Abstract:

Studies were conducted to determine the phosphorus status and sorption characteristics of three Vertisols of Uganda collected from Kamutur in Kumi District, Olweny Swamp Rice Irrigation Project in Lira District and Sironko in Mbale District. Soil site and profile descriptions were done to confirm that the soils in the selected areas are Vertisols. The phosphorus status and sorption characteristics of the soils were determined by sequentially partitioning their different inorganic phosphorus fractions and establishing their ability to sorb phosphorus. The effects of organic matter application on the phosphorus sorption parameters, and on the availability of native phosphorus was determined. Changes in the inorganic phosphorus fractions after incubation with superphosphate were also determined. There were quantitative differences in the status of native inorganic phosphorus the soils. Bray I phosphorus in the composite samples of the surface soils (0-20 cm) was high (39.5 ppm P), medium (11.6 ppm P) and low (6.6) in soils from Sironko, Kamutur and Olweny, respectively. Total phosphorus was highest in Kamutur soil (1322 ppm P), followed by Sironko soil (374 ppm P) and lowest in Olweny soil (335 ppm P). Of the active inorganic phosphorus fractions (aluminum-bonded phosphorus, calcium-bonded phosphorus and iron-bonded phosphorus), aluminum bonded phosphorus (Al-P) was the predominant fraction in all the soils, forming 62.2%, 89.7% and 65.8% of the total active inorganic phosphorus fractions in Kamutur, Olweny and Sironko soils respectively. Application of 30 mg P kg'1 soil as single superphosphate in Kamutur and Olweny soils significantly increased all the active inorganic phosphorus fractions; the increase in Al-P being significant only at 90 days of incubation. The soils differed in their ability to sorb added inorganic phosphorus and the standard phosphorus requirements (SPR) were of the order: Kamutur soil (46.61 ppm P) > Olweny soil (36.08 ppm P) > Sironko soil (8.62 ppm P). Phosphorus sorption in the soils from Kamutur and Olweny deviated from the conventional Langmuir equation and conformed to the binary (two-surface) Langmuir relationship. This is an indication of the existence of two types of sorption sites in the soils having different affinity for phosphorus; and this results from creation of new sites due to displacement of structural silicates of clays at high concentrations of phosphorus in equilibrium solution. For the Sironko soil, phosphorus sorption conformed to conventional Langmuir equation suggesting sorbed phosphate formed a monolayer on the sorbent surfaces. Application of organic matter, in form of cellulose and starch, at 20 g kg'1 soil increased phosphorus sorption maxima in the soils from Kamutur and Olweny but decreased that for the soil from Sironko. The increases in Kamutur soil were 5.5, 6.5 and 12.4 ppm P at 30, 60 and 90 days of incubation respectively. For Olweny soil the increases were 39.9, 42.4 and 47.8 ppm P at 30, 60 and 90 days of incubation respectively. In Sironko soil the decrease (of 4 ppm P) in phosphorus sorption was significant only at 60 days of incubation. No immediate phosphate fertilization is recommended for Sironko soil because of the high level of available phosphorus, but for Kamutur and Olweny soil there is need for phosphate fertilization for higher crop yields. The standard phosphorus requirements indicate that Kamutur soil requires 93.22 kg P ha'1 and Olweny soil 72.16 kg P ha'1 for optimum crop growth. However, the actual economic rates should be confirmed from field experiments. Incubation studies showed that most of the inorganic phosphorus added in Kamutur and Olweny soils was recovered as calcium-bonded phosphorus followed by aluminum-bonded phosphorus at 30 and 60 days of incubation. This implies that most of the applied phosphorus would be available to crops in the short term.