PHOSPHORUS SUPPLYING CAPACITY OF HEAVILY FERTILIZED SOILS .2. DRY-MATTER YIELD OF SUCCESSIVE CROPS AND PHOSPHORUS UPTAKE AT DIFFERENT TEMPERATURES

Nine heavily fertilized soils were collected from southern and central Norway. A greenhouse experiment in the phytotron was conducted to eva luate the P supplying capacities of these soils at different temperatu res (9, 12 and 18 degrees C). The crops were grown in succession and t he sequence was oat, rye grass (cut twice), oat, rape and oat. Effect of temperature on dry matter (DM) yield and P uptake was more marked u p to the fourth crop but the effect varied among crops. The DM yields of oat and rape increased with increasing temperature but the opposite was the case with rye grass. The yield differences among soils at 12 OC were highly significant (p<0.01) in contrast to 9 and 18 degrees C. The amount of P taken up by plants in these soils was highest at 18 d egrees C. The P supplying capacity was highest in the soils with highe r content of organic P. Generally, the soils of very fine and coarse t exture classes failed to supply enough P to crops to avoid P deficienc y in the successive crops. Soil P test (P-NH4-lactate) values in most of the soils increased with increasing temperatures. The highest tempe rature effect was seen in the Saerheim sand soil. Soil P test extracta nts P-AL, Bray-1 and Colwell-P were used to determine P in the soil af ter each harvest and the soil P test values were compared with P uptak e by crops. Only the P-AL extractant was significantly correlated to c umulative P removal (CPR) by plants in most of the soils. Regression e quation was calculated for each soil. The value of removed P per harve st (RPH) varied from 10.33 to 20.87 mg P kg(-1) soil. Phosphorus drawd own slope was determined for each soil and the number of consecutive h arvests necessary to reduce the P-AL value to a normal level (110 mg P kg(-1) soil) was calculated. The drawdown slope varied widely (1.257- 2.801) and this reflected the P buffer capacity and the number of crop s required to lower the soil test P value to a normal level. The highe st drawdown slope was found in the soils with higher P supplying capac ities. The Bray-1 extractant was significantly correlated in the soils with higher buffer capacity but the Colwell-P method did not show sig nificant correlation in any of the soils.