Effect of control lower limit of subsurface irrigation on phosphorus distribution in soil profiles of protected land

A subsurface irrigation experiment was conducted in a five-year tomato-grown protected land in the bases for scientific research of Shenyang agricultural university during 2000-2004. To different treatments, the irrigation allowable lower limits were set to 10, 16, 40, and 63 kPa, respectively, but the irrigation allowable upper limits of them were set to be 6 kPa. Results showed that the distribution and contents of phosphorus in soil profile were affected significantly by subsurface irrigation and irrigation lower limit. Soil total phosphorus, inorganic phosphorus and available phosphorus were higher at the depth of 0－10 and 10－20 cm than those at other depths, and were lowest at the depth of 30－40 cm. Changes of available phosphorus was mainly at the depth of 0－30 cm. Soil organic phosphorus leached down into the soil layers with irrigation water, and reached the highest contents at the depth of 40－60 cm. Soil inorganic phosphorus was the dominant form of phosphorus in different soil layers for all treatments. The results indicated that irrigation could be applied to facilitate the transformation and availability of phosphorus in soil.