Soil Nitrogen Contents as Affected by Composts Enriched with Organic Nitrogen Sources

Abstract

Information is scanty on the potentials of agricultural wastes to enrich composts whose low nitrogen (N) content limits their use in organic farming. This study involved enriching composts - cow dung + sawdust (CDSD) and poultry droppings + sawdust (PDSD) with N from bone, blood, hoof and horn meals; and neem leaf and tithonia leaf meals and their incubation in the soil for 16 weeks. Cow dung and sawdust (CDSD), poultry manure and sawdust (PMSD) in 1:1 ratios were composted in separate heaps and samples taken for nutrient analysis at 2 and 22 weeks of composting. The composts were enriched with Bone, Blood, Hoof and Horn of cattle; Neem leaf and Tithonia leaf meals to obtain the following: CDSD + bone (CDSDBN), CDSD + blood (CDSDBM), CDSD + hoof (CDSDHM), CDSD + horn (CDSDHN), CDSD + neem (CDSDNM) and CDSD + tithonia (CDSDTM); PMSD + bone (PMSDBN), PMSD + blood (PMSDBM), PMSD + hoof (PMSDHM), PMSD + horn (PMSDHN), PMSD + neem (PMSDNM) and PMSD + tithonia (PMSDTM). The enrichment was madeto attain 100, 200, 300 and 500g kg-1 N. Each treatment at 30 t ha-1 was incubated for sixteen weeks to monitor the nutrient release at four-week intervals. Total N contents monitored at four-week intervals of incubation showed enrichment in 74.0 and 83.0% of CDSD and PDSD compared to the respective controls. N contents were highest in 21, 0, 1 and 28 composts incubated for 4, 8, 12 and 16 weeks respectively. At 4 weeks, CDSD enriched to 500 g kg-1 N from blood meal (CDSDBM) and bone meal (CDSDBN) contained 10.5 and 9.2 gkg-1 total N while PDSD enriched to 50 g kg-1 N from bone meal (PDSDBN) and tithonia leaf meal (PDSDTM) contained 9.3 and 8.6 g kg-1 total N. These are suitable for the cultivation of short-season leaf vegetables. N content reduced at 8 weeks of incubation and increased at 12 and 16 weeks. Only CDSD enriched with neem leaf meal (CDSDNM at all N rates) showed increase in N content with time of incubation. CDSDNM and PDSDBM at 50 g kg-1 N contained the highest total N at 16 weeks of incubation and should be recommended for the cultivation of long-season vegetables.