Zambczia (1078), VI (ii).THE DRYLAND CASH-CROP PRODUCTION POTENTIALOF NATURAL REGION IIP. IVYDepartment of Conservation and Extension, Ministry of Agriculture, SalisburyTHS-: Ac-Ro-rcoiOGiCAL SURVEY of Rhodesia, published over 20 years ago,provides a very good guide to the natural factors governing agricultural pro-duction in Rhodesia: and generally the recommended farming systems pro-posed for the different Natural Areas still hold good today. However, a con-siderable amount of additional information on rainfall distribution, topo-graphy, soil and vegetation types, and, more particularly, crop and livestockperformance within the various Regions, is now available to the land-useplanner.Some modification to the Agro-ecological Survey was therefore in-evitable and so a revised map, showing slight changes in the delineation ofthe Natural Regions has recently been produced by the Planning Branch ofthe Department of Conservation and Extension. This makes use of the rainypentad analysis developed by Griffith in East Africa which appears to givea better indication of rainfall distribution and its effect on crop yield thandoes effective rainfall, which was one of the main criteria used in definingthe agro-ecological regional boundaries.A rainy pentad is defined as the centre one of three five-day periods(pentads) which together receive more than 40 mm rainfall and two of whichreceive at least 8 mm of rainfall. Areas which have the highest crop yieldsreceive on average at least 18 rainy pentads per season, whereas five-yearaverage crop yields of maize, flue-cured tobacco and cotton are generally sub-economic in areas which receive, on average, less than 16 rainy pentads perseason. These rainy pentad criteria were therefore used when revising theAgro-ecological Survey map, and the definitions of regions lla, lib and IIIwere amended accordingly. The definitions of the Natural Regions nowread:Natural Region 1: Specialized and Diversified Farming Region:Rainfall in this region is high (more than 1 000 mm per annum inareas lying below 1 700 m altitude, and more than 900 mm perannum at greater altitudes), normally with some precipitation inall months of the year. Temperatures are normaly comparatively* This is an edited version of a paper prcscntod to the Fourth Rhodesian ScienceCongress in September 1()77.147148 THE DRYLAND CASH-CROP PRODUCTIONlow and the rainfall is consequently highly effective enablingafforestation, fruit and intensive livestock production to be prac-tised. In frost-free areas plantation crops such as tea, coffee andmacadamia nuts can be grown; where the mean annual rainfall isbelow 1 400 mm, supplementary irrigation of these plantationcrops is required for top yields.Natural Region II: Intensive Farming Region: Rainfall is confinedto summer and is moderately high (750-1 000 mm). Two sub-regionshave been defined. Sub-region Ha receives an average of at least18 rainy pentads per season and normally enjoys reliable condi-tions, rarely experiencing severe dry spells in summer. The regionis suitable for intensive systems of farming based on crops and/or livestock production. Sub-region Kb receives an average of16-18 rainy pentads per season and is subject either to ratlier moresevere dry spells during the rainy season or to the occurrence ofrelatively short rainy seasons. In either event, crop yields in certainyears will be affected, but not sufficiently frequently to change theoverall utilization from intensive systems of farming.Natural Region III: Semi-Intensive Fanning Region: Rainfall inthis region is moderate in total amount (650-800mm), but, becausemuch of it is accounted for by infrequent heavy falls and tem-peratures are generally high, its effectiveness is reduced. Thisregion will receive an average of 14-16 rainy pentads per season.The region is also subject to fairly severe mid-season dry spells andtherefore is marginal for maize, tobacco and cotton production,or for enterprises based on crop production alone. The farmingsystems, in conformity with the natural conditioning factors,should therefore be based on both livestock production (assisted bythe production of fodder crops) and cash crops under good mange-ment on soils of high available moisture potential.Natural Region IV: Semi-Extensive Farming Region: This regionexperiences fairly low total rainfall (450-650 mm) and is subjectto periodic seasonal droughts and severe dry spells during the rainyseason. The rainfall is too low and uneeriain for cash cropping ex-cept in certain very favourable localities, where limited drought-resistant crops can afford a sideline. The farming system, in accordwith natural factors, should be based on livestock production, butit: can be intensified to some extent by the growing of drought-resisiant iodder crops.Natural Region V: Extensive Farming Region: The rainfall in thisregion is too low and erratic for the reliable production of evendrought-resistant fodder and grain crops, and farming has to bebased on the utilization of (he veld alone. The extensive form ofcattle ranching or game ranching is the only sound farming systemfor this region. Included in this region are areas of below 900 m al-titude, where trie mean rainfall is below 650 mm in the ZambeziValley and below 600 mm in the Sabi-Limpopo valleys.This paper indicates the distribution of crop yields by Natural Region,and the work in progress on mapping the major soil types (Natural Areas)P. IVY149within Natural Regions II and III. Crop yields from as many farms as possi-ble within each soil type arc being analysed, so that the yield potential canbe more accurately assessed.FREQUENCY DISTRIBUTION OF CROP YIELDS BY NATURALREGIONThe Farm Management Section, Planning Branch, Department of Conserva-tion and Extension, has compiled and published three editions of a hand-book, incorporating the area (Intensive Conservation Area) average yieldsfor all dr\!and crops from 1963-4 onwards. These yields are the weightedaverages, that is, the tola! production from the area divided by the numberof hectares grown.Five-year average yields from 1970-1 to 1974-5 are given for eachIntensive Conservation Area (I.C.A.), The choice of five years was originallymade in order to reduce the effect of technological improvements on crop pro-duction over a number of years. It does, however, have a disadvantage in thaisuccessive five-year averages may vary considerably owing io the effect olsporadic drought years. A longer term average would tend to reduce this varia-tion and there may well be a case in future for the use of a longer period,especially in the case of the major and longer established crops where tech-nological improvement may be expected to be slowing down.The frequency distribution of maize, flue-cured tobacco and cottonyields by Natural Region arc given in Table I which illustrates the high pro-portion cf I.C.A.s in Natural Regions lib, ill. IV and V which obtainaverage maize yields of less than 4 000 kg/ha. Production of maize aisuch low yield levels is not economic for commercial sales, but may be justi-fied for the supply of labour rations and slock feed. Practically no drylandflue-cured tobacco or cotton is grown in Natural Region IV and ihc numbercf farmers growing thcte crops in Natural Region III is very limited. Onlythe more successful fanners have been able to continue growing those cropsin Region IN, and even then, many are likely to become insolvent at suchlow yield levels.Tills information highlights the need for fanning in Natural Region illto be bafed on livestock production, with dryland cash crop production oniyunder good management on soils of high available moisture potential.THE MAPPING OF SOIL TYPESGeological Survey maps and farm plans which cover over "55 per cent of thearea were used to map the different soil types in the Karoi, Sinoia and BanketGroups of I.C.A.s. The soils were then classified into groups and sub-groups in accordance wiih Dr. f, G. Thompson's thesis 'The Soiis of Rhode-sia and Their Classification'.The mapping units refer to soils occurring on normal or upland relief,15OTHE DRYLAND CASH-CROP PRODUCTIONc/3sCUwQ.-I- sS3 »zS 1o i ««2 ^^OO vo O O O OO 1" O O O Ooo o r-i o o©ŁŠ o oj io oo oo o o oo ^rCN O OtO Ł<}Ł CNO Š <NOO1300-1400 1400-1500 > 1500/ 200-1 3001 100-1 200900 900-1 000 1 000-1 100NaturalRegion00 CN r-CN O *~CN Š O0O CN Šr- CN **O Š <NO O TP. !VY151and catenary associations are not shown. However, major areas of very un-dulating to broken country, resulting in a high proportion of lithosols havebeen mapped. In the description of mapping units, data relating to basesaturation, clay mineralogy and E/C value (cation exchange per 100of clay) refer to subsoil horizons in which the effect of organic matter isnegligible. The soil groups and sub-groups occurring within the area coveredby this report are:Calcimorphic Soils:Unleached soils, generally with large reserves of weatherable minerals; basesaturation usually over 80 per cent; clay fractions predominantly 2:1 lattice;E/C values over 40 m.e.Vertisoi Group: Moderately deep to deep, dark self-churning clays;clay fractions mainly monlmorilionite: E/C values over 60 m.e.3X Dark brown to black vertisols without appreciable watersoluble salts or exchangeable Na; but generally with inverseCa/Mg ralios and often with toxic quantities oi' heavy metals(Ni and/or Cr); formed on ulirabasic rocks.Sallitic Group: Soils in which the clay fractions are predominantlyillite or illite-montmorillonoid mixed-layer minerals: E/C valuesare usually over 40 m.e.: with or without illuvial calcareous ac-cumulation in the lower solum or underlying material.4E Shallow to moderately shallow brown or reddish-brown clays;formed on basic rocks.4S Mainly shallow relatively silty, sandy loams to clay loams;formed mainly on argillaceous sediments.Kaollnitic Soils:Moderately to strongly leached soils; clay fractions mainly kaolinite togetherwith appreciable amounts of free sesquioxides of iron and aluminium.Fersiailitie Group: Soils with appreciable reserves of weatherable mine-rals; base saturation over 40 per cent, usually between 60 and 80 percent; clay fraction contain some 2:1 lattice minerals; E/C values rangefrom about 15 to about 40 m.e.5E Moderately deep to deep, reddish-brown granular clays; formedon basic rocks.5X Moderately deep clays similar to those of 5E but usually withinverse Ca/Mg ratios and often with toxic quantities ofheavy metals (Ni and/or Cr); formed on ultrabasic rocks.152 THE DRYLAND CASH-CROP PRODUCTION5S Moderately shallow to moderately deep, reddish-brown togreyish-brown, relatively silty clay loams over reddish-brownto yellowish-brown similar clays. The latter are frequentlymottled but the soils are not markedly hydromorphic. Formedon argillaceous sediments.5A Moderately shallow, greyish-brown, relatively silty sandy loamsover similar yellowish-brown sandy clay loams; frequentlyMottled but not pronouncedly hydromorphic; formed on cer-tain arenaceous sediments.5F Shallow to moderately shallow, reddish-brown, highly mica-ceous, fine-grained sandy loams over similar clay loams; formedon highly micaceous materials.5P Moderately shallow to moderately deep, brown to reddish-brown sandy loams over sandy clay loams; formed on gneis-ses of various ages and origins.5G Mainly moderate shallow, greyish-brown, coarse-grained sandsthroughout the profile, to similar sandy loams over reddish-brown sandy clay loams; formed on granitic rocks.5M Moderately shallow to deep, greyish-brown sands or loamysands over light reddish-brown sandy loams; formed mainlyon Triassic and, to a lesser extent, Permian formations.At the scale of mapping of 1:250 000 small pockets of different soiltypes could not be mapped. Some of the variations will however be mappedat a scale of 1:100 000. No attempt will be made to delineate soil types downto the soil series level.CROP YIELDS BY SOIL TYPEThe average yields of flue-cured tobacco, maize (long season varieties) andcotton (Albar) from 1971-2 to 1975-6 were calculated for all farms in thethree groups of I.C.A.s. The mean crop yields of farms falling predominantlywithin each soil group were then extracted. Farms containing two or moresoil types in approximately equal proportions were excluded from the survey.Initially, soil groups were further sub-divided acording to variations inparent material, e.g. the granites were subdivided into porphyritic, mus-covite and biotite granite, while the gneisses of various ages were split intogranitic and sallimanite gneisses etc. It was observed however that crop-yields from farm to farm within a single soil type tended to vary more thanmean yields from one soil type to the next. Consequently the crop yields re-corded below refer only to the major soil groupings. A set of 1:100 000 mapsis however being produced which incorporates the sub-groups.Because of the very great differences in managerial ability from farmP. IVY153to farm, average yields for the top third have also been calculated; suchyields could be used as a target for crop extension programmes and theassessment of crop production potential.MakeTable 11 illustrates the average and the top third maize yields (mainly S.R.52)from the various soil groups. The top third averages are generally approxi-mately 1 000 kg/ha higher than the overall mean yields.The top third yields on the granite sands (5G) range from 6 000 to6 500 kg/ha in the Trelawney and Darwendale l.C.A.s to the south, to6 800 to 7 200 kg/ha in the Karoi, Doma, and Ayrshire l.C.A.s to the north.Slightly more favourable rainfall distribution might be expected to the northwhere the influence of the inter-tropical convergence zone is greatest, butdifferences in altitude arc possibly equally important.Maize yields on the gneisses (5P), arkoses (5A) and epidiorites (4E)range from 6 500 to 6 700 kg/ha, while the lowest yields, .5 800-6 100 kg/ha,are recorded on soils which have a tendency to compaction and sealing ofthe soil surface. This surface crusting results in poor emergence of seedlingsand reduced infiltration of rainfall; on ploughing, large clods are turned upwhich are not easily broken. The unfavourable characteristics in questionare found on the micaschists and phyllites (5F), slates and shales (5S) andmetasediments (5SE). These soils have a high content of silt or fine sand anda moderately high clay content.Flue-cured tobaccoThere are a number of factors which must be considered when comparingaverage tobacco yields from farm to farm, and season to season on thedifferent soil types.In order to control the availability of soil nitrogen, medium to heavytextured soils are usually ploughed from late July to September, when thesoil is dry. This allows little decomposition of organic material before thefollowing rains and consequently more fertilizer nitrogen is required thanwhen the soils are ploughed early. The poorer tilth resulting from late plough-ing affects ridging, nematode control and subsequent waterplantfng unlessthese operations are delayed until after the first rains. Tobacco yields aretherefore seldom as high as on soils which can be early ploughed. This wouldaccount for the top third tobacco yields (see Table III) from the micaschistsand phyllites (5F), slates and shales (5S) and metasediments (5SE) rangingfrom only 1 400 to 1 550 kg/ha, whereas the yields on the granites (5G),gneisses (5S) and arkoses (5A) range from 1 600 to 1 850 kg/ha.In the warmer, lower altitude areas to the north, many farmers producean early and a late tobacco crop; the late crop being planted approximatelysix weeks after the last plantings of the early crop. This delay reduces theoverlap in reaping between the two crops and enables the farmers to cope154THE DRYLAND CASH-CROP PRODUCTIONTable IIMAIZE YIELDS (LONG SEASON VARIETIES)Soil Type5G5P5F5S5A5M5SE4EI.CA.KaroiDomaAyrshire NorthAyrshire SouthBanket andEldoradoTrelawney andAngwaDarwcndalcKaroiKaroiAngwaAngwaAngwaAngwa andEldoradoBanketNumber ofFarms8540232527396161502744262714AverageYield kg/ha5 4745 6835 9745 9855 64751045 2355 3824 6774 7585 4205 62051325 620Top ThirdYield kg/ha6 9886 79171977 0226 7836 0366 3316 6665 7875 9026 5156 97261466 563with an increased area under tobacco without engaging extra labour or in-creasing the curing facilities. The late crop does., however, have a signific-antly lower yield potential. Crop yields obtained from the Central Statis-tical Office give no indication of the proportion of early to late plantings.In the warmer lower altitude areas, leaf ripening rates are generallymuch faster than at higher altitudes, and so a narrower hectares per barnratio is required in such areas to reduce leaf losses in the field during periodsof peak leaf ripening. It is possible that the higher yields obtained in theV,.P. IVY155Darwendale area are the result of more favourable ratios of early to lateplantings and hectares planted per barn. This requires further investiga-tion.Erratic or sub-economic yields have caused many farmers to give uptobacco production on the less suitable soil types, with only the more effi-cient farmers continuing to grow tobacco.This feature is reflected in the lownumber of tobacco farmers in the slates, arkoses, metasediments, (5S and 5SE)and dolomites, which in the mid 1950s would have equalled the number ofmaize farmers on these soils today.Table IIIFLUE-CURED TOBACCO YIELDSŁ *ŁSoil type5G5P5F5S5A5M5SEAreaKaroiDomaAyrshire NorthAyrshire SouthBanket andEldoradoTrelawney andAngwa SouthDarwendaleKaroiKaroiAngwaAngwaAngwaAngwa an ,EldoradoNumber ofFarms833325202141585242418118AverageYield kg/ha13861399127715031354r ''"137514941 3341181128413061207" " ~1156Top ThirdYield kg/ha1698165916751764158116651845169914581554171814741383156THE DRYLAND CASH-CROP PRODUCTIONCotton:Well drained sandy clay loam or heavier fertile soils, but without excessivesoil nitrogen, are best for cotton production. Soils lighter than sandy clayloam are not generally recommended. Drainage is more important than soildepth which, however, should be not less than 0,6 to 0,9 m.Mean minima temperatures should not generally be less than 15°C forprolonged periods. The lower temperatures prevailing at altitudes in excessof about 1 200 m are at present unsuitable for cotton growing.Practically the whole of the Karoi group of I.C.A.s and the Darwendale.Trelawney and Banket I.C.A.s are unsuitable for cotton production on accountof the cool climatic conditions and the light textured soils.Apart from the cotton produced on the metasediments (5SE) in theEldorado area, there are insufficient farms on which cotton is grown on theother soil types tor a reliable estimate of yield potential to be obtained fromthe figures given in Table IV.Table IVCOTTON YIELDSSoiltype5G5A555E/M5SE4EDo maAIV,TW:IAngwaAntiwa and DoinaŁŁŁ'Ł -~ """" " "Eldorado"Ł 'BanketNumber ofFarmsAverageYield kg/ha10 | 1 4101,3 j 1 321782331166128114151 367Top ThirdYield kg/ha194114971769156017931 872*The granites on which cotton is grown in the. Dotna I.G.A. are all contaminated,giving rise to sandy loam to sandy clay loan: soils.CROP PRODUCTION POTENTIAL: KAROI GROUP OF I.C.A.sLand Capability Classification: Over 40 per cent of the farms in the Karoiarea have been planned. Many of the farms, however, were planned beforethe date when the land capability classification criteria were modified toallow for the inclusion of moderately shallow (0,50-1,00 m) sands on 2-5 percent slopes into Class III. Previously such soils were placed in Class IV.Using the present land class criteria, the percentage of Class III land wouldP. IVY157be higher than that recorded in Table V, and the percentage of ClassIV correspondingly lower.Table VPERCENTAGES OF EACH CLASS OF LAND WITHIN THE MAJORSOIL TYPESSoil type5G5P5F5F125PI2TOTAL jTotal Area (ha)92 000132 00082 00096 00013 000415 000Arable Land Classes III 1116 j 35Ł162827IV262612V242615"leis Non-ArableVI7920VII229Litholsols, mainly classesV, VI and VIII.iVIII001Net Arable Area: The land capability classification gives an inflated assess-ment of the amount of land available for crop production: 25 per cent mustnormally be deducted from the arable area recorded on the land class mapto allow for land lost in 'squaring off lands, areas for homesteads, roadreservations, headiands. inaccessible pockets of arable land, marginal soilareas, etc. A fmuhcr 10-25 per cent must be deducted from the area of arablelands to cater for land taken up by contours, access roads within the lands,artificial grass waterways and anthills. The steeper the land, the narroweris the horizontal interval between contours (thus 10 per cent must be de-ducted from ihe gross arable area where the slope is 0-2 per cent; 15 per centwhere 2-5 per cent: 20 per cent where 5-8 per cent: and 25 per cent whereo-12 per cent.i in ihe Karoi area ihe land Mopes generally range from 2-5 percent.Table VINET ARABLE AREASoil ŁType !5G5F 'Ł5? 'ŁŁTot-i! /:,/92 000132 00082 0006763; 55.andi(}lms Arablelin61 6^083 16045 100Gro>s Are- ofLai:ds i ha 146 23062 37033 825Net Arable Area(ha)39 29553 01528 750\,.U : Land > L-irrv.v. :irca of larul^. 1;\-; 15 JHT C( I,I153THE DRYLAND CASH-CROP PRODUCTIONCrop Rotation: In arriving at what are considered reasonably safe and toler-able crop rotations, cognisance has been taken of the amount of soil lostfrom arable lands within the rotation cycle. There is evidence to prove, forexample, that soil loss from second and third-year tobacco on sandveld issignificantly higher than soil loss from first-year crops.Table VIIRECOMMENDED CROP ROTATIONS FOR NATURAL REGIONSII and IIISoil TypeSands, with or without heaviertextured subsoilsSandy loam tosandy clay loamsClay loam to heavy claysMarginal vlei soilsSlope7c0- 22- 55- 80- 22-55- 88-120- 55- 88-120-5Land ClassII and IIIIVIIIIVIVII and illIVIIIIIIVIIIIVIV1 and IIIII and IVIVIVw% Crop inRotation1005050332510050665033332525100503350With the 50 per cent to 100 per cent crop rotations at least half thecrops grown should be capable of providing an adeaquate cover early in theseason, as maize does. Crops such as cotton, tobacco, and late plantedlegumes have a high erosion risk and should not be grown too frequently inthe rotation.On 0-2 per cent slopes there are no restrictions on land use exceptthat farming operations should normally be carried out on the contour.«MP. IVY159Where there are contour ridges, crop ridges may be at 90° to the contour. Nospecial rotational requirements are specified other than those necessary forcultural considerations, such as eelworm-suppressing grasses grown in rota-tion with tobacco. Examples of recommended rotations would be:100 per cent crop rotation:Continuous maize or maize/cottonrotation.66 ,, —50 .. f,33 ., ,.25 — .,: Three cycles of a maize/cotton rota-tion followed by three years'grass.: Maize/Soyas/Maize/three years'grass ley.or Tobacco/Maize/Maize/three years,grass ley.: Tobacco/Maize/four years' grassleyor Cotton/Soyas/four years' grass ley.: Tobacco/three years' grass ley.Annual Cropping Areas: The net annual cropping area is obtained by apply-ing the recommended crop rotation for the soil type to the net arable area.Soil groups 5G and 5P are predominantly sands, with or withoutheavier texture subsoils, on slopes of 2-5 per cent, usually falling into landClass 111. Thus a 50 per cent cash crop rotation such as three years' cashcropping followed by three years' established grass ley would normally berecommended for these soils. Flue-cured tobacco production is however themost profitable enterprise in the area, and therefore to maximize tobaccoproduction, a tobacco, maize, three-year ley rotation would be better in anuncontrolled export market situation. The soils in group 5F, are pre-dominantly sandy loams to sandy clay loams with heavier subsoils. Wheresurface crusting is severe the land is down graded to Class III. Recommendedcrop rotations are continuous maize on land slopes not exceeding 2 per cent,and tobacco, maize, three years' grass ley or three years' maize, three years'grass icy on land slopes of 2-5 per cent.Table VIIICROP PRODUCTION POTENTIAL. KAROl GROUP OF I.C.A.sSoil 'Type5G5P5FNet Arable39 29553 01528 750RotationT.MiGGGT.M.lGGGT.M.MiGGGha7 86010 6004 790TobaccoPotentialYield (tonnes)13 34618 0096 984Maizeha7 86010 6009 580PotentialYield (tonnes)54 92670 66055 439160THE DRYLAND CASH-CROP PRODUCTIONSUMMARYThis paper outlines the work currently in progress on the mapping of themajor soil groups within the intensive farming regions, and the calculationof crop yields by soil type. The arable land percentage for each soil type willbe assessed from an analysis of farm plans. Based on sound crop rotationpractices, the annual cropping areas and crop yield potential will be esti-mated for each group of l.C.A.s within Natural Region II.ReferencesFARM MANAGEMENT SECTION, PLANNING BRANCH OF THE DEPART-MENT OF CONSERVATION AND EXTENSION 1976 Area & Farm CropYields (Salisbury, Govt Printer, 3rd edit.).IVY, P. 1977 A Guide to Soil Coding and Land Capability Classification for LandUse Planners (Salisbury, Min. of Agriculture, Dep. of Conservation and Extension).McKINSTRY. A. II. 1976 An Agro-Ecological Survey of Cotton Production in Rho-desia (Salisbury, Rhodesia Cotton Growers' Association).THOMPSON, J. G. 1965 The Soils of Rhodesia and Their Classification (Salisbury,Min. of Agriculture, The Rhodesia Agricultural Journal Technical Bulletin No. 6).VINCENT, V. and THOMAS. R. G. 1957 Agricultural Survey of Southern Rho-desia : Part I : Agro-Ecological Survey (Salisbury, Federal Printer).