Chemical characteristics of phosphorus in some representative benchmark soils of Zambia

The availability of native phosphorus (P) in Zambian Alfisols, Ultisols and Oxisols, the dominant agricultural soils in the country, is relatively low. Given that crop production in the country is dominated by low external input practices, native P remains important to plant P nutrition in many locations. There is a dearth of knowledge about soil P distribution, transformations and fertility in Zambia. The distribution of total P, organic P, inorganic P [aluminum plus iron P (Al + Fe–P) and calcium P (Ca–P)] and occluded P was determined in twenty soils of Zambia belonging to several soil orders and originating from different agro-ecological zones. Total P in the soils were observed to be similar to those found in more productive regions, ranging from 126 mg kg− 1 in the Bulozi Inceptisol to 742 mg kg− 1 in the Malashi Alfisol. There was overlap in total P contents determined among soil orders. This is attributed to interruptions in the pedogenesis process whereby new material was deposited on more weathered old basement complex. Soil organic P content was relatively low (3–175 mg kg− 1) and constituted an average of 15% total P. The ratio of organic carbon to organic P was generally less than 200, suggesting the potential for significant mineralization of organic P. The Al + Fe–P was the dominant inorganic P pool because of high Al and Fe oxides contents of these soils. The range of Al + Fe–P was 1–246 mg kg− 1 while Ca–P was 1–78 mg kg− 1. A disproportionately high fraction of total P (18–88%) was tied up in the occluded P pool. It was observed that clay content, and Fe plus Al oxides contents of the soils explained 62 and 51%, respectively of the variation in occluded P. Therefore the presence of high amounts of oxides and the rate of soil weathering due to high temperatures and moisture in part explain the low availability of P in Zambian soils.