Available Phosphorus in Soils Amended with Organic N-Enriched Composts during Periods of Incubation

Inorganic nitrogen (N) fertilizers, microbial inoculum and biologically-active substances are used to fortify composts which characteristically contain low amounts of N. The potentials of organic wastes from agriculture for N enrichment of composts were indicated by significant increase in soil N while there can be fortuitous improvement in the available phosphorus (P) contents on which information about the extent is lacking. In this study, composts: cow dung + sawdust (CDSD) and poultry droppings + sawdust (PDSD) were enriched with meals from bone (BN), blood (BM), hoof (HF) and horn (HN); and neem leaf (NM) and tithonia leaf (TM) to attain 150, 300, 450 and 600 g kg-1 N and the available P was monitored at four-week intervals during 16 weeks of incubation in soil. Available P increased with all N sources and enrichment rates slightly at week 4 but highly from week 8 and for each source, the enrichment to 600 g kg-1 N gave the highest values. The composts enriched to 450 and 600 g kg-1 N gave 15-20 and 20-29 mg kg-1 available P in week 12 and 16 respectively. CDSDBM and PDSDHF enriched to 600 g kg-1 N at week 4 and 8 respectively can be used for short-season crops while PDSDBM and PDSDNM enriched to 600 g kg-1 N in week 12 and 16 respectively would be suitable for long-season crops. The PDSDNM enriched to 600 g kg-1 N gave the highest available P (29.0 mg kg-1) and was followed by PDSDBN and PDSDHF at the same enrichment level, with 28.0 mg kg-1 each. Although the PMSDNM enriched to 600 g kg-1N had the highest available P in week 16, the enriched composts with values exceeding 20 mg kg-1 also have potentials for the cultivation of long-season vegetables.