The fertilizer equivalence of phosphorus and potassium in organic manures applied to arable soils

Previous papers Soil Use and Management (2004) vol. 20, 410; (2007) vol. 23, 162] derived relationships for different soil types in south England of change in topsoil K (ΔKex) or P (ΔPres) against K or P balance over 3–5 years for fields in commercial farms given variable rates of fertilizer but no manure. Shown here is how ΔKex or ΔPres on manured fields can be converted to a fertilizer equivalent (fertilizer substitute) K or P value of the manure (FEK or FEP). There were significant increases in Pres and Kex when animal manures were applied, and in Pres using sewage sludge cake or liquid. Median FEP and FEK values for one application were – cattle farm yard manure (FYM) 36 kg P per hectare and 153 kg K per hectare, pig manure 70 and 149, digested sludge cake 62 P and digested liquid 31 P. Poultry manures and slurries also showed a significant benefit. When <12 months (only one cultivation) elapsed between application and soil sampling, FEP was much lower than in the second year and further increased over 4 years, whereas the maximum FEK was attained within a year of manure application. Release of P is slow compared with K. After 2–4 years FEP and FEK per tonne of FYM calculated as 0.95 kg P and 4.5 kg K, but farmer-reported application rates may be inaccurate. For biosolids <60% of the total P showed as FEP within 4 years. Even single manure applications register a large benefit in soil P and K supply (currently worth >£225 per hectare as fertilizer), but are variable in effect, which must be evaluated by soil analysis: at least one season and two cultivations should elapse before sampling; ideally more than a 2-year interval.