Role of compost in the organic amendment of vegetable crops

Luigi Morra [Council for Agricultural Research and Economics (CREA) - Research Centre for Cereal and Industrial Crops, Laboratory of Caserta]

Soil degradation processes can be counteracted paying attention to the role of organic matter as key factor for soil fertility and CO2 planetary balance preservation. Large agricultural areas in Mediterranean Europe are interested by soil carbon depletion to critical levels (= 1%) and the composting of organic, source separated urban wastes, agro-industrial wastes, livestock effluents represent an efficient tool to recycle exogenous biomass in agricultural soils. The Author describes the Italian net of industrial composting plants, the amount of wastes treated and the amount of compost produced. In Italy, a law regulates the kinds of compost and the analytical characteristics the classification as compost of quality exploitable in agriculture depends on. The use of compost in horticulture would represent an innovation that, in conjunction with the adoption of other agro-ecological practices, could trigger changes in the vegetable crop systems toward agro-ecosystems managed according to ecological criteria. The amount of compost to supply to vegetable crop systems mainly depends on the dual target: a) to obtain an organic carbon balance positive or equal to zero (in relation to the soil organic carbon content higher or lower than 2%) and 2) to manage N balance such as to feed crops and to avoid hazards due to Nitrogen losses for leakage or volatilization. Experimental trials carried out to study the effects of growing rates of compost in open field and under tunnel horticultural systems, demonstrated that rates between 10 and 20 t ha-1 as dry matter achieve the above mentioned target with an efficiency conversion (unit of soil C per unit of supplied C) of 20-25%, higher than the one achievable with rates upper to 20 t ha-1. Mineralization of N provided by compost in soil is low (max 10%) in the first three years of amendment, while from the third year on it can reach 20% too. Mineral N released in annually amended soils, satisfies the needs of the spring-summer vegetables able to yield in such a way similar or higher to the ones fostered with mineral fertilizers; instead, vegetables cultivated in autumn-winter season may need an integration of reduced doses of mineral fertilizers. Mineral N released in soils rarely exceeds threshold of optimal availability for plants in open field, while under tunnel-greenhouse uncontrollable releases were detected. The use of compost of quality minimizes the hazards to augment total and bioavailable contents of trace elements in the soil.

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