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 (=U10), 50 (=U50, 110 (=U110) and 230 (U230) years together with a barren site in a nearby desert (=U0) all on sandy soils, were analysed for total P (TP), organic P (OP), inorganic P extracted by 0.5 m H2SO4 (Ca-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 increases slowed. In contrast, TP continued to accumulate steadily (4.8 g P m−2 a−1). AtU0, TP to 160 cm depth was very small (= 88 mg kg−1) and dominated by Ca-P with OP as a minor component. At U10, OP and Ca-P were in equal proportions. The latter acquired increasing dominance with increasing period of treatment whereas OP reached a steady state. NP increased at a rate which decreased with time. It is likely that most of OP compounds have been leached down the profile where mineralization and subsequent immobilization, probably by Ca2+ ions from irrigation waters, have occurred