The status of phosphorus in some Tanzania soils

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University of Dar es Salaam
A detailed study was conducted on the fractionation of inorganic phosphorus from the main soil orders found in Tanzania. The results obtained showed that the extracting time of one hour for Al-P with NH4F solution offered the least dissolution of iron-bound and calcium-bound phosphates in the soil. Ammonium fluoride extracting solution of pH 8 was Shawn to be the most selective extractant for A1-P as it had the least attack on the Fe-P and Ca-P in the soils studied. The inorganic phosphate fractions were determined in 23 soil profiles from eight soil orders commonly occuring in Tanzania. The distribution percentages for Ca-P, AI-P and Fe-P were found to be sensitive indicators of the weathering environment. Where soils were young, calcarious or the parent material rich in phosphorus bearing minerals Ca -P was the dominant fraction of inorganic phosphorus. In areas where the soils were highly weathered, A1-P and Fe-P were the dominant fractions. The distribution of organic phosphorus fractions in soil from three soil orders showed. that organic phosphorus was highest on the clay followed by the silt separates. The clay and silt were also found to contain the highest amount of Al-P and Fe-P, while the Ca-P fraction was highest on the sand. In another study on the distribution of organic P in soil profiles it was found that the` organic phosphorus content decreased with depth, except in a few cases where the accumulation. of organic phosphorus tended to occur in the second horizon. A highly significant correlation was obtained between the organic carbon and organic phosphorus content on the top soils of the profiles. The C:P ratio of the organic matter in the A horizons of the soil orders ranged from 25.7 to 493.3. The majority of the C:P ratios were below 200. The lack of response to applied P on some of the soils in Tanzania is attributed to the large amounts of organic phosphorus and the accompanying low C:P ratios. Decomposition of organic matter in these soils results in mineralization of significant amounts of organic phosphates which are then available for plant use. The activity of the enzyme phosphamonoesterase as measured by the amount of phenol liberated in soils was found to be highly significantly correlated with altitude and organic phosphorus. A highly significant negative correlation was obtained between the enzyme activity and the C:P ratio. The enzyme activity decreased as the C:P ratio increased. The enzyme activity decreased as the C:P increased and would appear to tail off around a C:P value of 200.
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Soils, Phosphorus content, Tanzania
Uriyo, A.P (1972) The status of phosphorus in some Tanzania soils, PhD dissertation, University of Dar es Salaam. Available at ( )