Oxisols and ultisols are the major soils of Malawi but the P sorption of these soils has not been studied intensively. As of now, in terms of P fertilizer application these soils are treated the same. This research was conducted to determine the phosphate sorption properties and external P requirements (EPR) of these soils and to derive sorption values from selected soil properties. Physical and chemical analysis showed difference in these soils. Soil exchangeable aluminium, soil pH and soil organic matter is some of the soil properties that affect P sorption and these were different in these two soils. The sorption maxima were 315.5 and 413.0 mg P kg-1 for ultisols and oxisols respectively and EPR values were 37.8 and 52.2 mg P kg-1 of soil for ultisols and oxisols respectively. Exchangeable aluminium, soil pH (H2O), and organic matter found to be highly correlating with sorption maxima. The results from this study showed that, blanket application of phosphate fertilizer may not be a good strategy because ultisols and oxisols demand different amounts of P fertilizer and this blanket application may lead to under-application or over-application of P in some areas. The results also show that farming practices which improve soil organic matter and soil pH should be encouraged.
Key words: External P requirement, P-sorption, Langmuir equation, sorption affinity, Malawi, oxisols, ultisols.