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Agricultural soils in northwest Europe as well as in parts of the USA have excessive P levels. To meet European environmental standards on nutrient leaching in soils with a high P load, the total P content of manures and composts limits their application rate. The aim of the present study was to determine the effect of feedstock composition on C/P ratio and P availability in mature compost. A functional compost with a high C/P ratio and a high organic matter content is a valuable tool for maintaining soil fertility. We produced composts with C/P ratios of 60–180 and high organic matter contents, i.e., 43–72% on dry matter basis. P concentration in the compost increased and C/P ratio decreased due to mass reduction during the composting process, which was determined by the biochemical composition, i.e., the biodegradation potential of the feedstock mixture. The feedstock selection also had indirect effects on P availability due to its effect on compost characteristics such as pH, which strongly reduced the readily available P concentrations at $$\text{pH}_{\text{H}_{2} {\text{O}}}$$ > 8.5. Compost pH was found to be an important indicator for readily available P concentrations in the studied composts with $$\text{pH}_{\text{H}_{2} {\text{O}}}$$ between 7.1 and 9.4, and was positively related to total Ca content in the compost, indicating an interaction between Ca and P. In contrast, P availability was not related to the organic matter content of the composts. Mixing biochar (pyrolyzed organic matter) into the compost further reduced the readily available P in two types of compost by 10–64%, even after addition of a small amount of additional mineral P. This was observed for two types of biochar with different chemical composition. The reduction was higher for the biochar with the highest Ca, Fe and Al concentrations, indicating the role of these elements for P binding in the biochar-blended compost.

The application rate of poultry manure as organic fertilizer is restricted due to its low C/P and N/P ratio. This research was set up to find out optimal feedstock composition for composting chicken manure in order to create a soil improver with a higher value as organic fertilizer, and to assess the effects of this process on P availability in the end products. The research is based on two compost experiments with three treatments each, and the chicken manure composts were compared with the end products of green waste compost used as bedding material in chicken houses, centrally processed chicken manure and a stockpiled mixture of green waste compost and chicken manure. In the first compost trial, feedstock materials were compared for composting with fresh chicken manure. The chicken manure compost with 42.5 vol% bark in the feedstock had the highest quality as soil improver in terms of organic matter content, C/P ratio, C/N ratio and stability. Results of the second compost trial indicate that 10 vol% of fresh chicken manure is the upper limit in small-scale on-farm windrow composting above which nutrient losses become too high and the N/P ratio of the obtained fertilizer becomes too low. Stability of the product (expressed as oxygen uptake rate) had a major effect on P availability in the compost. Blending the chicken manure compost with biochar based on holm oak resulted in a more than proportional decrease in easily available P in the biochar-blended compost. P in poultry manure can be recycled through composting and be applied as an organic fertilizer with optimal nutrient ratio and organic matter content when appropriate feedstock materials are selected and the amount of fresh manure in the feedstock mixture is restricted.