PHOSPHORUS FORMS IN A SANDY SOIL (TORRIPSAMMENT) UNDER PALM ORCHARDS OF DIFFERENT AGE IN SAUDI-ARABIA

Flood irrigated date-palm orchards annually manured for 10 (= U-10), 5 0 (= U-50) 110 (= U-110) and 230 (U-230) years together with a barren site in a nearby desert (= U-0), all on sandy soils, were analysed for total P (TP), organic P (OP), inorganic P extracted by 0.5 M H2SO4 (C a-P), inorganic P not extracted by 0.5 M H2SO4 (NP), total N (TN) and organic C (TC). Watering resulted in continuing CaCO3 accumulation in the soils. TC and TN to 160 cm depth increased rapidly over the first 50 years (120 g C m(-2) a(-1) and 9.7 g N m(-2) a(-1)), then the incre ases slowed. In contrast, TP continued to accumulate steadily (4.8 g P m(-2) a(-1)). At U-0, TP to 160 cm depth was very small (= 88 mg kg(- 1)) and dominated by Ca-P with OP as a minor component. At U-10, OP an d Ca-P were in equal proportions. The latter acquired increasing domin ance with increasing period of treatment whereas OP reached a steady s tate. NP increased at a rate which decreased with time. It is likely t hat most of OP compounds have been leached down the profile where mine ralization and subsequent immobilization, probably by Ca2+ ions from i rrigation waters, have occurred.