The effect of organic and inorganic amendments on phosphorus release and availability from two phosphate rocks and triple superphosphate in phosphorus fixing soils

Abstract

Phosphorus deficiency limits crop production in western Kenya due to low native soil P, continuous cultivation without adequate P fertilization and often high soil P fixation. The use of commercial P fertilizers by small scale farmers is limited by their cost. The use of finely ground phosphate rock (PR) is viewed as an attractive option to alleviate phosphorus deficiency. Two 16-week laboratory soil incubations were carried out to evaluate the effect of organic and inorganic amendments on the dissolution of two PR materials (Minjingu and Busumbu) in an Oxisol from western Kenya. Treatment with triple superphosphate (TSP) served as a reference. The effect of Tithonia diversifolia leaves (tithonia) added at 2 or 10 tonnes ha' on dissolution of moderately reactive Minjingu PR (Tanzania) and unreactive Busumbu PR (Uganda) was investigated in the first incubation experiment. The effect of inorganic amendments (CaCOj or CaCl2) on dissolution of the moderately reactive Minjingu PR was investigated in the second incubation study. Dissolution was determined from the increase in anion resin (AER), NaHCOj and NaOH-extractable P in soil amended with PRs, compared with the TSP treated soil. In the third study maize (Zea mays L.) and cowpea [Vigna unguiculata (L.) Walp] were grown as test crops in pots on soils treated with 30 mg P kg" soil as Busumbu PR and TSP applied either singularly or in combination with 5 or 10 tonnes ha' of tithonia to determine P uptake and dry-matter yield. In soils without added tithonia, AER-P followed the order TSP > Minjingu PR » Busumbu PR. By week 16, AER-P from Minjingu PR and TSP were similar. Busumbu PR solubility was low, and did not increase signifcantly in 16 weeks. ACER-extractable P was generally greater than AER-P. The difference was greater for PRs than for TSP. The ACER extraction may be a better estimate of plant P availability, particularly when poorly soluble P sources are used. This may be due either to the ability of the Ca sink (CER) to remove accessory CaCOj and Ca from the decomposition of tithonia, or to act directly to dissolve solid PR in the soil. Application of tithonia at 10 tonnes ha' in combination with Minjingu PR caused a small but significant depression in AER-P relative to Minjingu PR alone at all sampling dates except week 16 while ACER-P was not significantly affected by tithonia addition. v Soil samples treated with TSP had the highest NaHC03-Pi and NaOH-Pi (labile and moderately labile inorganic P) in the sequential extraction, followed by Minjingu PRo The tithonia + Minjingu treatment showed decreases in NaOH-Pi and NaHC03 Pi fractions, and increases in HCl-P (undissoved Ca-P) relative to Minjingu PR alone. Solubility of Busumbu PR was uniformly low, and was unaffected by the addition of either tithonia or TSP. The addition of inorganic amendments (CaC03 and CaCI2) showed a depression in AER-P as compared to application of Minjingu PR alone. This was presumably due to the Ca common-ion effect. The addition of CaC03 gave a significant increase in ACERP relative to all other treatments. The higher extractability of P on inclusion of both AER+CER may be due to direct interaction of CER with the Ca of undissolved PR, which presumably was the dominant form of PR at the higher pH of the CaC03 treatment. The lowest extracted P (both AER and AER+CER) was in CaCl2 treatments. It is possible that the decreased P extraction was due to Ca common ion effect and strong preference for CI- over orthophosphate by the anion exchange resin. In the pot study, addition ofTSP up to 100 kg P ha' gave a linear increase in dry-matter yield and P uptake for both maize and cowpea. At any given P rate, highest yields were obtained with tithonia alone. Organic-inorganic combinations resulted in yield decreases in most cases. Combination of Busumbu PR with TSP or tithonia did not enhance the effectiveness of the PR. MPR was very reactive in this soil whilst BPR was the least reactive. Addition of tithonia showed no effect on the solubility of the less reactive BPR. The solubility of the more reactive MPR was depressed.