"WI T l l l 1 HM ‘ m «u I \ 1 WI H 2% WI SREUGATION AND NEYRO‘GEE EFFfiCTS ON THE FACTGRS WRICH INFLUENCE ’{fiE WELD ”3;? ii; mm. 0? it’OTATC‘IES Tho-3%: far rim imam“ an: $91. 5, :‘AECHEISAN flhfi U§‘{}‘\"ER$§‘E"1‘ 'a’kilt‘ffizfis “i. wam T§éfi LIBRARY Michigan State University IRRIGATION AND NITROGEN EFFECTS ON THE FACTORS WHICH INFLUENCE THE YIELD OF A HILL OF POTATOES By Thames H. Obourn AN ABSTRACT Submitted to the School of Graduate Studies of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Farm Crepe 1962 / Approved fl" 1“"7“'3&)LJVLAL/@1\I/// 1 Thomas H. Cbourn Yield components of the potato consisting of tubers per stem, stems per hill and weight per tuber were studied at Lake City in 1961. Three varieties, Kennebec, Katahdin and Onaway, were evaluated under several combinations of irrigation and nitrogen. ' When the data was arranged by number of stems per hill, the tuber number, the gross tuber weight and the weight of U.S. Number 1 fancy tubers increased as stems per hill increased. Added nitrogen increased tuber number per hill in the Katahdin variety but had no effect on the Kennebec variety. The most marked effect of nitrogen was in the increased weight per tuber of the Katahdin variety. Irrigation increased both tuber set and size for the Onaway variety. Simple correlation coefficients between tubers to stems (T:S), weight to tubers (W:T) and weight to stems (WtS) revealed that for the most part they were correlated at above 1% level. Treatment of irrigation or nitrogen did not affect these relationships. The Kennebec variety was greater than either the Katahdin or the Onaway variety in (T:W) - It is proposed that yield components may be of value to both the potato breeder and production.manager in evaluating varieties. IRRIGATION AND NITROGEN EFFECTS ON THE FACTORS WHICH INFLUENCE THE YIELD OF A HILL OF POTATOES By Thomas H. Obourn A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Farm Crops 1962 ACKNOWLEDGEMENT The author wishes to express sincere gratitude to Doctors D. R. Isleib, N. R. Thompson, C. M. Harrison and Professor H. M. Brown of the Farm CrOps Department for their help with this problem and the completion of the thesis. TABLE OF CONTENTS INTRODUCTION REVIEW OF LITERATURE MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY LITERATURE CITED 13 52, 35 LIST OF TABLES Whpietfegl used, nitrogen practices and irrigation treatments at Lake City Experiment Station. 1961. Analysis of variance of hill yield of Kennebec and Katahdin varieties under treatments of five levels of nitrogen and two levels of irrigation. The frequency of number of stems per hill by variety. Average tubers per stem of the Kennebec and Katahdin varieties with treatments of no nitrogen and nitrogen and the Onaway variety with treat- ments of no irrigation and irrigation. Number of tubers per stem in different weight classes of under 50 grams, 50 to 500 grams and over 500 grams. Data taken from spacing trial of the Kennebec variety spaced at 7.5, 9, l2 and 15 inches. The simple correlation coefficients between the number of tubers (T), number of stems (S) and weight per hill (W) for the Katahdin and Kennebec varieties grown at Lake City in 1961. The hills per plot were harvested from each of three repli- cations. The correlation coefficients are the averages of the coefficientsobtained from the three replications. The simple correlation coefficients between the number of tubers (T), number of stems (S) and weight per hill (W) for the Onaway variety grown at Lake City, 1961. Fifteen hills per plot were harvested from each of three replications. The correlation coefficients are based on 45 hills. iv 14 15 22 26 29 31 10. 11- 12- 13- LIST OF FIGURES Growing season temperature, Lake City Experiment Station, May to Sept., 1961. Rainfall inches per week, Lake City Experiment Station, May to Sept., 1961 Number of tubers of the Kennebec variety with treatments of no-nitrogen and nitrogen. Number of tubers of the Onaway variety with treatments of no-irrigation and high irrigation. Number of tubers of the Katahdin variety with treatments of no-nitrogen and nitrogen. Gross weight per hill of the Kennebec variety with treatments of no-nitrogen and nitrogen. Gross weight per hill of the Katahdin variety withxtreatments of no-nitrogen and nitrogen. Gross weight per hill of the Onaway variety with treatments of no-irrigation and high irrigation. Weight per hill of tubers between 50 to 300 grams of the Kennebec variety under treatments of no- nitrogen and nitrogen. weight per hill of tubers weighing over 300 grams of the Kennebec variety with nitrogen treatment. Weight per hill of tubers between 50 to 500 grams of the Katahdin variety under treatments of no- nitrogen and nitrogen. weight per hill of tubers between 50 to 500 grams of the Onaway variety under treatments of no- irrigation and irrigation. Number of tubers per hill weighing between 50 and 300 grams of the Kennebec variety under treatments of no-nitrogen and nitrogen. 10 10 16 16 16 16 18 18 18 18 20 20 2O 14. 15. 16. 17. 18. vi Number of tubers per hill weighing between 50 and 300 grams of the Katahdin variety under treatments of no-nitrogen and nitrogen. Number of tubers per hill weighing between 50 and 300 grams of the Onaway variety with treat- ments of no-irrigation and irrigation Ayerage weight per tuber of the Kennebec variety under treatmentsof no-nitrogen and nitrogen. Average weight per tuber of the Katahdin variety under treatmentsof no-nitrogen and nitrogen. Average weight per tuber of the Onaway variety under treatments of no-irrigation and irrigation. 20 23 23 23 23 INTRODUCTION The potato industry is in a dynamic economic price- cost squeeze which.makes it necessary for the potato pro- ducer to obtain high yields to realize the highest returns on his investments. Potatoes are marketed as freshemarket table stock, for processing, or as seed with each.use having its particular requirements. Size is an important factor in the marketing of potatoes and.most buyers are familiar with the U.S. Number 1 grade which.is regulated by federal market grades and, in some states, by state marketing laws. Seed and processing potatoes have size standards of their own. The processors are not satisfied with.the minimum.quality. standards of U.S. Number 1 and additional factors are re- quired, such.as dry matter and reducing sugars, which:may affect the processed products. The yield of a potato plant is the product of the number of tubers and the weight per tuber which.may be con- sidered as components of yield. ‘When potato yield is con- sidered on the unit of the hill, which is actually a group- ing of individual plants, the number of stems per hill, the tubers per atom and the weight per tuber become the yield components. These components are governed by both.environ- mental and hereditary influences. The yield potential of any variety is determined by the number of tubers set per plant and the potential tuber size, while actual yield is determined by environmental influences which.may prevent full development of the tubers set. It is the objective of this thesis to explore some of the environmental and hereditary differences between varieties on a component basis. With an understanding and use of yield components, a.manager may adjust production practices for increased.yield and the potato breeder may assess seedling pOpulations. REVIEW OF LITERATURE Though.the literature contains many references to the potato, little information is available on just how:the potato plant reacts to hereditary and environmental factors which.inf1uence the yield. Arthur (1), in 1891, was among the first to report any data pertaining to plant factors affecting yield of potatoes. He found that the number of tubers per hill is determined, within certain limits, by the number of stems per hill. Claypool andMorris (8) reported the greatest yield of U.S. Number 1 tubers occurred with two to three stems per hill. The number of tubers per hill increased as the stems per hill increased. The hills with four stems offset the yield of those with one stem even though the latter had more tubers per stem and a higher average tuber weight. They concluded that potato size can be controlled either by seed piece or hill spacing. Bushnell (6) showed that the number of stems varied from one to five per hill. The hills with the fewest stems produced the lowest yield. The hills with.two to four stems gave the highest yield of potatoes over one and seven-eighths inches in diameter. He concluded that the number of stems per bill was not a factor affecting the yield of potatoes within the range of one to five stems. Burton (4) found that the hills with.more than one stem per hill produced.more tubers than one stem hills, but this relationship was not an arithmetical progression. Seed piece size has been reported by Bates (2) and Clark (9) to influence the number of stems. Clark (9) con- cluded that larger seed pieces generally have more eyes and therefore produce more stems. Greater stem number increased the tuber yield per hill. Stem number by itself may be increased by some of the following practices: (1) Larger seed pieces generally have more eyes. Consequently, more stems per seed piece will be produced. Bates (2) and Clark (9). (2) Michener (17) treated the tubers forty-six days after harvest with ethyl- enechlorohydrin to break apical dominance. The non-treated tubers produced 1.6 stems while the treated tubers produced 4.7 stems per tuber. (3) Bushnell (6), working with Russet Rural tubers in Ohio, found that as the planting season progressed from.April to June there was an increase in stem number. The stem increase was about one stem.per tuber during this period. Bushnell (6) suggested that as the season progressed, wider spacing differences could be used. To take advantage of the increased stem number, 9 inch spacing for the early planting and 12 inch spacing for the later planting could be used. Warren (19) cited many cases in which closer spacing produced a higher yield. In his investigations, 6 inch spacing consistently produced higher yields of 3 to 12 ounce tubers with.smaller yields of tubers over 12 ounces; while a 12 inch spacing produced more tubers over 12 ounces and less of the 3 to 12 ounce class. These findings agree with Biship's (3) work. Bishop also reviewed the work of several other investigators whose conclusions supported his results. Nitrogen and water are both essential for the pro- duction of a potato crop. If either is limited, the yield will be depressed; but when supplemental nitrogen or water is added, the yield may be increased. The influence of nitrogen or irrigation on the potato crop is a subject of much controversy in the earlier literature. King (15), in 1886, stated that the percentage of large tubers was increased by irrigation and this increase in the tuber size added to the yield. Clark (9), in studying the ontogeny of the potato tuber, declared that: (1) when irrigation was applied after tuber formation, there is only a size increase and (2) when applied before tuberization, there is both a set and size increase. Fussing (11) worked with the Irish Cobbler variety in irrigation investigations. He found that this variety set 3.0 tubers per stem when supplemental irrigation was applied at 1.0 and 1.5 inch levels. Irrigation was applied after water had evaporated one inch from an open pan. A total of 12 inches was applied for the season. An average of 1.0 tuber per stem was set with no irrigation. Fussing concludes that added irrigation increased the tuber set, but it did not materially increase yield as not all the tubers increased in size. Pratt, et al., (18) found that irrigation.may increase the tuber set as well as the tuber size, but these results varied with location and season. Early-season irrigation in rm m 1951 and 1952 produced 21 and 29 more tubers which were over 1.25 inches in diameter per 25 feet of row, respectively. In g 1949 irrigation produced no increase in yield of the tubers over 2 inches in diameter. The irrigated potatoes averaged .41 of a pound, and the non-irrigated .42 of a pound per tuber. Harris (13), working with flood irrigation, found that one inch of water per week for 12 weeks produced tubers averaging .22 of a pound. Treatments of O, 2.5, 5 and 7.5 inches per week produced tubers weighing less. The 2.5 and 5.0 inch levels produced more tubers but the 1 inch level resulted in the highest yield. Early-season irrigation increased the tuber number while later applications increased the tuber size. Harris found the average yield per hill and per acre were very closely related to the application of supplemental water. Bushnell (5) found that nitrogen, applied either in an organic form or as sodium.nitrate, increased the tuber set; but it had no appreciable affect on the tuber size. Martin (16) found the largest tubers were produced by mixtures of fertilizer containing large amounts of nitrogen and little or no potassium.or phosphorous. When nitrogen was absent from the fertilizer, the yields were the lowest. Bradley, et a1., (7), investigating the influence of ' earlybseason irrigation and nitrogen application, concluded: (l) the greatest effect of moisture was earlier maturity r rather than an increase in number of tubers set and (2) nitrogen was not effective in increasing tuber set. L The environmental factors over which.man has some _ control have been discussed.- Two factors over which.he has ' ‘ little control are light and temperature. Hardenburg (12) states that the potato plant yields best in areas where the mean temperature of the warmest month of the growing season is not above 65° Fahrenheit. Intensity, quality, and duration of light are important. However, according to Hardenburg, these factors are not considered to be of any great consequence during the normal growing season. MATERIALS AND METHODS An experiment was set up at the Lake City Experiment Station in 1961, which.was designed to measure the effect of irrigation and nitrogen levels on 3 different varieties of potatoes. Table 1 lists the materials used in this experiment. All varieties shown in Table 1 received combi- nations of irrigation and nitrogen. A split plot experi- mental design was used. Table 1. Varieties used, nitrogen practices and irrigation 'treatments at Lake City Experiment Station. 1961. Elemental nitrogen Irrigation applied Varieties _pounds per acre aty% field capacity Kennebec 0 0 Katahdin so P.U.l 50% Onaway 50 P.U. - 50 3.13.2 75% 50 P.U. - 100 S.D. 150 P.U. lP.U. = Plow under. 2S.D. 8 Side dress - July 5. Cut potato seed pieces of 1.5 ounces were planted on May 16 and 17. The seed pieces were planted in 56 inch rows spaced 9 inches apart in the rows The initial fertilizer rate of 1000 pounds per acre of 0-10-40 was broadcast before plowing. Three-hundred pounds of 5-10-20 was applied in bands at planting. Additional nitrogen was incorporated as reported in Table l. The soil type on which the potatoes were grown is classed as a Montcalm loamy sand. The pH of the plow layer was variable throughout the area but averaged 5.5. Fertilizer requirements were determined by soil tests. Irrigation was applied from July 5 until harvest, with soil moisture determinations governing the application of water. When soil moisture decreased to 50% or 75% of field capacity, irrigation was applied. Yield data from irrigation were evaluated by comparison with plots receiving only normal precipitation. weather data from the 0.8. weather Bureau recording station on the Lake City Experiment Station are shown in Figs. 1 and 2. Temperatures during the May to September growing season were favorable for the growth of the potato plant. Rainfall was well distributed over the entire grows 'ing season. Data were collected from plots of the Onaway, Kennebec and Katahdin varieties. The data for the Onaway variety were taken from 0 and 75% irrigation levels with nitrogen treatments of 50 and 50 plus 100 pounds nitrogen per acre. The data from the Kennebec and Katahdin varieties were obtained from 50% and 75% irrigation levels with all five levels of nitrogen. The early variety, Onaway, was harvested on.August 16, as which time the taps showed signs of maturity. Fifteen Temperature Inches of Rain 80 7O 60 50 40 30 10 ~Maximum // \“x M an // e. Minimum May I June I July I Aug. T Sept. 1 Fig. 1. Growing Season Temperature,Lake City Experimental Station May to Sept., 1961 1fi[ b .H .na ma n.a m.oa n.m n.o o.m o.¢ m.» m.a e.H m.a .nH ma 0.0 o.b n.m n.m o.m o.» o.H &.m .na 0 n.H H.m m.w o.» o.H o.H .aa m.> wnaodmm b w m e n m H o m w n m an r o m e n m nonsnn nepME n madam con hobo easy com on on madam on noon: .nonona ma one ma .0 .m.r we became heeded» ooponnom on» go dean» wndosme son“ send» spam .naanw oon nope use madam com on on .usdam on nouns no nommdao pnwaoh unchanged ca Haas non muons» no nopapz .m capes 27 Fingey and Stewart (10) found that as the number of stems increased, the number of tubers per stem increased; but the yield of the marketable tubers decreased. The stem number, Bushnell (5) concluded, is a factor that can act much the same as seed piece spacing. A potato hill with.many stems may produce the same effect as seed pieces spaced at inter- vals of approximately 6 inches. A potato hill with 2 to 3 stems is much the same as wide spacing, for instance 12 to 15 inches between seed pieces. Warren (19), and Bishop and Wright (3) have documented these facts. Weight per tuber The average tuber weight for the Kennebec, Katahdin and Onaway varieties is shown in Figs. 16, 17 and 18. When there was more than one stem per hill in the Kennebec and Katahdin variety, there was little decrease in the average tuber weight. The addition of nitrogen.markedly increased the average tuber weight of the Katahdin variety (Fig. 17). In the Kennebec variety (Fig. 16) the relationship between nitrogen and no nitrogen and weight per tuber was not con- sistent; the nitrogen application producing little if any increase in the average tuber weight. Where nitrogen was applied to the Kennebec variety it generally had a higher average tuber weight than Katahdin. The Kennebec variety averaged approximately 25 grams more per tuber beyond the one stem level than did the Katahdin variety. When observations were:made on the Onaway variety, I ! 28 it may be noted that there was little difference between the one and seven stem level in the average weight per tuber. Irrigation produced a larger average tuber weight at all stem levels. With irrigation, the average tuber weight of the Onaway variety increased 25 grams. Correlations 4 v“ Simple correlation coefficients were determined between number of tubers and number of stems (T:S), gross weight per hill and number of tubers (W:T) and gross weight per hill and number of stems (W:S). (Tables 6 and 7). Ten i hills were used for the analysis for each replication and the replication r values were averaged to obtain the values in the body of Table 6. Of the 60 correlation coefficients shown under the Kennebec and Katahdin varieties for 5 levels of nitrogen and 2 levels of irrigation, 52 exceeded the 1% level, 4 exceeded the 5% level and 4 were not significant. The 4 that are not significant are under the‘wss classification. In the classification of T:S, of 20 correlation co- efficients all but 2 exceed the 1% level and are greater than .500. This indicates a rather consistent relationship. The correlation coefficients of W:T were fairly con- sistent with.but one (.472) significant at only the 5% level. The correlation coefficients of W:S, though for the most part significant at the 1% level, are somewhat less than the corresponding coefficients under T:S and wsr. 29 Table 6. The simple correlation coefficients between the number of tubers (T), number of stems (S) and weight per hill (W) for the Katahdin and Kennebec varieties grown at Lake City in 1961. Ten hills per plot were harvested from each of three replications. The correlation coefficients . are the averages of the coefficient obtained from the three replications. ' Irrigation high Nitrogen Katghdin Kennebec Ave. nitrogen treatments T:S W:T :W:s T:S War ‘Wes T:S wzr ‘w:s 0 .664 .655 .582 .746 .860 .662 .705 .747 .602 50 .444 .656 .514 .690 .728 .598 .567 .682 .556 50 + 50 .906 .695 .588 .771 .802 .542 .808 .748 .565 50 + 100 .795 .757 .488 .564 .745 .580 .678 .741 .454 150 .545 .576 .220 .765 .791 .189 .655 .685 .204 Ave. high irrigation .666 .645 .478 .706 .785 .478 .686 .715 .478 Irrigation low 0 .656 .552 .454 .485 .729 .470 .569 .640 .462 50 .805 .807 .652 .708 .862 .709 .755 .854 .685 50 + 50 .810 .666 .625 .767 .654 .411 .788 .650 .527 50 + 100 .656 .786 .475 .779 .845 .591 .717 .825 .552 150 fl3621 .472 .565 .687 .697 .580 .654 .584 .472 Ave. low irriggtion .709 .656 .519 .685 .755 .552 .697 .704 .555 ‘Average " variety .687 .650 .498 .695 .769 .515 7 Average 0 nitrogen .657 .695 .552 Average 50 nitrogen .666 . .6 Tverage 50 + 50 nitrogen Averag6“50 +‘lOO nitrogen Average150 nitrogen General average 5% level 0.581 1% level 0.487 ‘fiflflf‘fififif‘nfifif' .597‘7755'7255' .§fi§ ifififTEfiEF' .691 .709 .507 30 Harvest observations may give some insight into why these correlation coefficients are lower than the correlation coefficients of T:S and‘W:T. It was observed, when digging the hills, that some stems had a very few or no tubers while other stems might have many tubers. The response is very similar for the Kennebec, Katahdin (Table 6) and Onaway (Table 7). All varieties had correlation coefficients of T:S, W:T and w:s that exceeded the 1% level in significance. Kennebec had a higher'W:T correlation than Katahdin. The w:s relationship was lower for each variety but it again was not significantly lower than the other r values. Irrigation by itself on the Kennebec and the Katahdin varieties is not significantly different in either case. All nitrogen applications when viewed independently of variety or irrigation exceed the 1% level in significance except the 150 pound application in.W:S (.558). The same general relationship exists in T:8, WtT and wss for each nitrogen level as has been noted for variety and irrigation (Table 6). In the Onaway variety (Table 7) the 0 level of irrigation gave higher correlation coefficient values than the high level of irrigation. However, this difference between 0 irrigation and high irrigation was not enough to be significantly different. The coefficient of determination (r2 x 100) gives another slant to the data in Tables 6 and 7. .An example 51 would be of .691 under general average for T:S which when squared and.multiplied by 100 equals 47.7%. This means that 52.5% of the squared variability in number of tubers per hill is unaccounted for by the squared variability in stem number. Shmilarly, the coefficients of determination of squared variability in gross weight of tubers per hill due to varia- bility in tuber number is 50.5% and in gross weight per hill due to variability in stem number is 25.7%. Many factors, such as, fertility, moisture, temperature, soil aeration, etc., could affect these relationships and must be studied further to get at the sources that influence the varia- bilities that affect the correlations. Table 7. The simple correlation coefficients between the number of tubers (T), number of stems (S) and weight per hill (W) for the Onaway variety grown at Lake City, 1961. Fifteen hills per plot were harvested from each of three replications. The correlation coefficients are based on 45 bills. Witrogen , Irri fiion high 13 irrigation gye. nitroge: treatment T:S W:T W;S T:S W:T WtS T38 WtT wzs 50 + 50 .550 .477 .495 .798 .745 .655 .664 .611 .565 50 + 100 .552 .746 .679 .774 .734 .496 .663 .740 .587 Irrigation .541 .611 .587 .786 .739 .565 ‘General average .665 .675 .576 5% level 0.581 1% level 0.487 SUMMARY Data were taken from an experiment measuring irri- gation and nitrogen responses of Katahdin, Kennebec and Onaway at the Lake City Experiment Station in 1961. Hill data were compiled on the number of stems per hill, the average number of tubers per stem and the weight per tuber per hill. The investigations may be summarized under the following: Stem and Tuber Number 1. As the stem number per hill increased, tuber number and gross weight per hill increased. 2. When the data were divided into classes, only numbers of the U.S. Number 1 fancy tubers (50-500 grams) in- creased with stem number. Tuber number classes for small (under 50 grams) and large tubers (over 500 grams) were inconsistent. Tuber'Weight 1. Average tuber weight decreased as the number of stems increased. Correlations l. Coefficients of correlations between T:S, WtT and W;S were, for the most part, statistically significant, 52 35 regardless of treatment. 2. The three relationships were somewhat stronger in Kennebec than in Katahdin on the average. 5. The‘Wss relation had the lowest r value of any of the relationships studied. General rm 5i 1. The component of yield hypothesis may be very useful to the potato breeder. Stem number, tuber number and tuber weight may be important considerations for the breeder in selecting new varieties. Any one of these factors or a combination may have a definite bearing on the marketable yield of a new seedling. Components of the tuber number and the tuber weight might be used to project the yield of a seedling from a few hills to an acre basis. 2. The potato producer may be able to predict yield in his fields early in the growing season. If yield expectancy is not up to par, he may be able to:manipu- late some factor, such.as nitrogen fertilization or irrigation, to increase his yield by influencing one of the components. There was a marked varietal response to nitrogen and irrigation for each of the three varieties. There is also a difference between varietal response to different components. A case in point is the Kennebec variety in its response to 54 added nitrogen. The Kennebec was not very responsive to any of the nitrogen levels applied during the growing season. It may be concluded from this that large applications of nitro- gen are not beneficial when applied to the Kennebec variety. It was shown that the Onaway responded very well to irriga- tion in that it increased the tuber set and also increased the tuber size. Some yield-influencing factors have been investigated and are reported in the literature, but many of these factors have not been investigated on the component basis. B‘W‘t -.“’ _ . “'3’". ' ' .-- . . .. When these component factors have been studied and the varietal responses are better known, recommendations may be made to manipulate the crop environment for the most beneficial yield. In this way blanket recommendations will not have to be made for all varieties in all locations, but they may be broken down by variety and by area. LITERATURE CITED 1. Arthur, J. C. .A physiological basis for the comparison of potato production. Agr. Sci. 6:201-216. 1892. 2. Bates, G. H. A study of the factors influencing size of potato tubers. Jour. of Agr. Sci. 25:297-515. 1955. 5. Bishop, J. 0., D. N. wright. The effect of size and spacing of seed pieces on the yield and grade of White Rose potatoes in Kern County, California. Amer. P0130 Joure 35 3235-239. 1959. 4. Burton, C. L. The value of size of vine, set and shape of tubers as indexes for selecting potato tubers in the breeding program, Masters Thesis, Michigan State University. 1952. 5. Bushnell, J. Effect of fertilizers on number of and size of potato tubers. Proc. of the Pot. Assoc. of Amer. 11:108-115. 1924. 6. . Effect of number of plants per hill on the yieId of potatoes in Ohio. Amer. Pot. JOur. 19: 119-125. 1942. 7. Bradley, G. A., A. J. Pratt. The effect of different combinations of soil moisture and nitrogen levels on early plant development and tuber set of potatoes. Amer. Pot. Jour. 52:254-258. 1955. 8. Claypool, L. L., 0. M. Morris. Some responses of ’ potato plants to spacing and thinning. Amer. Soc. of Hort. Sci. 28:255-256. 1951. 9. Clark, C. Development of tubers in the potato. U.S. Dept. Of Agr. B11110 9580 1921e 10. Fingey, D. 0., G. Stewart, Effect of seed set on yield - ~ and on certain other characters in potatoes. Agron. JOur. 20:710-721. 1928. ll. Fussing, E. B. Irrigation--its effect on pore space, set, and yield of potatoes. Proc. Ohio Veg. and Pot. Growers Assoc. 25:154-162. 1940. 55 12. 13. 14. 15. 16. 17. 18. 19. 36 Hardenburg, E. V. Potato production. Comstock Pub. Company, Inc., Ithaca, New'York. p. 270. 1949. Harris, F. S. The irrigation of potatoes. Utah Agr. Exp. Sta. B1110 1570 19170 Hougland, G. V. 0., R. V. Akeley. Effects of seed spacing and fertilizer rate of potato varieties and on financial returns. Amer. Pot. Jour. 56: 227-254. 1959. King, F. H. Influence of varying amounts of water in the yield of potatoes. Wis. Sta. Rep. 1886: 189-204. 1886. Martin, W. H. The influence of nitrogen, phosphoric . acid, and potash on the number, shape and weight of E potato tubers. Jour. of Agr. Res. 45:251-260. 1951. Michener, D. H. Dormancy and apical dominance in potato L~ tubers. Am. Jour. Bot. 29:558-568. 1942. Pratt, A. J., J. Lamb, J. D. Wright, G. Bradley. Yield, tuber set and quality of potatoes. Cornell Agr. Exp. Sta. B111. 8760 19520 warren, G. C. The influence of type of set and planting distance on grades and yield of potatoes for seed. Amer. Pot. Jour. 55:755-757. 1958. Wu- HILIUHIH l l I I'll- l I I l I III III III III- II. I l I» III 93 03169 07 312 IIHIIHHIHIIIII