Organic amendments and organic vegetable farming under plastic cover: Observations after six years of application

During an experiment carried out over a six-year period, annual applications of large quantities of two types of compost led to an increase in the carbon level in the ground but had little impact on its physicochemical characteristics. No significant effect was revealed on successive vegetable crops. In contrast, it was shown that these cropping systems had specific characteristics. Cropping conditions under plastic cover lead to a high level of mineralisation of the endogenous organic matter and a limited lixiviation of nutrients. As a result of frequent over-fertilisation practices in the past in conventional farming systems, deep soil horizons may contain large stocks of minerals, part of which will be tapped by crops over the years following a conversion to organic farming. In our experiment, this contribution of the soil to subsequent crop nutrient supply largely compensated for the reduction of inputs applied to summer crops up to the sixth year. We therefore hypothesize that these types of effects sustainably mask the impact of changes in fertilisation practices linked to the conversion to organic agriculture.