Phosphorus dynamics and soil P-fertility constraints in Nepal

There are significant differences between the red and non-red soils in the Middle Mountains of Nepal, and inherent differences in the parent materials and soil management seem to have the greatest influence on fertility and P dynamics, The red soils with Munsell Hue color of 2.5 YR and 5 YR, were fo und to have higher P sorption capacity than the non-red soils (Hue 7.5-10 Y R), and the Al(AAO) content was found to be the best predictive variable fo r P sorption. Using a 2 x 2 x 2 x 3 factorial design and GIS overlay techni ques, a P-sorption map for a case study watershed was produced. The results may be used for determining where agricultural intensification with chemic al fertilizer applications is most effective, The amorphous Fe and Al conte nt is critical in determining P dynamics, and without high organic matter i nput, the red soils are highly deficient in available P, Management influen ces the P-sorption capacity in that the Al-(AAO) content decreased in irrig ated red soils whereas the Fe-(AAO) content increased significantly. Irriga tion reduces the amorphous Al content and this results in a 10 to 30% reduc tion in P sorption, Since most red soils are acidic and are deficient in ca rbon, the reduction of amorphous Al via irrigation has a positive effect on management by significantly reducing P sorption, The reducing conditions c reated by flood irrigation during the monsoon season and the addition of ex changeable bases from sediments and irrigation water increase pH values by up to 0.5 units and alter the amorphous Al and Fe content in these soils an d, hence, P availability.