To efficiently improve planosols in Sanjiang Plain in the northeastern part of China, the physical chemical properties of planosols have been investigated since 1986. The results showed that the AW layer, about 18-40cm in depth, was the key problem in planosol. In the subsoil layer, the water regime is poor with only 5.39% available water. The soil permeability is only 6.08×10^-5cm/s in saturation. Meanwhile, the soil hardness in over 25kg/cm². Because the Aw layer is so close to the surface soil that many problems occur in the ploughing layer, such as shallow available soil layer for crop roots, heavy water logging and drought. When this layer was ploughed into surface soil, the mixed soil could became harder and tighter than before. Nearly 65% of total phosphorus was observed to be organic phosphorus in Ap layer and the contents of p fractions are in the order: Fe-P > Al-P > Ca-P. During the soil incubation, 50%-60%, 10%-20% and 2%-10% of the applied p were changed into Ca-P, Al-P and Fe-P, respectively. According to above-mentioned results, emphasis of the soil improvement should be placed on the Aw layer.