Soil test phosphorus as affected by phosphorus budgets in two long-term field experiments in Germany

Phosphorus (P) is a limited resource but plays an important role in crop production. Because of the high P binding capacity in soils, changes in soil P availability due to different P management practices generally occur slowly. This study evaluated data of two long-term field experiments in Germany, which both focus especially on P management strategies. The Rostock field experiment (established in 1998, north-east Germany) considers organic and inorganic P sources in single and combined application and the Freising field experiment (established in 1978, south Germany) comprises different levels of inorganic P sources in combination with liming. Soil tests for available P were performed using double-lactate (DL) and calcium-acetate-lactate (CAL) extraction. For calculations of P budgets, the P removal with the harvest and the P supply through fertilizers were considered. Crop yields depended only partly on P supply and differences regarding crops sensitivity to P supply were found in the following order: beet > maize > spring cereals > winter cereals. Omitted P supply resulted in reductions of soil test P from about 42 to 29 mg kg(-1), within 18 years in Rostock and from about 44 to 17 mg kg(-1) within 38 years in Freising. Contents of soil test P were positively related with P budgets (Rostock: R-2 = 0.70, Freising: R-2 = 0.69). However, variations of soil test P occurred at both sites, a finding which could not be explained by P budgets and might be reasoned with vertical P transport. Our study shows that results of long-term experiments are important for the interpretation of soil P tests. The different responses of crops to P supply and the temporal variations in soil test P should be considered more in P fertilizer recommendations.