Changes in soil organic matter and phosphorus fractions under planted fallows and a crop rotation system on a Colombian volcanic-ash soil

Acquisition of soil and fertiliser phosphorus (P) by crops depends on soil and plant properties. Soil processes determining P availability to plants are P solubility/sorption, P transport, root/soil contact and mineralisation/immobilisation. Plants have evolved properties contributing to a more efficient use of plant-available soil P and to mobilise P from less available soil P fractions. Agronomic measures may affect P availability to crops through the modification of soil properties or through direct quantitative and qualitative crop impact on soil P dynamics. Among the agronomic measures, the application of organic matter such as green manure and crop residues to maintain or increase soil organic matter content and to enhance soil biological activity, and the incorporation into the cropping system of P-mobilising plant species are particularly beneficial.Our experimental activities have concentrated on the characterisation of the P mobilising capacity of different leguminous grain and cover crops, and their effect on P availability to less P-efficient cereals grown in mixed culture and in rotation. Fractionation of P in the rhizosphere soil revealed the capacity of some legumes to better use P from sparingly soluble soil P fractions than maize. Field experiments conducted on 2 sites in the Northern Guinea Savannah of Nigeria and accompanying green-house pot experiments revealed a positive rotational effect of P-efficient cover crops on maize growth and grain yield with and without the return of crop residues. This could unequivocally be attributed to a better P supply to maize, especially on strongly P-fixing soil. However, the residual effect was small compared to the application of water-soluble P fertiliser. This clearly indicates the need for a maintenance application of fertiliser P in addition to the agronomic measures for sustainable crop production.