Investigation the effects of FYM on phosphorous availability and recovering.

Phopsphorus as one of the most important essential nutrients for plants has a low mobility in soil. Most part of the applied phosphorus is fixed in the soil and a little percentage of it is used by the first year plant. Some part of the fixed phosphorus can be recovered by the plants of the following years. There are many factors that affect the residual phosphorus recovering which one of the most important one is soil organic matter. This expriment was designed to evaluate the rate of phosphorus depletion from the soil and its residual availability and recovering by later plants and the effect of FYM as a main source of organic matter on it. Expriment was conducted on Randomized Complete Block Design in a factorial manner with three replications in four years. Treatments were consisted of FYM in three levels ( 0, 15, and 30 ton.ha-1) and three levels of phosphrus ( 0, 90 and 180 kg.ha-1 as P2O5 from TSP). Phosphorus fertilizer applied only in the first year of expriment, but FYM used in all of the years. Onion, canola, bean and spinage were planted in the four years of experiment, respectively. Results showed that phosphorus application had significant increasing effect on soil phosphorus availability in the first year of experiment even in the absence of FYM. But, in the following years, residual phosphorus showed no effect on P availability in the soil in the absence of FYM. Although FYM usage increased crop yield and P availability in the soil in all of the years and treatments, but applied or residual phosphorus excelerated it. It should be noticed that the highest rate of FYM was more effective, paticularly in later years. Also, the plant type was important in phosphorus absorption from the soil. Although it is necessary to study it in the futhre but it seems that spinage is more able and onion is less able than others in P absorption from the soil.