Use of mijingu rock phosphate fertilizer in acid soils of central Kenya

Acid soils cover about 13 % of the total land area in Kenya where many cash and food crops are grown. In humid and sub-humid regions of central Kenya, maize performs poorly due to problems associated with acidity in soils. While many options for correcting soil acidity exist, rock phosphate has been recommended elsewhere because of its low costs. Greenhouse work was conducted in 1997 to determine the major fertility constraints in acid soils, compare Mijingu phosphate rock (MPR) with other amendment options and generate recommendations that can be adopted by farmers. Fertility constraints were determined by laboratory analysis of soil samples. Mijingu phosphate rock was compared with other amendment options in a pot experiment with maize as a test crop. The options were triple super-phosphate (TSP), MPR, and TSP mixed with lime (fSP+L) or farmyard manure (TSP+FYM). Lime and FYM were applied at 4 and 5 t ha-l respectively. fu all the options, P levels were applied at increasing rates of 0, 22, 44, 66, 88 and 132 kg P ha-l. Maize dry matter (DM) yields were used to assess the treatment effects and data generated was analysed as a two factor, RCBD. Results showed that the soils had high exchangeable acidity and were deficient in Ca, P, and Mg. All the options gave same maximum yields of 37.5 gIpot but differed significantly on the amount of P required to achieve this yield. Triple super- phosphate and MPR gave highest yields at a P rate of 66 kg P ha-1 but adding lime or manure to TSP lowered the P requirement by 33%. This was associated with the release of Ca and Mg and neutralisation of Al toxicity after the dissolution of lime in acid soils. Mijingu phosphate rock performed better than TSP at all the rates of application because of its high Ca and Mg content. Triple super- phosphate applied alone had the lowest efficiency because of P fixation by Al +3. Mijingu phosphate rock was the favoured option due to low costs but it was premature to recommend this option without a further on -farm evaluation.