In areas with high livestock density it can be advantageous to export a solid manure fraction after slurry separation to avoid overload of P. By combustion or gasification of solid manure energy is produced and nutrients are concentrated and therefore less expensive to transport. However, some studies have indicated that the plant availability of P and K is decreased by combustion. The dynamics of extractable P and K in soil was compared during 16 weeks after application of equal amounts of P in ashes, solid slurry fractions and superphosphate to a sandy soil. Concentrations of water- , bicarbonate- and resin-extractable P and exchangeable K were measured after incubation. The ashes/biochars studied derived from gasification (ca 730°C) of poultry manure, gasification of solid manure, co-combustion of solid manure with straw (ca 700 and 900°C) and pyrolysis of solid manure (250, 400 or 500°C, biochar). Resin-extractable P in soil decreased from superphosphate > solid manure=pyrolysis ash 250-500°C >poultry gasification ash>solid manure gasification ash>manure co-combustion ash. Only 20-60% of ash K was water-soluble, but soon after application to soil 58-88% of the applied K was exchangeable compared to a KCl reference. The heavy metal content of the tested ashes was below the Danish threshold value for wastes like ash, except for Ni in the poultry ash.
Main Research Area:
Managing livestock manure for sustainable Agriculture, 2010