RELATIONSHIP BETWEEN AVAILABILITY INDEXES AND PLANT UPTAKE OF NITROGEN AND PHOSPHORUS FROM ORGANIC-PRODUCTS

A better knowledge of the plant-availability of nitrogen (N) and phosp horus (P) in organic products may help to improve the efficient use of these products as fertilizers. In the present study, availability ind ices for N and P of nine widely differing organic products obtained by different fractionation methods were compared with the plant uptake o f N and P from these products. The fractionation methods included CaCl 2 extraction, thermal fractionation (heating of organic products), and pepsin extraction, for N, and extraction with diluted sulphuric acid, P-Bray-I, P-Olsen, and extraction using an iron oxide coated filter p aper, for P. The results of pot experiments with ryegrass using a doub le-pot technique (Janssen, 1990) over 62 (N experiment) and 93 days (P experiment) were used as reference for plant-available N and P. The 0 .01 M CaCl2 extractable inorganic N reasonably predicted plant-availab le N only in organic products with a high inorganic N fraction. Therma l fractionation and pepsin extraction provided a reasonable index for mineralizable N in organic products having a high fraction of minerali zable N. Of the P fractionation methods, the extraction using iron oxi de coated filter paper was the best indicator of plant-available P in the products.