Abstract
The distribution of total, organic and the various inorganic phosphorus fractions was determined for samples from three Gezira profiles. Total phosphorus values were generally high throughout the profiles (406 to 700 p.p.m.), increasing with depth to reach a maximum at 50 to 145cm and then declining. They decreased in magnitude from the south to the north of the area in the order Hag Abdulla > Tayiba > Turabi. As expected in these alkaline soils, most of the inorganic phosphorus was present as calcium phosphate which varied between 280 and 510 p.p.m. Residual phosphate came next in abundance while aluminium and iron phosphates were present in relatively small amounts. Sodium bicarbonate extractable phosphorus was low, varying between 1·4 and 4·5 p.p.m. Organic phosphorus was generally low and the values decreased with depth. There was also a decrease in organic phosphorus from south to north, related to a decrease in the organic carbon content. The C : P ratios of the profiles were less than 200 indicating the possibility of mineralization. Superphosphate added to the soil was mostly retained as calcium phosphate, followed by aluminium and iron phosphates. A small amount was retained as residual phosphate and none as organic phosphorus. There was very little downward movement of applied phosphate in this heavy-textured soil
