Elias Gichangi, Pearson Mnkeni, Phil Brookes:
Changes in Inorganic and Microbial P Fractions Over Time Following Goat Manure and Inorganic Phosphate Addition to a High P Fixing Soil

ELIAS GICHANGI¹, PEARSON MNKENI¹, PHIL BROOKES^2
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

Full paper: http://www.tropentag.de/2007/abstracts/full/33.pdf

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Contact Address: Elias Gichangi, University of Fort Hare, Department of Agronomy5700 Alice, South Africa, e-mail: gichangimyahoo.com