Inorganic soil nitrogen distribution in relation to soil properties in smallholder maize fields in the Kenya highlands

We investigated the influence of soil charge properties on the distribution of inorganic soil N in maize (Zea mays) fields on smallholder farms in the subhumid highlands of Kenya. There was little or no fertilizer use in thes e subsistence fanning systems. Soil NO3- was measured to 2 m depth in maize fields on 96 farms, sampled in transects at three physiographic positions (ridgetop, midslope and lower slope) in each of six soil map units (4 or 8 transects per soil map unit) covering four parent materials: acid igneous, various igneous, basic igneous, and sedimentary. Soil NO; concentrations va ried from < 0.1 to 10 mg N kg(-1) in the top 0.25 m and from < 0.1 to 16 mg N kg(-1) in the subsoil (0.5-2.0 m depth). Mean subsoil NO3-; contents wer e about two times higher in two soil map units (Ferralsols on sedimentary m udstones, mean = 57 kg N ha(-1)) than in the other soil map units (means ra nged from 20 to 30 kg N ha(-1)). Subsoil NO; concentration was not related to effective anion exchange capacity (AEC), plant N content (taken as an in dicator of plant N demand) or anaerobic N mineralization (taken as an indic ator of N supply). Effective cation exchange capacity (CEC) of less than or equal to 7.3 cmol(c) kg(-1) clay correctly classified 79% of the farms wit h high subsoil (0.5-2.0 m) NO3- contents of > 70 kg N ha(-1). It may be dif ficult to accurately predict the spatial distribution of subsoil nitrate ac cumulation in maize from routinely measured soil variables, but our results indicate that information on parent material and subsoil CEC and clay cont ent may help to provide broad generalizations. (C) 2001 Elsevier Science B. V. All rights reserved.