In situ estimation of water and fertilizer losses under cropping with a view to optimizing water and fertilizer management

Extensive pollution of water tables has become a major environmental problem. It is henceforth imperative to optimize the addition of water and fertilizer to crops so as to minimize losses beyond the root zone, while maintaining agricultural practices consistent with sustainable development. A rapid review is made of the methodologies used for gauging the transfer of water and elements in solution in the unsaturated zone of the soil. Two main topics are dealt with: (1) measurement of flows of water (drainage) and solute (losses) passing beyond the root zones; and (2) determination of the relationship between hydraulic conductivity and water content of the soil at the estimated depth of these flows. An illustration is given of the method on the basis of the results obtained during three consecutive years of experiments on maize crop irrigated with fertilizer varying from 'traditional' application of 260 kg N/ha to 'optimized' application of 160 kg N/ha, without significant loss of production. A plot was also fertilized with ammonium nitrate labelled with 15N to determine both the origin of N losses and the utilization rate of the fertilizer. Owing to a combination of extraction of soil solution by means of porous soil solution samplers and termination of the variation in soil water content, it was possible to determine over the whole growing cycle the water and N balance in the root zone. It was also shown that the irrigation method employed was very effective, since losses of water (and of N in soil under cultivation) were always zero or negligible during the irrigation periods. On the other hand, the situation was quite different at the end of the cycle, when strong autumn rains specific to the region tended to cause very high N (nitrate) losses if large residues were present in the root zone after harvesting. This was the case with traditional application, which resulted in a residue of 150 kg N/ha after harvesting. Owing to good adaptation of fertilizer application to the needs of the plant ('optimized' application), it was possible to reduce this residue and the associate nitrate losses to zero. 17 refs, 8 figs