Composting for increasing the fertilizer value of chicken manure: effects of feedstock on P availability

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.