Effects of goat manure and inorganic phosphate addition on soil inorganic and microbial biomass phosphorus fractions under laboratory incubation conditions

Changes in inorganic phosphorus (P) and soil microbial biomass P following the addition of goat manure and different amounts of inorganic P to an Umbric Ferralsol were assessed over 12 weeks under laboratory incubation conditions using a sequential fractionation procedure. Triple superphosphate was added at rates equivalent to 0, 45, 90, 135 and 180 mg P kg-1, with or without goat manure at a rate of 10 g kg-1soil on a dry-weight basis and incubated moist. Resin P, 0.5 mol L-1 NaHCO3 extractable inorganic P (NaHCO3-P-i), 0.1 mol L-1 NaOH extractable inorganic P (NaOH P-i) and soil microbial biomass P concentrations were determined on days 7, 14, 28, 56 and 84. The magnitudes of the inorganic P fractions extracted were: NaOH P-i > Resin P > NaHCO3 P-i. Thus, NaOH P-i was the major sink for the applied P. The dynamics of the three labile P fractions (resin P, NaHCO3 P-i and microbial biomass P) varied considerably during the incubation period. The resin P fraction consistently declined with time in all treatments, whereas the NaHCO3 P-i fraction changed little with time in the control and goat manure amended soil, but increased rapidly with time when inorganic P was applied alone or in combination with goat manure. Microbial biomass P increased with time in all treatments, peaking on day 28 and declining thereafter. The co-application of inorganic P with goat manure produced up to twofold more microbial biomass P than either inorganic P or goat manure applied alone. Therefore, the combined application of manure with low rates of P fertilizers may be a cost effective strategy for increasing the efficiency of fertilizer P use through enhanced biological cycling of P in small-holder farms in South Africa.