MODELING AND MEASUREMENT OF THE EFFECTS OF FERTILIZER-N AND CROP RESIDUE INCORPORATION ON N-DYNAMICS IN VEGETABLE CROPPING

An easy-to-use simulation model was developed with the aim of improvin g fertilizer practice when crop residues are incorporated instead of r emoved. It was tested against data from a well-monitored N fertilizer experiment in which three successive brassica crops were grown followe d by barley. Experimental findings included: (a) that fertilizer-N gre atly increased yield of 3 crops without increasing residual soil miner al-N at harvest unless supply exceeded crop demand; (b) that, by contr ast, fertilizer-N increased both yield of and residual soil mineral-N left by the remaining crop throughout the range of applications; and ( c) that at each harvest the apparent disappearance of fertilizer-N by immobilization and other processes was almost proportional to fertiliz er-N. These phenomena were simulated by the model. Overall the model g ave estimates of soil mineral-N, plant weight and % N in the crop for each crop that were either in close agreement with or linearly related to the measured values. Deviations from this pattern are shown to res ult almost entirely from experimental error. In addition the model gav e simulations of the time course of soil mineral-N and soil water that were in good agreement with measurement. Simulations with the model i ndicate that appreciable benefits from residue incorporation of crops will only be obtained when fertilizer-N is also applied, unless plant masses at harvest are small.