the Symposium on Wheats for More Tropical Environments - cimmyt

250Alleviating <strong>the</strong> Constraintsof Acid Soils on Rainfed Wheat in ZambiaR. Little, Zambia-Canada Wheat Research Project, Mount MakuluResearch Station, Chilanga, ZambiaAbstractRainjed wheat cultivation in Zambia is still in <strong>the</strong> early stages ofdevelopment.The major problems ofproduction are diseases and acid sOils. However, acombination ofdisease-resistant and aluminum-tolerant varieties, with limingand optimum seeding date, are now giving yields ofover 2 tlha in trial plots andin afew commerclalftelds. Areas where aluminum toxicity is not a problem arebeing identifled, and <strong>the</strong> size of <strong>the</strong> land area availablefor wheat production isbeing quantifled.Very little rainfed wheat is grown inZambia at present. Attempts to growwheat between 1975 and 1982 werelargely unsuccessful, with yields below1 tfha. due mainly to high diseasepressure (particularly Helminthosporlumsativum). aluminum toxicityand <strong>the</strong> lack of varieties with adequateresistance and tolerance (10). Thesituation is improving with <strong>the</strong> recentintroduction of PF7748 from Brazil. <strong>the</strong>use of lime on acid soils and <strong>the</strong>identification of soils with loweraluminum content.SoilsThe areas of Zambia with reliablerainfall and reasonable rainfalldistribution are mainly in <strong>the</strong> north of<strong>the</strong> country. with an annualprecipitation of more than 1,000 mm.Most of <strong>the</strong> soils in this area are reddishbrown in color. highly leached andchemically poor, with a pH (CaCI2) ofabout 4.0. CEC of 6 to 13 mellOO g clayand BSP of usually less than 25%;aluminum satUration may exceed 60%in <strong>the</strong> subsoil. The soil texture is asandy clay loam. <strong>the</strong> clay fraction beingmainly kaolinite (1,3.4.6). The soils areclassified as typic haplustox or xanthic.orthic or rhodic ferralsols (1.3). Theirhigh aluminum content makes <strong>the</strong>mparticularly problematic for wheatproduction.There are smaller areas of soil developedfrom basic parent materials which arered in color; <strong>the</strong> precise location and sizeof <strong>the</strong>se is not accurately known. Theyhave a higher CEC. approximately 12.20me/100 g clay. a BSP of 25 to 50%. anaverage pH of about 5.0 (2) and analuminum content that is variable, butoften quite low. The iron content in <strong>the</strong>B horizon is very high (average 19%),which probably accounts for <strong>the</strong> goodmicro-aggregation of <strong>the</strong>se soils (4); <strong>the</strong>yare classified as ferralic or cambicarenosols (3).Aluminum toxicitySymptoms of aluminum tOXicity arecommonly expressed as a swelling andthickening and/or darkening of <strong>the</strong> roottips and lateral growth of <strong>the</strong> roots.resulting in shallow rooting. This. intum, usually results in short. weakplants that are very susceptible todrought. since <strong>the</strong> shallow rooting allowslimited access to nutrients and moisturebelow. The pH of <strong>the</strong> soil at <strong>the</strong> Zambia­Canada Wheat Project farm at Katito.Mbala. is about 4.0. with approXimately20 ppm aluminum (extracted in 0.20 MCaCI2). which is highly toxic to wheat.The relationships between pH andaluminum and between aluminum andwheat yield were studied on a field thathad been limed at 2 tfha and plantedwith <strong>the</strong> nontolerant wheat varietyJupateco 73. The distribution of<strong>the</strong> lime