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Testing the APSIM Model with Experimental Data from the Long-term Manure Experiment at Machang'a (Embu), Kenya

A 27 season, long-term study (1989‐2002) was conducted at Machang’a, near Embu in the semi-arid lands of eastern Kenya, to assess the effect of manure application on soil nutrient status and crop productivity. The experimental treatments comprised a control (no inputs); 5 and 10 t ha‐1 rates of high quality manure (26% C; 2.0% N; 0.48% P) from a single source and applied annually in October; residual manure treatments where manure application ceased from 1993; and a NP fertiliser treatment on previously unfertilised plots from 1993. The data from this experiment have been used to test the performance of the Agricultural Production Systems Simulator (APSIM) in predicting the crop dry matter yield, extractable soil phosphorus and soil organic carbon on a P-deficient soil. The experiment was simulated using the APSIM P-aware maize module, even though maize was not the test crop before the November 1999 season. Agreement between model predictions and measured data was generally satisfactory for all three of the variables tested. The APSIM modelling framework described elsewhere in these proceedings now has a capability to simulate the dynamics of phosphorus in addition to nitrogen, and the effects of both nutrients on crop growth (Probert 2004). In order to validate that the model predictions are credible, there is a need to compare the model outputs against measured data. Unfortunately, few studies have been carried out in the tropics that provide suitable data sets for such purposes. In this paper, we describe an experiment that does provide suitable data for testing the model, and to allow comparison of the observed data with model predictions.