Effects of Compost on Soil Fertility Parameters in Short-, Mid- and Long-Term Field Experiments

To evaluate the influence of compost on soil fertility and plant growth, several medium term and long term field experiments with composts were conducted in different crops. In two maize experiments, one in a sandy and one in a loamy soil, the influence of different composts and digestates on soil parameters and plant growth were investigated. All products increased pH of the soil and improved the biological soil activity (e. g., enzymatic activities). Immature compost immobilized nitrogen and reduced plant growth. Organic nitrogen fertilizer added during cultivation, could compensate the growth depression. A full factorial experiment in a 2-years-old organic apple orchard was conducted from 2001-2007. The factors tested in all 9 possible combinations were: i) biowaste compost, ii) commercial organic N-fertilizer, iii) foliar N-fertilizer. In spring, the highest values for mineralized N (N-min) in the tree strip were found in the treatment with commercial organic N-fertilizer, with addition of compost it was 75%, and biowaste compost alone reached 50% of this value on average, whereas unfertilized plots had the lowest but still sufficient values for the same tree performance and fruit nutrition as fertilized plots. In the DOK long-term field trial, three farming systems are compared since 1978: i) mineral and organic fertilisers, synthetic pesticides; ii) organic fertilisers, mechanical weeding and biological disease and pest control; and iii) composted manure and bio-dynamic preparations. A conventional system with mineral fertilisers only and an unfertilized treatment serve as controls. Soil fertility mirrored by soil biological parameters, soil biodiversity and soil organic matter are higher in the organic systems and render these systems less dependent from external inputs.