Soil Phosphorus Pools as Affected by Application of Poultry Litter Ash in Combination with Catch Crop Cultivation

Biomass ashes from energy production are a source of phosphorus (P), and their reutilization in agriculture could help to close nutrient cycles and save natural P resources. To analyze the P fertilizing effect of biomass ashes, a pot experiment with a loamy sand, originated from a long-term field experiment without any P supply, was carried out. As P source, poultry litter ash was compared with high soluble mineral P (potassium phosphate; KH2PO4). Four catch crops, in particular phacelia, buckwheat, ryegrass, and oil radish, were cultivated. The soil P-fractionation method was used to follow the transformation process of ash P in the soil. Oxalate-soluble P, iron (Fe), and aluminium (Al) were determined to assess the effect of ash on P sorption parameters. In general, a high P-fertilizing effect of biomass ashes was found. Ash application resulted in an increase of plant P uptake and the most bioavailable resin P fraction and was even comparable to a high soluble mineral P source. No enrichment of ash P in hardly plant-available P fractions could be detected. Crops mainly influenced the readily available P fractions. Buckwheat and oil radish exhausted the resin P and sodium bicarbonate (NaHCO3) P fractions most. Phacelia cultivation led to an increase of the highly available resin P content, which is probably due to mobilization processes from the hardly available residual P fraction. The obtained results indicate that ashes may have a high P availability to plants and may provide an adequate substitute for commercial P fertilizers.