OPTIMAL DRAINAGE DEPTH OF 5 CULTIVATED ORGANIC SOILS

Optimal drainage depth with regard to crop growth of five cultivated o rganic soils, including moss peat, fen peat and gyttja soils, was dete rmined in lysimeter experiments in a greenhouse with supplementary inv estigations in the field and in the laboratory. The variation range in physical and chemical properties of organic soils is much greater tha n in mineral soils and drainage recommendations have to vary according ly. Gyttja soils in general are well aerated, due to their permanent c rack system, but root growth in the subsoil can be restricted by low p H and toxic levels of soluble aluminium. Water levels should therefore be kept high (40-50 cm under the surface), during periods of high eva porative demand, to avoid shortage of water for plant growth and devel opment of soil water repellence due to dehydration. Moss peats and com pacted fen peats often suffer from aeration problems and ground water levels should be kept lower (60-70 cm). On well structured fen peats, with high hydraulic conductivity, ground water levels may be kept slig htly higher. At low evaporative demand, during spring operations and a t harvest time, ground water levels should be lowered to improve beari ng capacity and avoid aeration problems. Low water tables will bring m ore air into the profile, allow more microbial activity with more nitr ogen available for the plants but also induce a faster subsidence rate .