Enhanced biological cycling of phosphorus increases its availability to crops in low-input sub-Saharan farming systems

• Published in: Soil biology & biochemistry 2006, Vol.38 (1), p.81-90
• Main Authors: Ayaga, G., Todd, A., Brookes, P.C.
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; bioavailability; Biochemistry and biology; Biological and medical sciences; cattle manure; Chemical, physicochemical, biochemical and biological properties; corn; crop yield; Fundamental and applied biological sciences. Psychology; microbial activity; Microbial biomass P; Microbiology; nutrient availability; P cycling; phosphorus; phosphorus fertilizers; Physics, chemistry, biochemistry and biology of agricultural and forest soils; Small scale farming; soil microorganisms; soil nutrient dynamics; Soil science; Zea mays
• ISSN: 0038-0717; 1879-3428
• DOI: 10.1016/j.soilbio.2005.04.019

Many soils in sub-Saharan Africa, which are farmed by smallholders, are P deficient and highly P fixing. Furthermore, P inputs supplied as farmyard manure (FYM) or inorganic P fertilizer are normally too small to replace P offtakes by crops. Consequently most soils are in a negative P balance, which is reflected in small, and often declining, crop yields. The obvious solution of simply applying adequate P is seldom an option due to shortages of manure, which is usually low in nutrients in any case, and the high cost of inorganic P fertilizer relative to the likely cash value of the harvest. Our aim was to see if we could devise practical methods to increase soil P availability in this situation and to investigate the mechanisms involved. Two approaches were adopted. Firstly, to attempt to saturate the P-fixing sites in the soils by applying a large annual application of P (75 kg P ha −1), which should serve for several seasons. Secondly, to attempt to keep the fertilizer P in biological forms by supplying fertilizer P and cattle manure (FYM) in combination. Here, the aim was to promote the cycling of P through the soil microbial biomass and associated metabolite pools, with the expected result of decreasing P fixation and increased plant availability of this P. These treatments were investigated using two field sites on smallholder farms in Kenya: one, considered a ‘high P fixing’ soil at Malava (Kakamega District) and one considered a ‘low P fixing’ soil at Mau Summit (Nakuru District). The following treatments were applied in 1997 and 1998: nil; 75 kg P ha −1 as super phosphate (P); 25 kg P ha −1; FYM at 1.9 t ha −1 dry matter; FYM+25 kg P ha −1. All treatments also received 100 kg inorganic N ha −1. Maize was the test crop. There was no significant correlation in either year at either site between soil P, measured as NaHCO 3-extractable P, resin P or NaOH-extractable P and maize yield. However, the different soil P fractions were closely correlated with each other. Yields at the high P rate (75 kg ha −1y −1) were often little better than the control. There was, however, a significant positive relationship ( P