Ionic contents of leachate from grassland soils: a comparison between ceramic suction cups and drainage

Ceramic suction cups were used to obtain samples of soil solution from permanently grazed swards receiving 200 kg N/ha/y. The suction cups were installed in 1 ha plots at 10, 30 and 60 cm depth in a poorly drained, heavy clay soil in S.W. England. The plots were hydrologically isolated from each other by perimeter drains which channelled surface runoff water into v-notch weirs. In one treatment, artificial drainage by a system of field and mole drains also converged to outfalls through v-notch weirs, which enabled samples to be taken. Nitrate and a range of other ionic constituents were examined over a 12 month period in soil solutions taken from the suction cups and compared with leachate obtained from the field drains and surface channels. Field drain samples frequently exceeded the EC limit of 11.3 mg nitrate-N/l, but concentrations in suction cups obtained during the same period did not, and were up to ten-fold less. Although correlations for ions were found between different sampling depths and drainage samples, no clear patterns emerged. It was concluded that suction cups were inappropriate for the determination of the overall leaching losses in this soil type, but provided useful data on changes in ionic concentrations which occurred in different soil horizons through to drainage outfalls.