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The sustainability of cereal/legume intercropping was assessed by monitoring trends in cereal or legume grain yield, soil organic C (SOC) and soil extractable P (Olsen method) measured over 13 years of field experimentation on a P-deficient soil in semi-arid Eastern, Kenya. Goat manure was applied (annually for 13 years at 0, 5 and 10 t ha-1 and trends in grain yields. were not identifiable because of season-to-season variations. SOC and Olsen P increased for the first seven years of manure application and then remained constant. The residual effect of manure applied for four years only lasted another seven to eight years when assessed by yield, SOC and Olsen P. Mineral fertilizers provided the same annual rates of N and P as in 5 t ha-1 manure and initially gave the same yield as manure, declining after nine years to about 80%. Therefore, manure applications could be made intermittently and nutrient requirements topped-up with fertilizers. Grain yields for sorghum with continuous manure were described well by correlations with rainfall and manure input only, if data were excluded for seasons with over 500 mm rainfall. A comprehensive simulation model should correctly describe crop losses caused by excess water

Long-term indicators of soil fertility were assessed by measuring grain yield, soil organic carbon (SOC) and soil Olsen phosphorous for a P-deficient soil. In one set of treatments, goat manure was applied annually for 13 years at 0, 5 and 10 t ha−1, and intercrops of sorghum/cowpea, millet/green gram and maize/pigeonpea were grown. Yield depended on rainfall and trends with time were not identifiable. Manure caused an upward trend in SOC, but 10 t ha−1 manure did not give significantly more SOC than 5 t ha−1. Only 10 t ha−1 manure increased Olsen P. Measurements of both SOC and Olsen P are recommended. In another set of treatments, manure was applied for four years; the residual effect lasted another seven to eight years when assessed by yield, SOC and Olsen P. Treatment with mineral fertilizers provided the same rates of N and P as 5 t ha−1 manure and yields from manure and fertilizer were similar. Fertilizer increased Olsen P but not SOC. Management systems with occasional manure application and intermediate fertilizer applications should be assessed. Inputs and offtakes of C, N and P were measured for three years. Approximately 16, 25 and 11% of C, N and P respectively were stabilized into soil organic matter from 5 t ha−1 a−1 manure. The majority of organic P was fixed as soil inorganic P.

The long-term experiment was initiated in 1989 on a P-deficient soil in semi-arid Kenya where sustainability of the cereal/legume intercropping was assessed by monitoring trends in grain yield, soil organic C (SOC) and soil extractable P (Olsen method). Goat manure was applied annually at 0, 5 and 10 t ha−1 and trends in grain yield were not identifiable because of season-to-season variations. SOC and Olsen P increased for the first 7 years of manure application and then remained constant. The residual effect of manure applied for 4 years only lasted another 7–8 years when assessed by yield, SOC and Olsen P. Mineral fertilizers provided the same annual rates of N and P as in 5 t ha−1 manure and initially gave the same yield as manure, declining after 9 years to about 80%. There was a wide agreement between observed and predicted dry matter yields, soil labile P, but less for organic C using the Agricultural Production Simulator (APSIM) model. Soil nitrate was highest at the onset of the season and was highest where manure had been applied and was subject to loss through leaching or runoff

RothC and Century are two of the most widely used soil organic matter (SOM) models. However there are few examples of specific parameterisation of these models for environmental conditions in East Africa. The aim of this study was therefore, to evaluate the ability of RothC and the Century to estimate changes in soil organic carbon (SOC) resulting from varying land use/management practices for the climate and soil conditions found in Kenya. The study used climate, soils and crop data from a long term experiment (1976–2001) carried out at The Kabete site at The Kenya National Agricultural Research Laboratories (NARL, located in a semi-humid region) and data from a 13 year experiment carried out in Machang’a (Embu District, located in a semi-arid region). The NARL experiment included various fertiliser (0, 60 and 120 kg of N and P2O5 ha−1), farmyard manure (FYM—5 and 10 t ha−1) and plant residue treatments, in a variety of combinations. The Machang’a experiment involved a fertiliser (51 kg N ha−1) and a FYM (0, 5 and 10 t ha−1) treatment with both monocropping and intercropping. At Kabete both models showed a fair to good fit to measured data, although Century simulations for treatments with high levels of FYM were better than those without. At the Machang’a site with monocrops, both models showed a fair to good fit to measured data for all treatments. However, the fit of both models (especially RothC) to measured data for intercropping treatments at Machang’a was much poorer. Further model development for intercrop systems is recommended. Both models can be useful tools in soil C predictions, provided time series of measured soil C and crop production data are available for validating model performance against local or regional agricultural crops.