NITRATE LEACHING AFTER CUT GRASS CLOVER LEYS AS AFFECTED BY TIME OF PLOWING/

Nitrate leaching after one year of a cut grass/clover ley was measured in two succeeding years to investigate how the postponing of ploughin g leys from early to late autumn or spring, in combination with spring or winter cereals affected leaching of nitrate. The experiment was co nducted as three field trials, two on a coarse sandy soil and one on a sandy loam soil. For calculation of nit:rate leaching, soil water sam ples were taken using ceramic suction cups. The experiments started in spring in a first year ley and ended in spring three years later. Tot al nitrate leaching for the three year periods for each trial ranged b etween 160-254 and 189-254 kg N/ha on the coarse sand and 129-233 kg N /ha on the sandy loam. The results showed that winter wheat (Triticum aestivum L.) did not have the potential for taking up the mineralized N in autumn after early autumn ploughing of grass/clover leys, and tha t the least leaching was generally found when ploughing was postponed until spring, and when winter rye (Secale cereale L.) was grown as the second clop rather than spring barley (Hordeum vulgare L.). Neverthel ess, leaching was generally high in the winter period even when winter rye was grown. On these soil types ploughing out should be postponed, whenever possible, to spring. Crop systems that maximize the utilizat ion of mineralized N and thereby minimize nitrate leaching need to be further developed. Based on N balances, the data were further used to estimate the biological N fixation by the clover.