Fig. 7 Nitrate concentrations in the soil water for the blocks at different distances from the tile-drain, measured by soil sampling on 17 Dec. 1991 (t = 0; a) and 8 Apr. 1992 (t = 114 d; b), assuming all nitrate was dissolved in the soil water. On 8 Apr. 1992 the data of the topsoil are subject to errors due to the interference of fertilizer granules in the samples (see text)