Fosforgödslingseffekt av olika restprodukter

Phosphorus fertilizer effect of fifteen potential fertilizers based on residues was tested in a pot experiment and in an incubation. The possibility to relate phosphorus effect to extractable phosphorus with different extracting agents was also tested. Fertilizers included in the study were e.g. three types of sewage sludge, three kinds of livestock manure, two biogas residues, three ashes and meat and bone meal. In a pot experiment, ryegrass was grown which was cut after 5 and 11 weeks to measure the dry matter yield and the uptake of phosphorus. As phosphorus availability is strongly dependent on pH, the effect was tested in both limed and unlimed soil in order to distinguish the effects of added phosphorus and any liming effects of the residues. Results showed that yield and phosphorus uptake was reduced because of liming in this study. Residues with liming effect did therefore not perform well. Products with a low organic content, e.g. ash and biogas residues, had a faster impact than many of the other products with higher organic content such as manure and slurry. These differences leveled off over time. The total harvest and phosphorus uptake over two months were similar for all products and similar to a treatment receiving half as much phosphorus with mineral fertilizer. In the incubation, the P-AL value in soil increased about half as much as with mineral fertilizers. There was no connection between which products gave a maximum phosphorus effect on yield and a high P-AL value. When extracting phosphorus from the fertilizers, we found that there was a relationship between the extracted phosphorus and phosphorus effect in the first cut. P-AL extraction gave the best correlation. Citrate extraction gave misleading values for sewage sludge, but worked well if those were excluded. Water extraction gave to low extraction levels in comparison to the plant available phosphorus for several of the products.