The phosphorus status of German cropland - An inventory of top- and subsoils

Background: In search for more sustainable crop production, the subsoil has recently come into focus as considerable reservoir of nutrients and water. Aims: Dimensions of subsoil phosphorus (P) reserves are yet largely unknown but crucial for identifying regions suitable to include subsoil into sustainable management strategies. Methods: We analyzed stocks of total and plant-available (calcium acetate lactate-extractable) P in 96 representative soil profiles of German arable land down to 1 m depth. Results: We found that the German arable soils stored, on average, 8 t ha^–1 of total P, of which nearly 500 kg ha^–1 were readily plant-available. Notably, one third of plant-available P was located below the plow layer and one fifth even at depths below 0.5 m. The depth gradients of plant-available P stocks were affected more by major reference soil group than by texture. Generally, Chernozem but also Anthrosol, Gleysol and Fluvisol exhibited the largest P stocks in German cropland. The contribution of plant-available P to total P stocks was larger in sandy and extremely acidic (pH < 4.5) soils compared with more fine-grained and slightly acidic to alkaline soils, possibly because fertilization compensated for overall lower total P stocks at these sites. Generally, the more P was stored in topsoils, the more P was stored also in subsoils. Conclusions: A hypothetical crop utilization of 10% from plant-available P stocks and 0.1% from total P stocks from shallow subsoil could compensate for P fertilization by ca. 8 kg ha^–1, but the rate of plant-available P replenishment in subsoil likely remains the crucial factor for the role of subsoil P stocks in crop nutrition. Generally, the large P reserves found in subsoil could act as an ‘insurance’ system for crops.