Dissolution and extractability of minjingu phosphate rock fertilizer in some acid soils of Rift Valley province as related to soil chemical properties

Abstract

Dissolution and extractability of Minjingu phosphate rock fertilizer in some acid soils of Rift Valley province as related to soil chemical properties by Kowenje C.B.O. ABSTRACT ' Phosphorus (P) is a critical nutrient needed to improve soil fertility for sustainable crop production in large areas of developing countries. The use of phosphate rocks (PR5) as directly applied fertilizers in tropical acid soils is considered as a potentially cheaper altemative to the more expensive conventional water-soluble P fertilizers. Ilowcvcr, the major factor limiting the use ofthe PR materials is their low solubility in soils, The dissolution of PR5 in soils is affected by several factors including soil chemical properties. The dissolution and extractability of Minjingu PR (Ml>R) fertilizer from Tanzania in relation to some selected soil chemical properties were investigated in a laboratory incubation experiment. Six acid soils from Rift Valley Province of Kenya were used. Four soils (two Nitisols and two Ferralsols) were moderately acidic (pH 5.1-5.5) and had P adsorption capacities ranging from 1220-2487 mg?/Kg soil, while the other two soils (an Andosol and a Phaeozem) were slightly acidic (pH 63-6.8) and had P adsorption capacities of 836-904 mgP/Kg soil. The treatments consisted of two P sources (MPR fertilizer, and a water- soluble P fertilizer, Single Superphosphate (SSP)) and three rates of P addition (O mg,P/Kg soil, 200 mgP/Kg soil, and 500 mgP/Kg soil). The SSP fertilizer was included to compare the effectiveness of MPR as a fertilizer in maintaining plant- available P. The fertilizers were mixed separately with each soil at the different rates of P addition, and the soil treatments were brought to field capacity with distilled water. The _zimounts of MPR fertilizer that had dissolved were determined by measuring increases in 1 M BaCl2-triethanola.mine (TEA) extractable Ca (ACa) in soils amended with MPR fertilizer compared to the control (Zero-l’) treatments at various periods of reaction up to 60 days. Plant-available P was determined in both the PR-treated and SSP-treated soils by measuring increases in 0.5 M Na.HCO; - extractable P (Olsen P) over the control treatment after 15,30 and 60 days of reaction time. Simple linear correlation was performed to investigate the relationships between the selected soil chemical properties and the extent ofPR dissolution. The extent of MPR dissolution increased with the rates of application, incubation time and varied among the soils. In the moderately acid soils, 2.8 % ofthe total added P had dissolved after 4 days, and this increased to a mean value of 29.3 % ofthe total added P after 60 days. In the slightly acid soils, 2.3 % of the total added P had dissolved after 4 days, and after 60 days, 11.7 % of the total added P had dissolved. The increase in the dissolution of the PR material with time indicated that the dissolution reaction was continuing. The increase in the dissolution with the rate of fertilizer application was attributed to the greater amount of fertilizer input per unit weight of soil at the higher rate of application. The differences between the soils in the amount of MPR dissolved were attributed to their differences in the chemical properties. V Of the initial soil chemical properties selected, exchangeable.Ca ,(exch. Ca) Olsen P, % C, and P-sorption capacity were significantly related to the extent of PR dissolution time, with the most predictive property being P-sorption capacity. Soil pH and CEC were not significantly correlated to the extent of PR dissolution. ‘No single soil chemical property significantly related to the PR dissolution at the early stages of incubation. Also no single soil chemical property tested was significant in predicting the initial rate of MPR dissolution. The Olsen P values increased over the control treatment following P addition with the SSP-treated soils showing higher values than the MPR-treated soils. This was attributed to the differences in the solubilities of the P sources. The Olsen P values for the SSP treatments consistently decreased with time due to‘ fixation reactions by soil components. On the other hand, the Oisen P values for the MPR treatments initially increased with time due to dissolution reactions, and then declined subsequently because dissolution reactions had slowed Town, and fixation reaction 1 . . d become the dominant reaction. The Olsen P values for both the P sources differed among the soils. The moderately acidic soils indicated relatively lower values compared toythe values otthe slightly acidic soils. This was attributed to the differences in thc P-sorption capacities ofthe soils, Highly significant negative correlation was obtained in the PR treatments between the % P dissolved and the corresponding Olsen P values. This finding suggested that high PR dissolution does not necessarily produce proportional increases in Plant-available P in acid soils having high P fixation capacities. Key words: Minjingu, Phosphate rock, dissolution, extractability,chemical properties, incubation, correlation, Kenya, acid soils.