Maturity indices for composted dairy and pig manures

Bulking agents and bedding materials used on farms for composting manures affect the time required for composts to mature. The effects of these materials on guidelines for the use of composted manures in potting mixes are not fully known. Several chemical and biological compost characteristics were mentioned and a cucumber plant growth greenhouse bioassay was performed on samples removed from windrows during composting of: (i) dairy manure amended with wheat straw; (ii) dairy manure amended with sawdust (mostly Quercus spp.); and (iii) pig manure amended with sawdust and shredded wood (mostly Quercus spp.). Dry weights of cucumber seedlings grown in fertilized and unfertilized potting mixes amended with composts (30%, v/v) having stability values of <1 mg CO2–C g–1 dw d−1, did not differ significantly from those in a control peat mix. Only the most mature dairy manure-wheat straw compost samples consistently established sufficient N concentrations in cucumber shoots in unfertilized treatments. For the dairy manure-wheat straw compost, all possible subset regression analyses of compost characteristics versus cucumber plant dry weight revealed that any of several compost characteristics (electrical conductivity-EC, compost age, total N, organic C, C-to-N ratio, ash content, CO2 respirometry, Solvita CO2 index and the Solvita® Compost Maturity Index) predicted growth of cucumber in the unfertilized treatments, and thus maturity. In contrast, at least two characteristics of the dairy manure-sawdust compost were required to predict growth of cucumber in the unfertilized treatments. Effective combinations were EC with compost age and the Solvita® maturity index with total N. Even five compost characteristics did not satisfactorily predict growth of cucumber in the non-fertilized pig manure-wood compost. Nutrient analysis of cucumber shoots indicated N availability was the principal factor limiting growth in potting mixes amended with the dairy manure-sawdust compost, and even more so in the pig manure-wood compost even though the compost had been stabilized to a high degree (<1 mg CO2–C g−1 dw d−1). Maturity of the composted manures, which implies a positive initial plant growth response of plants grown without fertilization, could not be predicted by compost characteristics alone unless the bulking agent or bedding type used for the production of the composts was also considered.