The long term use of farmyard manure and compost: Effects on P availability, orthophosphate sorption strength and P leaching

This study was set up to identify the role of dairy farmyard manure and green waste or farm compost used as a source of stable organic matter on soil P availability and P leaching. We sampled two long term field trials (8 and 16 years) on silt loam soil, with continuous amendment of dairy farmyard manure (FYM) and 6 types of organic waste (VFG, BIO), municipal solid waste (MSW), sludge (GWS) or organic farm waste (CMC1, CMC2) composts. Soil P availability was measured as 0.01M CaCl₂ extractable P (P-CaCl₂) and hot water extractable P (HWP) and fresh subsamples were used to conduct a leaching experiment in unsaturated conditions in the laboratory. Since the P fertilization history was not equal among the treatments, P leaching concentrations (TP) were standardized with the total P content of the soil (Ptot). P-CaCl₂, HWP and the P leaching experiment revealed that in long term farmyard manure amended soils, more P becomes available and is susceptible to leaching than in long term VFG, BIO, MSW, CMC1 and CMC2 compost amended soils. We observed a seasonal trend in P-CaCl₂ with minimum and maximum at end of winter (February) and mid-summer (July), respectively. Although this trend was independent of the fertilizer type amended on the soil, the P-CaCl₂ significantly increased for farmyard manure amended soil, compared to compost amended soil at any time during the sampling period of >1 year. A sorption experiment with radiolabelled orthophosphoric acid (³³PO₄) revealed that increased soil P availability and P leaching are related to a decrease in orthophosphate sorption in farmyard manure amended soils, which was not observed in compost amended soils. It is concluded that farmyard manure derived P is more available and more prone to leaching compared to compost derived P due to differences in PO₄ sorption.