S.

Barrett for typing of the manuscript.

ii

CONTENTS

Page
ACKNOWLEDGEMENTS. . . . . . . . . . . . . . . ii
CONTENTS..................... iii
INTRODUCTION . . . . . . . ..... . . . . . . . 1
REVIEW OF LITERATURE . . . . . . . . . . . . . . 3
EXPERIMENTAL PROCEDURE . . . . . . . . . . . . 13
RESULTS...................... 18
Soil Characteristics . . . . . . . . . . . . . . . 18
Seasonal Changes in Available Soil Moisture . . . . 21
Influence of Soil Moisture on Total Yield of Fruit . 29
Influence of Soil Moisture on Fruit Size . . . . . . 31
Influence of Soil Moisture on Growth of Tree. . . . 38

Influence of Soil Moisture on Mineral Composition
ofLeaves................. 41

Influence of Soil Moisture on Pre- and Post-harvest
Fruit Quality and Mineral Composition of Fruit 44
DISCUSSION.. .. . . . 51
SUMMARY 62

LITERATURECITED................ 64

iii

INTRODUCTION

Few fruits are produced commercially over as extensive an area as
apples. In the United States thirty-five states produce apple commercially
and of these Michigan now ranks third. Also, apple production has increased
at a greater rate in Michigan than in any other state in recent years (10).

Michigan apple producing areas receive average annual precipitation
ranging from 29 to 35 inches. Monthly averages during a growing season
of May to October range from 3 to 4 inches (11). Much of this falls in
small amounts which may be of limited benefit to apple trees, especially
when grown under the prevailing sod system of soil management.

In recent years, sprinkler irrigation systems utilizing light weight
portable aluminum pipe have made supplemental irrigation feasible in

Michigan apple orchards. Also, either a surface or ground water supply
is generally available, so grower interest in supplemental irrigation is
increasing. However, little information is available as a basis upon
which to recommend suitable irrigation practices to commercial apple
growers. Whether or not normal precipitation in Michigan is adequate
for optimum apple tree growth, production, and fruit quality is not known.
Information is needed as to whether supplemental irrigation in Michigan
Will produce a response in apple tree growth and production.

The purpose of this investigation was to study the effects of various

soil moisture levels on the apple tree, asmeasured by total yield and
size of fruit, growth of tree, mineral composition of leaves, and pre-
and post-harvest fruit quality and mineral composition of the fruit. By
such measurements, an optimum range of soil moisture for apple trees

might be determined.

REVIE W OF LITERATURE

The influence of soil moisture supply on tree growth and yield has
been the subject of numerous investigations. Conclusions based on results
of certain research are contradictory to generally accepted principles of
soil moisture-plant response relationships, and the subject has remained
somewhat controversial. Since soil moisture treatments were often based
on irrigation intervals rather than upon percentage of available soil mois-
ture, a comparison of results between certain of these experiments is
rather difficult.

Much of the early work in the area of soil moisture requirements
of deciduous fruit crops was done by Hendrickson and/or Veihmeyer in
California. In 1927, Veihmeyer (34) used outdoor tanks to study water
use and growth of young French prune trees. This was an historic experi-
ment since it involved recognition of the fact that uniform soil moisture in
the available range in an active root zone can be controlled in plant experi-
ments only if the degree of moisture depletion is limited by bringing the
entire root zone to field capacity. Data from the soil moisture treatments
led to Veihmeyer's summary:

"Studies of young trees grown in tanks under controlled
conditions indicate that the use of water by these trees
was not influenced by the amount of water in the soil

above the wilting coefficient. Under comparable atmos-
pheric conditions, the rate of moisture extraction by the

roots of the trees was the same whether the moisture
content of the soil above the wilting coefficient was
high or low.

Apparently the roots of these trees were able to ob-
tain water as readily when the soil moisture had been
reduced almost to the wilting coefficient as when the
soil was filled with water to its maximum field capa-
city. "

"The results obtained from the controlled studies
made with prune trees in tanks indicated‘that not only
the use of water but the trees themselves were not
affected by variations in amounts of soil moisture
above the wilting coefficient. While these results
apply only to these young prune trees, it appears that
many of the current views regarding soil moisture
relations of other plants may also be questioned."

Again, work published by the same authors in 1955 (38) indicated
that water in the available range was equally available for evaporation,
but there was insufficient evidence to indicate that this water was equally
available for producing vegetative growth.

Veihmeyer and Hendrickson (3 S, 36, 37) made extensive tests to deter-
mine the most efficient methods for irrigation in California. On the basis
of their observations on perennial fruit crops they have reported that
moisture is equally available over the range from field capacity to near
the permanent wilting percentage. They state (36) that studies with grape
vines, peach, prune, apricot, apple and pear trees in field plots plus sun-
flowers in containers indicate that there is no single percentage of mois-
ture above the permanent wilting point at which plants grow better than at
another percentage and which therefore could be considered optimum for

plant growth.

Hendrickson and Veihmeyer in 1929, 1934 and 1942 published reports
(l7, 18, 19, 20) concerning irrigation of fruit trees which contained such
statements as:

"The term readily available moisture is applied to
the range between field capacity and the permanent wilt-
ing percentage since previous investigations by the
authors have shown no difference in the rate of soil
moisture extraction in this range. "

"Work at this station has indicated that the soil
moisture above the permanent wilting percentage is
readily available for use by plants. "

"Apparently, with the plants and soils studied, no
one moisture content (within the available range) could
be considered as optimum for plant growth."

 

Data concerning soil moisture extraction presented by these workers
appears to be linear down to a point near the permanent wilting percentage,
but this does not indicate that soil moisture is uniformly available for main-
tenance of plant turgor and growth over this linear extraction range. High
moisture treatments usually produced the largest trees and highest yields
of the largest fruits, but treatments providing a comparison between various
degrees of moisture depletion through the available range were not described.

The irrigation procedures recommended by Veihmeyer and Hendrickson
(35, 36, 37) were based on field tests on sandy soils and were well founded,
but conclusions regarding relation of the growth rate to soil moisture in the
available range were not in harmony with observations of other workers, es-
pecially with plants grown in containers. Considerable evidence is avail-
able to indicate that water is not equally available to plantsover the entire

range of available moisture.

Kenworthy (23) grew young apple trees in containers in a greenhouse
study of the effect of soil moisture on tree growth. Tensiometers and wilt-
ing were used to indicate soil moisture conditions and treatments were
spaced so that 20, 40, 60, 80 and 100 percent available soil moisture was
used prior to irrigation. Significant growth decreases occurred when soil
moisture was allowed to drop as low as 20 percent of available before irri-
gation.

Experiments reviewed to this point have emphasized the effects of
soil moisture level on plant growth. However, since the fruit itself is the
marketable product of the deciduous fruit trees, considerable emphasis
has been placed by various workers on the measurement of fruit size and
total yields as influenced by level of soil moisture.

Furr and Magness (13) reporting on the relation of soil moisture to
stomatal activity and fruit growth of apples, stated that their investigations
indicated that in apple orchards the trees could function at near a maximum
level as long as the moisture content of the entire root zone was "appreciably"
above the wilting percentage. They reported that as the permanent wilting
percentage was approached, stomatal closing occurred earlier in the day,
and growth rate of the fruit was reduced.

Work (39), in a report on the relation of soil moisture to pear tree
wilting in Oregon, stated that soil moisture becomes less readily available
to pear trees as the moisture in the adobe clay soil declined from field capa—

city toward the wilting percentage.

Work and Lewis (40), again working with pear trees in Oregon, re-
ported that differential amounts of soil moisture within the available range
exerted profound influence upon pear fruit size and consequent yield, and
that during periods of relatively uniform weather conditions, moisture was
lost from all soil depths at a decreasing rate as the moisture content de-
creased, beginning when from 50 to 60 percent of the available soil moisture
was still present.

Additional data led Aldrich, Lewis and Work (I) later to publish:

" With the heavy soil and climatic conditions at the
Medford Station, whenever a reasonably large number
of fruits are set, larger fruits will be produced by
maintaining soil moisture high in the available range

throughout the season, and particularly late in the season. "

Their treatments were:
(1) Irrigated when available soil moisture dropped to 50
percent in top 3 feet of soil.
(2) Irrigated when soil moisture dropped to 10 to 20 per-
cent.
(3) Same as (2) early, then (1) late.
_ (4) Same as (1) early, then (2) late.

For six years treatment I consistently produced larger fruits, and
treatment 2 produced the smallest fruits, with size 26 percent smaller than
treatment 1, and total yield 33 percent less than treatment 1. Neither
treatment 3 or 4 was consistently better than the other.

Their data showed that the diminished rate of fruit growth due to the

"dry" treatment was the result of a lowered rate of water accumulation and

also a diminished rate of dry matter accumulation. Fruit growth was
measured, and reduced rates of growth were observed when the average
moisture content in the top 3 feet of soil was reduced below 50 percent
available capacity.

The growth rate of apples in Maryland orchards (25) was reduced
when the driest part of the root zone approached the permanent wilting per-
centage.

Claypool (17) found reduced apple fruit growth while the average soil
moisture in the principal root zone was above the permanent wilting per-
centage.

Boynton and Savage (6) compared growth rates of apples at numerous
locations in New York State during 1936 (a dry year) and 1937 (a wet year).
In all reported cases, the rate of fruit growth was higher in the wet year,
even though soil moisture was seldom depleted to the wilting percentage in
any plot in either year.

Forshey (12) conducted tests in a New York (Hudson Valley) Golden
Delicious apple orchard in 1957, a dry year. Treatment A, maintained
above 50 percent available soil moisture, increased yields 75 percent over
the cheek treatment which was below 25 percent available soil moisture
for all but two weeks of the summer, although his data does not show how
far below 25 percent. Treatments B and C in Forshey's tests, which were

irrigated when the available soil moisture dropped to 37. 5 and 25 percent,

respectively, did not yield significantly less fruit than did Treatment A, but
all yielded significantly more fruit than the check plots. Also, the percentage
of fruit larger than 3 inches in diameter from treatments A, B and C was
significantly greater than from the check plot, but no one of these three
treatments differed from another in effect on fruit size.

Apparently conflicting results are reported by Harley and Masure (16)
working with apple trees in Washington. They stated:

"With a given leaf area, Delicious apples continued

to increase in volume in the 'dry' plots at about an equal

rate with those on trees growing in soil maintained at

approximately field capacity. After the wilting percent-

age was reached, however, the fruit growth rate decreased

rapidly until water was applied."
These workers claimed that this was further evidence to the effect that
varying the percentage of water in most soils is of minor importance in
plant growth as long as the root zone is above the permanent wilting per-
centage.

Studies relating soil moisture to mineral composition of the plant
have not been as widely reported as have been the effects on plant growth
and fruit yield.

Nour (28), working with seedling plants of peach, apple, mustard,
and strawberry, studied the effect of minimum moisture levels of 70, 40,
and 10 percent of field capacity. The growth of all test plants was pro-
portional to soil moisture, and phosphorus and potassium increased in leaf

tissue with increasing moisture.

10

Benedict (2) reported that irrigation applications did not significantly
affect the nitrogen composition of Jonathan apple leaves in comparison with
no irrigation.

Mason (26) placed East Malling VII apple rootstocks in pots with half
of them watered daily (wet treatment) and half allowed to dry to the incipient
wilting point (dry treatment). The "wet" treatment reduced the concentra-
tion of nitrogen in the whole plant, increased phosphorus, potassium, and
magnesium, and had no effect on calcium. In general, the effects of the
treatments on mineral concentrations in the various parts were the same as
on the whole plant.

Hibbard and Nour (21) found that regardless of the level of potassium
supply, concentration of potassium in the leaves was reduced with 3 mois-
ture stress. Moisture stress reduced leaf phosphorus, which was more
affected by soil moisture stress than by the available phosphorus supply in
the soil.

This relationship was not reported by Corgan (8) who found no effect
of drought at any period of the year on the percent of phosphorus in the
leaves of apple seedlings. He reported that the primary effect of moisture
stress was a very large reduction in uptake of the cations calcium, potas-
sium, and sodium.

Therefore, these reports concerning leaf composition could be sum-

marized by stating that one worker found nitrogen content not affected by

11

soil moisture stress and one report indicated a reduction in nitrogen on a
whole plant basis. Phosphorus was reported by three workers to decrease
with decreasing soil moisture, and to be unaffected by one. Four reports
were in agreement by stating that potassium decreases with moisture stress.
One report showed calcium decreasing with decreasing soil moisture, and
one indicated no effect. Magnesium and sodium were reported to be reduced
by moisture stress.

Some relatively early work concerning the effect of soil moisture on
fruit quality was reported by Overly _e_t 31.111 1932 (29). In Washington,
irrigation studies on Jonathan apples showed that the lightly irrigated plots
produced the highest quality apples. Medium and heavily irrigated plots
produced fruit with less red over color and a lower percentage of extra
fancy grade. This fruit was of larger size, but was more readily bruised
by handling and decayed more rapidly when stored at room temperature.

Ryall and Aldrich (31) found that Bartlett pears produced by trees on
a continuously high level of soil moisture were lower in percent dry matter,
less firm according to pressure tests, and higher in frequency of core
breakdown than were fruits from trees at a lower level of soil moisture
supply. These workers claimed that a moderate moisture stress during
the latter part of the season was conducive to production of higher quality
fruit.

Haller and Harding (15) stated that differences in soil moisture had

no effect on the susceptibility of apples to decay, but irrigated apples were
softer and showed greater breakdown after removal from storage than did
non—irrigated fruit. However, they stated that the benefits from the greater
yields and higher quality of the apples grown under ample soil moisture far
outweighed the detriment of shorter storage life.

Mochizuki and Hanada (27) studied the effect of soil moisture on the
grmvth and quality of apple fruits. They reported that the keeping quality
of jonathan apples grown in a relatively dry soil was greater than that of
apples grown in a wet soil, since the cell size was decreased and the cell
wall development increased. They stated that the nitrogen supply to the
fruit: was lower in the dry soil, and this reduced the rate of post-climacteric

respiration.

EXPERIMENTAL PROCEDURE

For the purpose of studying soil moisture effects on the apple tree,
experimental plots were established in 1957 in an 18-year-old planting of
McIntosh, Red Delicious and Northern Spy varieties. All trees had been
budded on seedling rootstocks, and were planted at a 40—foot spacing. The
soil type was described as a Bellefontaine sandy loam and soil management
practices consisted of a ladino clover—June grass sod, seeded in 1946 and
mowed at intervals during the summer. Pruning had been done uniformly
but lightly. Ammonium nitrate had been applied annually at the rate of
five pounds per tree.

During the five years the experiment was conducted, uniform moderate
pruning was done, and annual fertilizer applications consisted of ammonium
nitrate at 6 pounds per tree on McIntosh and Delicious varieties, and 7 pounds
per tree on Northern Spy in 1958, 1959 and 1960. In 1961 and 1962, 3 pounds
of ammonium nitrate per tree was applied to all varieties. Trees within the
plots varied in trunk circumference from 24 to 40 inches.

Soil moisture measurements were made by using gypsum blocks, as
described by Bouyoucos and Mick (4, 5). Gypsum blocks were employed
since they have been recommended by Kelley Still: (22) as being preferable
for the range of soil moisture over which measurement were desired.

Blocks were placed under the outer branches of each tree at a depth of

13

14

18 inches, since this depth represented an area of greatest root concentra-

tion (14). Insulated wire leads from the blocks were led underground to a stake
near the tree trunk to avoid the possibility of damage by orchard machinery.
Weekly soil moisture readings were taken, using a battery—powered meter (4)
calibrated to read directly in percentage of available moisture. Water was
applied by sprinkler irrigation.

The moisture retention characteristics of the soil were studied by re-
moving samples of soil at various depths for measurement of field capacity,
permanent wilting point, and soil density. Moisture release data were ob-
tained by taking weekly readings on tensiometers placed adjacent to the
gypsum blocks at nine locations. With these data, soil water tension at
various levels of available soil moisture was calculated.

Soil moisture treatments, established in 1958, consisted of three
moisture levels:

Treatment 70 - available soil moisture allowed to drop to 70
percent before being brought to field capacity.
Treatment 40 - available soil moisture allowed to drop to 40
percent before being brought to field capacity.
Control - received only normal precipitation. Available soil
moisture level fluctuated as shown in Figures 1 through 5.
Each treatment was replicated three times, and each of the three

varieties was represented by two trees in each replicate, involving a total

of 54 trees. Border rows were left between rows of trees involved in the
various moisture treatments.

Beginning in 1958, the following data were collected, on a per tree
basis:

Total yield of fruit, recorded in number of bushels per tree.

 

This figure usually included fruit from two separate pickings plus dropped
fruit recovered from the ground.

Average size of fruit, recorded as average number of fruits

 

per standard size bushel. Twenty-five percent of the total number of bushel
crates harvested per tree were selected at random and the number of fruits
per bushel was counted in each.

Since no difference in fruit size due to irrigation had been found through
1961, in 1962 12 fruits per treatment of each variety were selected and marked
soon after petal-fall and their diameters were measured at weekly intervals
until harvest, providing a record of fruit development throughout the season.

Tree growth was determined by making terminal growth measurements

 

annually. Ten terminal shoots per tree were measured and marked in the
autumn of 1958, and measurements were henceforth made annually on the
same branches. Trunk circumference measurements were made after the
second year and again after the fifth year, providing a measurement of trunk
circumference increase over a three year period.

The nutritional status of the tree was measured by collecting leaf

16

samples in mid-july of each year and making determinations of 12 elements
on a dry weight basis. Nitrogen was determined by a modified Kjeldahl
method, potassium was determined by flame spectrophotometer, and phos-
phorus, calcium, magnesium, manganese, iron, copper, boron, zinc,
molybdenum, and aluminum were determined by photoelectric spectrometer.

Fruit quality was judged by placing samples of fruit from each tree in

 

32° F. refrigerated storage after the 1960 harvest. To determine if there
was an interaction effect on fruit quality between soil moisture treatment
and type of refrigerated storage, samples of McIntosh from each plot were
placed in two types of controlled atmosphere (CA) environments in addition
to the regular 32°F. storage. Regular CA storage conditions were a 32°F.
temperature, 2 1/2 percent C02 during the first month, then 5 percent C02,
with oxygen held at 3 percent. A second CA storage treatment consisted of
a 38° F. temperature, with the same CO2 and O2 levels as listed for regu-
lar CA.

Again in 1962, since relatively large soil moisture differences were
obtained between treatments, fruit from the McIntosh plots were placed in
regular 32° F. storage. Prior to storage, samples from each storage lot
were tested for flesh firmness with a Magness-Taylor pressure tester and
for soluble solids with a Zeiss refractometer.

After removal from storage, the fruit was again tested for flesh firm-

ness, soluble solids, and ground color (9), and observations were made on

the following disorders, as described by Smock and Neubert (32): storage
scald, brown core, internal breakdown, internal browning, C02 injury,
and bitter pit.

Mineral composition of the fruit as influenced by soil moisture level

 

was determined by analysis of fruit samples from the 1960 harvest. Wedge-
shaped slices of fruit were collected for determinations of elements as pre-

viously described for leaf samples.

RESULTS

Analysis of variance was performed on all data presented. In the case
of total fruit yields, a covariance adjustment was used as a significant rela-
tionship was evidenced between yield and tree size as indexed by trunk cir-
cumference.

Significance is indicated by "‘* at the 1% level, * at the 5% level, and

*(10) at the 10% level.

Soil Characteristics

 

The Bellefontaine sandy loam had a soil layer of matted grass and leaf
mold two inches thick over a brown loamy sand containing organic matter and
averaging 10 inches in depth. The subsoil consisted of a slightly sticky mix-
ture of reddish-brown clay, sand, and small gravel. This layer in turn,
rested on a heterogenous mass of sand, gravel, boulders, and clay, contain-
ing a large amount of limestone gravel.

Undisturbed cores of soil at 6 inch (topsoil) and 18 inch (subsoil) depths
were removed for determination of wat er-holding capacity. It was found im-
possible to remove soil cores at greater depths due to the coarse, gravelly
subsoil. Cores from three separate locations in each of the three repli-
cates were removed at both a 6-inch depth (actually from 4 to 8 inches) and

at an 18-inch depth (from 16 to 20 inches). Thus, a total of 18 cores were

18

19

removed and brought into the laboratory. Field capacity was determined

by saturating the samples with distilled water, covering to eliminate evapor-
ation, and allowing them to drain for 12 hours. Percent moisture on an oven
dry basis was then determined. Bulk density (grams/cc of the undisturbed
cores) was calculated by dividing the oven-dry Weight by the total volume of
the sample. Sunflowers were used to determine the permanent wilting per-
centage, as described by Veihmeyer and Hendrickson (36). The formula
described by Richards and Wadleigh (30) and others for determination of the

capacity of a soil for storing available water was employed:

R _ FC - WP pb
a _ _-—-—_— X —
am 100 pw

Ram: maximum available water ratio.

FC-WP: difference in percent water at field capacity and wilting
point.

pb: bulk density of the soil, or mass per unit volume.
pw: bulk density of water.
A summary of the data obtained follows:

96 Water at 95» Water at Soil

 

 

 

 

 

R.
F. C. P. W. P. Density am

6 inch depth

Rep. 1 8. 9 l. 0 1. 73 .137

Rep. 2 8. 9 1. 0 l. 78 .140

Rep. 3 8. 9 I. O I. 75 . 138
18—inch depth

Rep. 1 8. 8 1. 0 1. 83 . 143

Rep. 2 8. 9 1. 0 1.86 . 147

Rep. 3 8. 9 1. O l. 75 . I45

20

Mean Ram values at 6 inches and at 18 inches are . 138 and . 145 res-
pectively. This corresponds to an available water-holding capacity of slightly
less than 1. 7 inches per foot at the 6-inch depth, and slightly over 1. 7 inches
per foot at the 18—inch depth. The samples at the 6-inch depth represented
the loamy sand topsoil containing organic matter, while the samples from the
18-inch depth represented the clay-sand-gravel subsoil.

Moisture release characteristics of the soil were determined by plac-
ing tensiometers adjacent to nine of the gypsum blocks for two growing
seasons. When plotted, data thus obtained showed the following relationships
between soil water tension and percentage of available moisture. Values of

one atmosphere and above were obtained by extrapolation.

 

 

Tension Available Moisture
(atmospheres) (percent)
0. 1 92
0. 2 82
0. 3 77
0. 4 71
0. 5 67
0. 6 62
0. 7 59
O. 8 56
1. 0 50
1. 5 40

The irrigation sprinklers, placed at a 30 by 40-foot spacing, delivered
8. 8 gallons of water per minute per sprinkler, for an application rate of 0. 70
inches per hour. Assuming that the surface three feet of soil held 5. .1 inches

of available water, when soil moisture dropped to 70 percent of available it

21

was necessary to operate the irrigation system for two hours and twelve
minutes. Thus, 1 1/2 inches of water were applied, the amount necessary
to bring the surface three feet of soil to field capacity (treatment 70). When
soil moisture was depleted to 40 percent of available (treatment 40), a water
application time of slightly less than 4 1/2 hours duration was required.
Since a slightly higher water—holding capacity of the soil had originally been
estimated, the system was run either 2 1/2 hours for treatment 70 plots,

or 5 hours for treatment 40 plots, resulting in wetting the soil to a depth of

3 1/2 feet whenever irrigation water was applied.

Seasonal Changes in Available Soil Moisture

 

Since precipitation contributed to soil moisture levels in all plots, wide
fluctuations in moisture level necessarily occurred within all treatments.

Relatively extensive changes in soil moisture also occurred due to
irrigation, since uniform soil moisture within the available range in an active
root zone can be controlled only if the degree of moisture depletion is regu-
lated by bringing the root zone to field capacity.

Soil moisture levels obtained for the various treatments during the
course of this experiment are represented in Figures 1 through 5. Soil
moisture and precipitation measurements were recorded at weekly intervals
throughout the growing season. Soil moisture data is presented as the per-
centage remaining of the available moisture range. Dates and amounts of

precipitation are indicated. Dates of irrigations are indicated, with amount

of water applied calculated to bring the surface 3 1/2 feet of soil to field

capacity.

Available Soil Moisture 1958-1963 (Figures 1-5)

 

Figure 1 indicates that although rainfall in 1958 was infrequent, timing
and quantity were such that only one irrigation in each of treatments 70 and
40 was needed to maintain the desired moisture levels at an 18-inch depth.
Irrigation equipment was not in working order until July 1, hence the soil
moisture level in treatment 70 was lower than desirable prior to that date.
Precipitation of less than 1/4 inch was not recorded. Note that the irri-
gated plots remained above 50 percent available moisture, and the control
plots dropped below 20 percent only for a brief interval.

Figure 2 represents soil moisture levels for 1959 at 18 inches depth.
Since no rainfall exceeding 1/4 inch was recorded from June 1 to late July,
the available moisture level in the control plots fell rapidly to about 20 per-
cent, where it remained during July. Rainfall of 1 1/2 inches was then
apparently suffieient to reach the moisture blocks at the 18-inch depth.

Two irrigations were sufficient to hold treatment 70 at a suitable level, and
one irrigation was required for treatment 40.

Well distributed rainfall of sufficient quantity in 1960 (Figure 3) caused
the control plots to remain above 40 percent available moisture except for
brief periods. Two irrigations in each of treatments 70 and 40 held soil

moisture at the 18-inch depth at the desired levels.

Figure 4 indicates that in 1961 treatments 70 and 40 required one
irrigation each. Above normal rainfall after July 13 caused the control
plots to remain above 50 percent available moisture throughout the entire
growing season.

Relatively little rainfall in 1962 (Figure 5) allowed soil moisture in
the control plots to be depleted to the lowest level of the five years studied.
Available moisture in the control plots remained below 10 percent for
about two weeks in late July. To maintain the desired moisture levels,
four irrigations were necessary in treatment 70, and two were required
in treatment 40.

Only in 1962, then, did soil moisture in the control plots drop to a
comparatively low level, that is, near the wilting point at the 18-inch
depth. In the years 1958 to 1961 available soil moisture remained above

20 percent, except for occasional very brief intervals.

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Total Yield of Fruit

 

Table 1 illustrates the effect of soil moisture level on fruit yields for
five years, with yields adjusted for tree size where applicable.

The average yields were not significantly different for the three treat-
ments for McIntosh and Red Delicious. For Northern Spy, however, the dif—
ferences were significant at odds of 9:1 with the irrigation treatments yield-
ing more fruit than the control treatments.

Significant differences between treatments for each year were never
above odds ()f 9:1. With Northern Spy such a difference occurred in 1961 when
either irrigation treatment yielded more fruit than the control treatment. There
were no instances of significant differences for McIntosh or Red Delicious
yields during any year.

Data presented in Table 1 indicate that available soil moisture levels
as low as 20 percent at an Iii-inch depth for relatively brief periods did not
reduce total yields. Even in 1962, when available moisture in the control
plots fell to from 4 to 8 percent for a period of two weeks, yields were not
reduced. Possibly such low moisture levels would have exerted a significant
effect on. yields were the trees not able to extract moisture from depths

greater than the 18-inch level. Due to a lower concentration of roots at

these greater depths, soil moisture is depleted at a slower rate, and

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therefore an ample moisture supply was undoubtedly available at depths of

two and three feet or more.

Fruit Size

Table 2 presents the effect of soil moisture level on the size of the
fruit.

Data for Red Delicious in 1959 and 1961 and Northern Spy in 1959 is
missing due to fruit being removed from the orchard before a size measure-
ment could be obtained. In 1962, Northern Spy was harvested in pallet boxes,
which precluded obtaining a random sample of fruit for a size count. No
fruit size record was obtained in 1958 on any variety.

Significant differences in fruit size for the three treatments occurred
with McIntosh only in 1962. Both irrigation treatments produced larger
fruit than the control. This situation was not evident in the three prior
years. Probably the greater differences in soil moisture between the three
treatments in 1962 than in other years contributed to these results.

Northern Spy showed significant differences at odds of 9:1 in 1961 with
treatment 70 producing smaller fruit than treatment 40 or the control.

No differences in fruit size were shown for Red Delicious in either
year that size records were obtained.

Figures 6, 7 and 8 are graphical presentations of fruit size measure-

ments made by vernier caliper at weekly intervals from anthesis to com-

mercial harvest date. Fruit diameter increase is depicted for McIntosh
(Figure 6), Red Delicious (Figure 7), and Northern Spy (Figure 8) subjected
to three different soil moisture levels during 1962. Each point at a weekly
interval represents a mean of 12 fruits (4 each on 3 trees) within a soil
moisture treatment and a variety. These figures represent data of 1962,
when rather wide differences in soil moisture between the various treat-
ments occurred. Although occasional relatively sharp increases in fruit
size due to irrigation or precipitation may be observed, in general, growth
of the fruit was a gradual, linear type.

Figure 6 indicates that at harvest, treatment 40 showed an 0. 3 cm.
diameter increase over the control McIntosh, which averaged 6. 0 cm. , or
slightly under a commercially acceptable 2 1/2 inches diameter. Treat-
ment 70 produced fruit measuring 0. 6 cm. greater in diameter than the
control fruit. The relationship of fluctuations in fruit growth rate in treat-
ment 40 and the control due to irrigation and rainfall may be noted.

Figure 7, showing Red Delicious fruit development in 1962, indicates
that both irrigation treatments produced fruit measuring 0. 5 cm. larger in
diameter than the control fruit. Although slight, some influence of rainfall
and/or irrigation on fruit development may be seen.

Figure 8 presents fruit development data for Northern Spy in 1962.
Treatment 40 produced fruit averaging 0. 5 cm. larger in diameter than the

control fruit, and treatment 70 produced fruit averaging I. 8 cm. larger

33

than the control. Increased growth of treatment 70 fruit became apparent
shortly after the first irrigation. The same relationship is seen for treat-
ment 40. Much of the difference in size between fruit of treatment 70 and
the other plots appeared relatively early in the growing season, prior to
August 1. Various fluctuations in rate of fruit growth due to irrigation and
precipitation may be noted.

Magness, Degman and Furr (25) found apple fruit growth to be linear
from six to eight weeks following bloom to near harvest. Fruit growth data
presented in Figures 6, 7 and 8 agree, in general, with this statement,
although the growth curve could better be described as curvilinear, increas-
ing in diameter more rapidly early in the season than later. Possibly
measurement of fruit volume would show a different type of curve.

It may be noted that the greatest rate of fruit diameter increase
occurred in all varieties prior to July 1. Fruit size data presented in these
figures agree with fruit size (at harvest) data of Table 2, except for Northern

Sp y.

34

TABLE 2. --Effect of Level of Soil Moisture on Fruit Size (Number Fruits
per Bushel).

 

 

Treatment

 

Difference

 

Variety Year 70 40 Control Significance Required
McIntosh 1959 131 139 144 ns

1960 178 176 191 ns

I961 159 161 161 ns

1962 208 I99 228 *’-‘ 17
Red 1960 124 127 132 ns
Delicious I962 169 177 187 ns
Northern 1960 127 121 141 ns
Spy 1961 125 107 106 "(10) 15

 

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38

Tree Growth

 

Terminal shoot growth (Table 3) was unaffected in all but one case by
soil moisture level. With Northern Spy in 1959, treatment 70 showed sig-
nificantly less growth than treatment 40 or the control, at odds of 9:1. This
generally insignificant effect of soil moisture treatment upon terminal shoot
growth may be explained by citing the growth habits of the apple tree. Gener-
ally, terminal shoot growth commences at time of bloom and progresses
rapidly for about three to four weeks, when a terminal bud becomes visible.
During the five years this experiment was conducted, shoot growth began
in late May and terminal buds were usually visible by July 1, the trees aver-
aging about 25 days of rapid shoot growth. During the month of June when
this rapid flush of growth was occurring, available soil moisture never fell
below 40 percent, except in 1959. Thus, soil moisture stress was appar-
ently not great enough early in the season to exert a significant effect on
shoot growth.

Trunk circumference increase (Table 4) for Red Delicious was not
affected significantly by soil moisture level. With McIntosh, both irriga-
tion treatments caused significant increases in trunk circumference, at
odds of 9:1, over the control. With Northern Spy, treatment 40 showed
greater trunk increase at odds of 19:1, than the control or treatment 70.
Treatment 70 did not show a significant growth increase over the control.

Apparently trunk circumference increase indicated greater response to soil

39

TABLE 3. --Effect of Level of Soil Moisture Level on Terminal Shoot Growth

 

 

 

(Cm. ).
Treatment _ _ _ Difference
Variety Year 70 40 Control Significance Required
McIntosh 1958 36. 7 36. 0 36. 3 ns
1959 29. 0 32. 7 26. 3 ns
1960 21. 3 18. 7 18. 7 118
1961 19. 7 18. 7 17. 7 us
1962 9. 7 10. 0 9. 7 ns
Total 115. 3 114. 7 108. 0 ns
Red 1958 29. 0 28. 7 28. 0 ns
Delicious 1959 26 3 27. 3 25. 7 ns
1960 27.0 29.0 25.0 ns
1961 31. 0 29. 0 28. 7 ns
1962 23. 0 22. 7 18. 7 ns
Total 135. 3 135. 7 125. 3 113
Northern 1958 28. 7 29. 3 28. 0 ns
Spy 1959 21. 0 23. 7 24. 0 * (10) 2. 3
1960 23. 0 23. 7 23. 3 ns
1961 14.3 17.0 15.3 ns
1962 10. 3 13. 3 12. 3 ns
Total 97. 3 107. 0 101. 7 ns

 

40

TABLE 4. --Effect of Level of Soil Moisture on Trunk Circumference In—
crease (Percent Increase 1959-1962).

 

 

 

 

 

_ Treatment . _ _ Difference
Variety 70 40 Control Significance Required
McIntosh 12. 8 12. 3 11. 0 *(10) 1. 2
Red
Delicious 10. 2 9. 5 8. 7 ns
Northern
Spy 11. 7 14. 0 9. 7 * 2. 3

 

41

moisture level than terminal shoot growth. The significant increases in
trunk growth due to soil moisture level presented in Table 4 are indicative

of proliferation of conductive and associated tissues of the vascular cambium.
Such tissues apparently were more responsive than shoot tissue to differ-
ences in soil moisture level, or else growth processes occurred for a greater

portion of the growing season.

Leaf Mineral Compo sition

 

Leaf samples were collected in mid—July of each year from each tree.
Table 5 represents means of the three varieties. No significant differences
in amounts of potassium, calcium, magnesium, copper, or zinc were found
in any year. Leaf nitrogen in 1959 showed higher levels, at odds of 9:1 in
treatment 70 and the control, than in treatment 40. No differences in nitro-
gen due to soil moisture treatment were noted for the years 1958, 1960, 1961
or 1962.

Leaf phosphorus in 1962 was higher in both irrigation treatments than
in the control, at odds of 19:1. No differences in phosphorus were found for
1958, 1959, 1960 or 1961. 1

Leaf manganese showed more response to soil moisture level than
other elements. In 1958, treatments 70 and the control showed lower leaf

manganese, at odds of 99:1 than treatment 40. In 1959 trees of treatment

70 had lower manganese than treatment 40 or the control, at 9:1 odds.

42

TABLE 5. --Effect of Level of Soil Moisture on Leaf Mineral Composition.

 

 

 

 

 

Treatment Difference
Element Year 70 40 Control Significance Required
1/
9:. N— 1959 2. 47 2. 38 2. 48 *(10) 0. 08
%P 1962 .191 .194 .173 * 0.015
Ppm Mn 1958 89 112 83 i" 20
1959 172 198 192 *(10) 20
1961 162 179 195 ’1‘ 21
1962 190 211 207 *(10) 21
Ppm Fe 1958 177 215 199 * 27
Ppm B 1958 37. 7 29. 8 26. 2 "‘(10) 2. 7
Ppm Mo 1958 3. 8 4.1 3. 8 * 0. 3
1962 4. 7 3. 9 4. 0 ’1‘ 0. 5
Ppm Al 1958 187 239 225 * 33
1960 361 413 408 *(10) 44
1/

7' Only elements and years in which significant differences occurred are
listed. No differences were found in any year for K, Ca, Mg, Cu or

Zn.

43

Treatment 40 did not differ from the control. In 1961, treatment 70, but
not treatment 40, was lower in leaf manganese than the control at 19:1 odds.
Treatment 70 in 1962 showed lower leaf manganese than treatment 40, at
19:1 odds, but not lower than the control. No difference existed between
treatment 40 and the control.

In 1958, leaf iron was lower in treatment 70 plots than in treatment 40,
but not different from the control at 19:1 odds. Iron in treatment 40 and the
control did not differ from each other.

Leaf boron in 1958 was lower in treatment 70 than in treatment 40, but
not lower than the control, at odds of 9:1. No difference existed between the
control and treatments 70 or 40.

Leaf molybdenum displayed inconsistent effects of soil moisture treat—
ment. In 1958, treatment 70 and the control showed higher molybdenum
content than treatment 40, at 19:1 odds. Treatment 70 showed no difference
from the control. In 1962, treatment 70 showed higher molybdenum levels
than treatment 40 or the control, which did not differ from each other, at
highly significant odds of 99:1. ‘

Aluminum in 1958 and 1962 showed lower levels in the treatment 70
plots than in treatment 40 or the control, which did not differ from each other.
Odds were 19:1 in 1958 and 9:1 in 1960.

In general, leaf mineral composition showed no consistent effects of

soil moisture level. Possibly soil moisture differences had not become great

44

enough by the mid-July sampling date to exert definite influences on nutrient
element accumulation. This may be illustrated by referring to Figures 1
through 5, where it may be noted that only in 1959 and 1962 were there any
moderate differences in soil moisture between treatments prior to july 15.
However, more differences in leaf composition occurred in 1958 than in
any other year, and there were relatively small differences in soil moisture
at the time of sampling. Therefore, it may be concluded that if soil mois-
ture levels exert an influence on mineral accumulation in apple leaves, the
moisture stress necessary to produce such effects must be greater than

that achieved in these plots by mid-July.

Fruit Quality

 

Fruit quality as affected by soil moisture level was judged by measure-
ments of soluble solids, flesh firmness, color, and development of certain
storage disorders of economic importance to the Michigan apple industry.

Storage scald, a disorder affecting McIntosh, and to a lesser degree,
Red Delicious, showed no significant relationship to soil moisture level.
McIntosh ground color, an index of maturity, was not affected by soil
moisture treatment. Bitter pit, a disorder affecting Northern Spy, and
occasionally other varieties, also was not affected by soil moisture level.

Tables 6, 7 and 8 present other data pertaining to fruit quality.

Brown core development (Table 6) was apparently related to soil moisture

 

 

 

 

TABLE 6. --Effect of Level of Soil Moisture on Brown Core Index—1]
Storage Treatment Signi- Difference
Variety Conditions Year 70 40 Control ficance Required
2/
McIntosh 32° F. Reg. (a)— 1960 60 34 35 ns
(b) 126 109 96 * 18
32° F. CA (a) 22 22 21 ns
(b) 54 48 49 ns
38° F. CA (a) 16 21 23 ns
(b) 11 6 7 ns
32° F. Reg. (a) 1962 72 23 6 ns
(b) 92 85 44 ** 29
Red
Delicious 32° F. Reg. (a) 1960 0 0 0
(b) 3. 0 l. 9 2. 5 ns
1/ r

— Degree of brown core severity x frequency.

2/
— (a) At removal from storage.

(b) One week after removal.

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46

stress in McIntosh. One week after removal from regular 32° F. storage,
fruit from treatment 70 trees showed significantly more brown core than
the control trees, at odds of 19:1 in 1960 and at odds of 99:1 in 1962. In

1962, fruit from treatment 40 also showed more brown core than did control

fruit at 99:1 odds. In both years, no difference in brown core existed R?
between treatments 70 and 40. Where McIntosh were stored under controlled I 7'
atmosphere conditions, development of brown core was materially reduced,

and significant differences due to soil moisture level did not occur. No dif- {

ferences in brown core development were found in Red Delicious.

Flesh firmness, as measured by a Magness-Taylor pressure tester,
is presented in Table 7. No differences in flesh firmness were noted for
any variety after storage. However, McIntosh fruit was tested in 1962
prior to storange, and it was noted that fruit from treatment 70 was softer
than control fruit. Treatment 40 fruit was not different in firmness from
either treatment 70 or the control. These data suggest that differences
in flesh firmness due to soil moisture may occur at harvest, although the
fruit may soften to a uniform degree during storage.

Data concerning the relationship of soluble solids to soil moisture
level are presented in Table 8. Soluble solids in Red Delicious and
Northern Spy were not affected by soil moisture, based on post-storage
measurements made on 1960 season fruit. McIntosh of 1960 kept in

regular 32" F. storage, or in the 38°F. controlled atmosphere conditions,

47

TABLE 7. --Effect of Level of Soil Moisture on Flesh Firmness.

 

 

 

 

. Storage Treatment Signi— Difference
Variety Year Conditions 70 40 Control ficance Required
McIntosh 1960 32° F. Reg. 9. 8 9. 7 9. 7 ns

32° F. CA 10. 4 10. 2 10. 4 ns

32°F. CA 10.4 10.6 10.6 ns

. l/ .
1962 At harvest- 16. 8 l7. 4 18. 4 "‘ 1. 1

32" F. Reg. 22. 5 22. 5 23. 1 ns
Red
Delicious 1960 32° F. Reg. 13. 7 13. 8 13. 7 ns
Northern
Spy 1960 32° F. Reg. 12.1 11. 9 11. 8 ns
1/

— Differences between pressure tests at harvest and after storage are not
significant, due to differences in methods of measurements.

48

TABLE 8. --Effect of Level of Soil Moisture on Percent Soluble Solids.

 

 

 

. Storage Treatment Signi- Difference
Variety Year Conditions 70 40 Control ficance Required
McIntosh 1960 32° F. Reg. 11. 0 11. 4 11. 3 ns

32° F. CA 10. 9 11. 2 11. 6 * (10) 0. 6
38°F. CA 11.0 11.4 11.3 ns
1962 At harvest 11. 5 11. 9 13. 0 ”‘ 1. 0
32° F. Reg. 11. 2 11. 4 12. 3 * 0. 5
Red
Delicious 1960 32° F. Reg. 13. 2 13. 7 14. 0 ns
Northern
Spy 1960 32° F. Reg. 12. 1 11. 7 12. 0 ns

 

49

failed to show differences in soluble solids upon removal from storage.
However, the 1960 McIntosh, upon removal from 32° F. controlled atmosphere,
showed less soluble solids in treatment 70 than in the control. Treatment 40
fruit showed no differences from the control. In 1962, McIntosh from either
irrigated plot contained lower soluble solids than the control plot at harvest
time and the same relationship held when the fruit was tested after removal
from storage on May 1. These inverse relationships, of both flesh firmness
and soluble solids, to soil moisture level could be explained on the basis of

a larger fruit cell size caused by high soil moisture levels. Such cells would
offer less resistance to penetration by the pressure tester, and the soluble

cell contents may be diluted by an increased water supply.

Fruit Mineral Composition

 

The mineral composition of the fruit (Table 9) in general was not in-
fluenced by soil moisture treatment. Only magnesium and zinc showed signi-
ficant effects of soil moisture, both increasing as soil moisture increased.
Fruit of treatment 70 contained more magnesium, at odds of 9:1 than the con-
trol. Treatment 40 showed no significant difference in magnesium content
from the control. Fruit from both treatments 70 and 40 contained more zinc
than the control, at 19:1 odds. Although these differences were significant
at the levels indicated, such slight differences in mineral content would

probably be of no practical significance.

r
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50

TABLE 9. --Effect of Level of Soil Moisture on Mineral Composition of

 

Fruit (Dry Weight Basis) - 1960..1./

 

 

 

 

 

Treatment Difference

Element 70 40 Control Significance Required
% N 0. 27 0. 26 0. 25 ns

% P 0. 082 O. 080 0. 076 ns

% K 0. 70 0. 74 0. 76 ns

% Ca 0.116 0.104 0.116 ns

% Mg 0. 060 0. 059 O. 058 *(10) O. 002
Ppm Na 152 156 156 ns

Ppm Mn 13 13 12 ns

Ppm Fe 31 40 47 ns

Ppm Cu 14. 7 12. 1 l3. 8 ns

Ppm B 17. 9 12.1 13. 8 ns

Ppm Zn 9 9 6 * 3

Ppm Mo 1. 06 1. 00 1. 00 ns

Ppm A1 16 13 17 ns

1/

_ All values represent mean of three varieties.

DISCUSSION

The literature review presents what may seem an apparent disagree-
ment among various workers concerning the availability to the plant of
water in the so-called "available" range. Veihmeyer and/or Hendrickson
(16, 17, 18, 19, 34, 35, 36, 37) published experimental results indicating that
soil moisture was equally available to plants throughout the available range.
Harley and Masure (15) reported essentially similar conclusions. Con-
versely, Aldrich, Lewis and Work (1, 38, 39) claimed that on the basis of
their work, larger fruits would be produced by maintaining soil moisture
high in the available range throughout the growing season, since soil mois-
ture became less readily available as the moisture in the soil declined
from field capacity toward the wilting percentage.

Baver (2) states that the amount of water required by a given crop will
vary more with the type of soil than any other factor. Data is presented
indicating that Chino clay, with its high specific surface, holds water rather
tightly. About 1/10 atmosphere suction is necessary to remove any water
from this soil. At 15 atmospheres tension, only 15. 7 inches of water from
a total of 50 inches of water held per 100 inches of soil have been removed,
or about 31 percent. At the other extreme, Hanford sand released 4. 1

inches of a total 11. 9 inches held: most of this was extracted before 0. 5

51

52

atmosphere suction had been applied. This data emphasizes that different
soils not only hold different amount of water for plant use, but also hold this
water with varying degrees of tenacity.

Richards and Wadleigh (30) present curves showing the percentage of
available water remaining in various soils at different soil-moisture ten-
sions. Experimental points are shown on the curve at the l-atmosphere
value; at this value it can be seen that the sandy soil retains only about 15
percent of the available water, as compared with 45 percent for the clay
soiL

Vegetative growth of some plants decreases significantly as the soil
moisture stress increases in the l-atmosphere range (30), and response of
economic significance may occur at tensions of l. 5 atmospheres or greater
(33). Therefore, it may be understood why certain workers would claim
that soil water was equally available to almost the wilting point, while others
showed decreased growth when 40 percent of the soil moisture was yet
available. Such was probably the case with Veihmeyer and Hendrickson,
and Harley and Masure, working with sandy soils while Aldrich, Lewis
and Work conducted their experiments on a clay adobe soil.

There is no doubt that more energy is required to move water from
soil to roots in a dry soil than in a moist soil. This may not immediately
decrease transpiration or growth because as the diffusion pressure deficit

of the soil increases, the osmotic pressure and diffusion pressure deficit

of the plant may at first increase proportionately. There is, however,
abundant data indicating that as the soil moisture decreases to the lower
half of the available moisture range, growth and yield are often decreased
before the permanent wilting point is reached.

Where soil is permeated thoroughly and uniformly by roots it is likely
that plants can reduce the average moisture content much nearer the per-
manent wilting percentage without suffering from a water deficit than in
heavy soils where root systems are inhibited in development and unevenly
distributed. Kramer (24) states that the contradictory opinions concerning
the availability of water held by various investigators results at least
partly from differences in the soil types used, differences in opinion re-
garding what constitutes permanent wilting, and differences in interpretation
of the data.

Probably the differences in soil moisture between the irrigated plots
and the control, as described in Figures 1 through 5, were not generally
sufficient to result in highly significant effects on yield or size of fruit, tree
growth, leaf composition, or fruit quality and composition. Evidently such
tensions as those existing in the driest year (1962), when available soil
moisture dropped to about 5 percent, were not sufficient to cause responses
in fruit yield (Table 1) although tree growth (Table 4) was affected. Certain
other crops respond to soil moisture differences in a similar manner, i. e.

soil moisture may affect vegetative growth without affecting yield of the

marketable product. As an example, the vegetative growth of guayule was
increased by soil moisture increases, while rubber production of guayule
was decreased (30). If the marketable portion of the plant, or other portion
utilized as the criterion of response, is of a highly vegetative type, favor-
able response to soil moisture increases in the available range may usually
be observed. On the other hand, if the basis of measurement is a non-
vegetative product, such as rubber, oil, starch, sugar or other elaborated
carbohydrate or protein, or fruit containing large amounts of these sub-
stances, response to changes in soil moisture level have often not been
detectable. Van Bavel (33) and others have shown that response (if economic
level to soil moisture tension may occur at tensions of about 1. 5 atmos—
pheres. Data presented in Figures 5 through 8 may illustrate such a re-
sponse. It may be noticed in Figures 6, 7 and 8 that early in July, fruit

in the control plots began to lag in size increase, compared to fruit in the
irrigated plots. This relationship may be observed for each of the three
varieties studied. By referring to soil moisture data for that year presented
in Figure 5, it can be observed that in early July, soil moisture dropped
below 40 percent of available. According to data on page 20 concerning

the relationship of available soil moisture to atmospheres of tension, this
point corresponds to slightly above 1. 5 atmospheres. Hence, it may be
inferred that this apparent retardation in the growth rate of the fruit may

be due to soil moisture tensions exceeding 1. 5 atmospheres. Only in 1962

U!
at

did available soil moisture (Figure 5) fall below 40 percent (about 1. 5 atmos-
pheres tension) for an appreciable length of time. It may be observed in
Table 2 that fruit size showed greater differences due to soil moisture level
in 1962 than in other years.

The fact that soil moisture tension greater than 1. 5 atmospheres in
late June of 1959 did not influence vegetative shoot growth of the tree was
probably due to shoot growth being practically completed by the time such
tension occurred. In no other year did soil moisture tension exceed 1. 5
atmospheres during this rapid period of shoot growth.

It has been pointed out by some researchers that the water needs of
apple trees are greater during the period of rapid vegetative growth than
at any other time. Although Figure 5 indicates that in 1962 approximately
one inch of rainfall per week was not sufficient to maintain existing soil
moisture levels during this period, rainfall and soil moisture data of the
other years (Figures 1 through 4) does not aid in identifying such a period
of higher water requirements. Rainfall in these years was either excessive
or insufficient during the period of rapid vegetative growth.

Supplemental irrigation seemed to have less influence on yields of
McIntosh and Red Delicious than on Northern Spy yields (Table 1). How-
ever, it may be noted in Table 1 that total yield of Northern Spy was in-

fluenced by increased yields in the irrigated plots in only one year, 1961.

56

Examination of the Northern Spy data indicate that possibly the choice of
particular trees in establishing the plots had an influence on the yield re-

sponse. In all cases the trees in the control plots were from three to nine

inches greater in trunk circumference than were trees in the other plots. ,

ua-_‘.i

Therefore, the actual yield data of Table 1, presented as the average
number of bushels of fruit per tree, does not represent the true effect of
the irrigation treatments on fruit yield. However, analysis of covariance
provided an adjustment of these average yields based on trunk circumfer-
ence, since positive correlation existed between yield and trunk circum—
ference. Significance related to differences in yield is based on the results
of analysis of covariance and indicates that certain differences in yield due
to soil moisture treatments do exist, after yields have been adjusted for
tree size. Therefore, although the data showing higher actual yields of
fruit in the control plots may be explained on the basis of larger trees in
these plots, nevertheless the significance listed is based on adjusted yields
and indicates that certain differences in yields may not be accounted for by
differences in the size of the Northern Spy trees, and must be due to irri-
gation treatment.

Data presented in Table 2 concerning fruit size of Northern Spy in 1962
show fruit of treatment 40 and the control plots to be larger than fruit from
treatment 70 trees. Figure 8 indicates that fruit from irrigated trees was

larger than that from trees not irrigated. Such controversial data might

57

be explained by recalling that the figures presented in Figure 8 represent
fruit size on the lower accessible branches of the tree, while Table 2
represents size of fruit from all portions of the tree. Possibly irrigation
water striking the lower branches of the tree may have resulted in in-
creased size of the fruit on these lower branches, while fruit on upper
branches was above the height reached by water from the sprinklers and
was therefore not affected.

Undoubtedly, plant response to any given level of soil moisture is in-
fluenced by such factors as planting distance. depth Of rooting, extent of
root penetration and permeation in this zone, the texture of the soil, and
evaporative capacity of the air. In the case of this experiment the trees
were situated on a deep soil that was probably fully penetrated by roots
giving a relatively large active root area compared to leaf area. Certainly
the 40-foot spacing of the trees allowed tree roots an extensive area in
which to range for moisture. Also the soil was relatively light, thus allow—
ing rapid water movement. On such a soil a water deficit in the trees
probably would not occur until the average soil moisture was depleted to
nearly the permanent wilting point. And, since a minor fraction of the root
system existed at depths of two, three and four feet or more, the available
soil moisture at 18 inches might be reduced to near zero before effects of

economic importance were measurable.

Jl

A summary of climatological data for the June through September

growing season for the five years in which this experiment was conducted

 

 

 

follows:

Year Average Average Monthly Total. Precipitation
__ Temperature Pan Evaporation (June to l-larvest)
1958 66 6. 1 11. 3

1959 70 6. 3 2. 4

1960 67 6. 2 10. 8

1961 67 5. 7 14. 4

1962 67 7. 0 9. 9

Temperature and pan evaporation data were taken from official U. 5.
Weather Bureau records, while precipitation data were collected at week-
ly intervals at the experiment site.

It may be noted that although the average temperature in 1961 and
1962 were identical, 1961 could be described as a season of relatively
high rainfall, and relatively low evaporative conditions. 1962 showed
the opposite conditions, that is, less total rainfall (that fell at frequent
intervals, but in small amounts), and relatively high pan evaporation.
Another relatively "dry" year occurred in 1959. However, yield data
of Table 1 show that yields in the non—irrigated plots were not signifi-

cantly lower in 1959 and 1962, and not higher in 1961. Therefore, in

general, not only did supplemental irrigation fail to increase yields in two
of three varieties, but climatological differences that affected soil moisture
between years also did not influence yields. Weather Bureau data indicate
that the years 1958 through 1962 were not abnormal in respect to tempera-
ture, evaporation, and precipitation, except for precipitation in 1959. Long
term averages for this location show mean temperature for the months of
June through September to be 68° F. , pan evaporation for this period as 6. 5
inches per month, and rainfall as an average of 3. 0 inches per month.
Therefore, it may be concluded that climatological conditions existing dur-
ing these five years were fairly typical for the area.

Probably the relatively low evaporative power of the air under central
Michigan conditions was a major factor in allowing continued growth of tree
and fruit at available soil moisture levels below 20 percent. Climatological
data indicate that during the five years in which this experiment was con-
ducted, average temperature during the June through September growing
season was 67° F. , and pan evaporation during this same period averaged
6. 3 inches per month. Under such conditions minor water deficits occuring
in the leaves during the day may be compensated for at night.

It is of interest to note that of the irrigation experiments on apples
reported by Hendrickson and Veilnneyer (20), no difference in size of the
fruit as related to irrigation regime was found in two of the experiments.

In the third, the larger fruit was found in the unirrigated plot. The latter

60

experiment was carried out on a sandy loam soil, in an area having heavy
winter rainfall, rather cool conditions during the growing season, and
early morning fogs. On the unirrigated plot, the moisture content never
reached the permanent wilting percentage below two feet of soil.

Leaf mineral composition data (Table 5) indicate that soil moisture
levels within the range measured in this experiment exerted little effect
upon the nutritional status of the trees. The literature review presents
findings of various workers indicating that levels of the nutrients nitrogen,
phosphorus, potassium, calcium and magnesium in apple leaves may be
influenced by various degrees of soil moisture stress. Data presented in
Table 5 do not support such previous findings. It is indicated that ferti-
lizer programs should not be altered when supplemental irrigation is intro-
duced in apple orchards.

On the basis of these findings it may be concluded that supplemental
irrigation may not be profitable in orchards similar to the one herein des-
cribed. The following conditions contributed to these results, and must
necessarily be taken into account when determining if responses described
herein might apply to other locations: (1) the type of tree studied, including
rootstock, variety and planting distance: (2) the nature of the soil involved,
including water storing and water release characteristics; and (3) climatolo-
gical factors. If these conditions were known, it should be possible to

predict response of apple trees to supplemental irrigation at locations other

61

than the one studied. 1f the factors contributing to these results are similar,
one may expect similar responses. Evidently in apple orchards located on
soils having capacities of water storage and water release equal to, or greater
than those in the orchard studied in similar climatic conditions and having
trees of similar type, age, and spacing, supplemental irrigation may show
only occasional significant effects. Possibly the use of systems of soil
management, such as mulching, designed to collect precipitation and con-
serve soil moisture to the greatest extent possible, may be more profitable
practice than investment in supplemental irrigation equipment. Of course,
on lighter soils of lower water-holding capacity, and/or where planting
distances are decreased as in "hedgerow" systems, the use of supplemental

irrigation may prove valid.

SUMMARY

For the purpose of studying the effects of irrigation practices on apple
trees under central Michigan conditions, experimental soil moisture plots
including three commercially important varieties were established in 1957.
Response of 54 18-year-old trees to soil moisture level was studied for five
years. Soil moisture was measured by an electrical resistance method at
an 18—inch depth, and was regulated by sprinkler irrigation. Treatments
consisted of (1) allowing available soil moisture to fall to 70 percent before
being brought to field capacity (treatment 70): (2) allowing available soil
moisture to be depleted to 40 percent before being brought to field capacity
(treatment 40); and (3) control (not irrigated).

Available moisture in the control plots was reduced to near 20 percent
for the first two years, to near 30 percent in the third year, to 50 percent
in the fourth year, and approached the permanent wilting percentage in 1962.

Effects of soil moisture level were studied by measuring yield and size
of fruit, tree growth, leaf composition, and fruit quality and composition.

Total yield of fruit for the period of the experiment was not significantly
increased by either irrigation practice. Occasionally, for certain years and
certain varieties, differences occurred. Northern Spy seemed to respond
more favorably to increased soil moisture than did Red Delicious and McIntosh.

Irrigated McIntosh fruit was larger in 1962, but no difference was noted in

63

the size of fruit of the other varieties. Terminal shoot growth was un-
affected by soil moisture level, but trunk circumference increase was
significantly correlated to soil moisture increase. In certain years, leaf
nitrogen and phosphorus increased as soil moisture increased, while man-
ganese and iron decreased, and boron, molybdenum and aluminum showed
variable effects. In other years these particular relationships failed to
appear. Magnesium and zinc in the fruit were higher in the irrigated plots.

Storage scald and ground color measurements on the fruit indicated
no relationship to soil moisture. Brown core was significantly related to
high soil moisture levels, but was reduced under CA storage conditions.
Development of bitter pit on Northern Spy was not influenced by soil mois-
ture increase. Both flesh firmness and soluble solids measurements in
McIntosh varied inversely with soil moisture, when tested at time of har-
vest.

In general, very slight response to the various soil moisture treat-
ments was achieved. It is suggested that a relatively high soil moisture
level in the control plots plus relatively low evapotranspiration rates may
have contributed to these results. Apparently, in comparable orchards on
similar soils and under similar climatic conditions, soil moisture may not

be a limiting factor in tree growth and productivity.

10.

11.

LITERATURE CITED

. Aldrich, W. W., M. R. Lewis, and R. A. Work. Anjou pear responses

to irrigation in a clay adobe soil. Ore. Agr. Exp. Sta. Bul. 374,
Part 1, 1940.

. Baver, L. D. Soil Physics. 3rd Ed. p. 308. John Wiley and Sons, Inc.

New York. 1 956.

. Benedict, W. D. Effect of supplemental irrigation upon the nitrogenous

composition of the leaves and stems adjacent to the fruit and upon
fruit size and quality of the Jonathan apple. Diss. Abst. Ohio State
University. 1957.

Bouyoucos, G. J., and A. H. Mick. An electrical resistance method
for the continuous measurement of soil moisture under field condi—
tions. Mich. Agr. Exp. Sta. Tech. Bul. 172. 1940.

. Improvements in the plaster of
paris absorption block electrical resistance method for measuring
soil moisture under field conditions. Soil Sci. 63: 455-465. 1947.

 

Boynton, D. , and E. F. Savage. Soils in relation to fruit growing in
New York. Part XIII: Seasonal fluctuations in soil moisture in some
important New York orchard soil types. N. Y. (Cornell) Agr. Exp.
Sta. Bul. 706. 1938.

Claypool, C. C. A study of certain reactions of the apple to soil mois-
ture and meteorological conditions. Res. Studies. State Coll. of
Wash. 5: 78-79. 1937.

Corgan, J. N. The relationship between moisture stress and the uptake
and translocation of phosphorus by plants. Diss. Abstr. Univ. of
Mo., Columbia. 1961.

Author unknown. Ground color for McIntosh apples. Supplement -
Cornell Extension Bul. 750. 1958.

Dennis, C. C. Michigan Fruit production - importance and location.
Mich. State Univer. Spec. Bul. 441. 1962.

Eichmeyer, A. H. Climates of the States - Michigan. Climatography
of the United States No. 60-20. U. S. Gov't Print. Off. May,
1959.

64

65

12. Forshey, C. G. Irrigating New York Orchards. Proc. N. Y. Sta. Hort.
Soc. 1958: 90-94.

13. Furr, J. R. , and J. R. Magness. Preliminary report on the relation of
soil moisture to stomatal activity and fruit grth of apples. Proc.
Amer. Soc. Hort. Sci. 27: 212. 1930.

14. , and C. A. Taylor. Growth of lemon
fruits in relation to moisture content of the soil. U. S. D. A. Tech.
Bul. 640. 1939.

 

15. Haller, M. H., and P. L. Harding. Relation of soil moisture to firm-

ness and storage quality of apples. Proc. Amer. Soc. Hort. Sci. 35:
205-211. 1938.

16. Harley, C. P., and M. P. Masure. Studies on the interrelation of leaf
area, soil moisture, and nitrogen to fruit growth and fruit bud for—
mation in the apple. Proc. Wash. State Hort. Assn. 28: 212-216.
1932.

17. Hendrickson, A. H., and F. J. Veihmeyer. Irrigation experiments
with peaches in California. Cal. Agr. Exp. Sta. Bul. 479. 1929.

18. . Irrigation experiments
with prunes. Cal. Agr. Exp. Sta. Bul. 573. 1934.

19. . Irrigation experiments
with pears and apples. Cal. Agr. Exp. Sta. Bul. 667. 1942.

20. . Readily available soil
moisture and sizes of fruits. Proc. Amer. Soc. Hort. Sci. 40:
13-18. 1942.

21. Hibbard, A. D. , and M. Nour. Leaf content of phosphorus and potas-
sium under moisture stress. Proc. Amer. Soc. Hort. Sci. 73: 33-
39. 1959.

22. Kelley, O. J., A. S. Hunter, H. R. Haise, and H. H. Clinton. A com-
parison of methods of measuring soil moisture under field conditions.
Jour. Amer. Soc. Agron. 38: 759-784. 1946.

23. Kenworthy, A. L. Soil moisture and growth of apple trees. Proc. Amer.
Soc. Hort. Sci. 54: 29-39. 1949.

24.

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' 36.

66

Kramer, P. J. Soil moisture in relation to plant growth. Bot. Rev. 10,
No. 9: 525. 1944.

Magness, J. R., E. S. Degman, and J. R. Furr. Soil moisture and
irrigation in Eastern apple orchards. U. S. D. A. Tech. Bul. 491.
1935.

Mason, A. C. The effect of soil moisture on the mineral composition
of apple plants grown in pots. Jour. Hort. Sci. 33: 202—211. 1958.

Mochizuki, T. , and S. Hanada. Effect of soil moisture on the growth
and quality of apple fruits. Bull. Fac. Agric. Hirosaki University
No. 6: 43—56. 1960.

Nour, M. The effect of different minimum soil moisture levels on
potassium and phosphorus uptake by seedling plants of peaches, apples,
mustard, and strawberries. Diss. Abstr., Univ. Mo., Columbia.
1956.

Overley, F. L. et al. Irrigation of orchards by sprinkling. Wash.
Agr. Exp. Sta. Bul. 286. 1932.

Richards, L. A., and C. H. Wadleigh. Soil physical conditions and
plant growth. Agronomy, Vol. 2, Chap. 3. Academic Press Inc.,
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Ryall, A. L., and W. W. Aldrich. The effect of water supply to the
trees upon water content, pressure test, and quality of Bartlett pears.

Proc. Amer. Soc. Hort. Sci. 35: 283-288. 1938.

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Interscience Publishers, Inc., New York, N. Y. 1950.

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by soil moisture conditions. Agron. J. 45: 611-614. 1953.

Veihmeyer, F. J. Some factors affecting the irrigation requirements of
deciduous orchards. Hilgardia 2: 125—288. 1927.

, and A. H. Hendrickson. Soil moisture conditions in

 

relation to plant growth. Plant Physiol. 2: 71-82. 1927.

Some plant and soil mois-

 

ture relations. Amer. Soil Surv. Assoc. Bul. 15: 76-80. 1933.

67

37. Veihmeyer, F. J., and A. H. Hendrickson. Essentials of irrigation and
cultivation of orchards. Cal. Agr. Exp. Sta. Circ. 50. 1936.

38. . Does transpiration decrease
as the soil moisture decreases? Trans. Amer. Geophys. Union 36:
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29: 41-46. 1937.

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tree wilting in a heavy clay soil. Jour. Amer. Soc. Agron. 28: 124-
134. 1936.

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