ELIAS GICHANGI1, PEARSON MNKENI1, PHIL BROOKES2
1University of Fort Hare, Department of Agronomy, South Africa
2Rothamsted Research, Agriculture and Environment, United Kingdom
Phosphorus transformations in untreated and manure treated soils at varying inorganic P application rates were assessed in controlled laboratory incubation experiments using a sequential fractionation procedure. Phosphorus was added at rates of 0, 45, 90, 135 and 180 mg P kg-1 as triple super phosphate with or without 20tha-1 of goat manure (dry weight) and incubated moist for 12 weeks. Resin P, soil microbial biomass P, 0.5 M NaHCO3 extractable inorganic P (NaHCO3-Pi) and 0.1 M NaOH extractable inorganic P (NaOH-Pi) concentrations were determined on days 1, 7, 14, 28, 56 and 84. Addition of inorganic P increased all P fractions but the increases were greater when goat manure was co-applied. The control treatments had only 17.2 and 27.5 mg P kg-1 of resin extractable P in the un"=amended and manure amended treatments, respectively which increased to 118.2 and 122.7 mg P kg-1 at the highest rate of P application (180 mg P kg-1) on day 28 of incubation. NaOH-Pi was the largest extractable Pi fraction and ranged from 144 to 251 mg P kg-1 and 108 to 213 mg P kg-1 in the unamended and manure amended treatments, respectively. Inorganic P addition increased the microbial biomass P concentration from 17 to 44 mg P kg-1 in P alone treatments but the fraction was greatly enhanced with manure addition, increasing it from 32.6 to 97.7 mg P kg-1. The largest improvement in microbial biomass P due to manure occurred at low rates of added P indicating the potential of goat manure to enhance the fertiliser use efficiency of low doses of P fertilisers. This increase in microbial biomass P following goat manure addition implies that the presence of goat manure increased the proportion of added P immobilised in microbial cells that would be subsequently released into the soil solution and be available for plant uptake following microbial turnover.
Keywords: P fractionation, goat manure, microbial biomass, resin-P