Abstract

The investigations of the tile drain water composition in areas with a clayey soil consisted of taking two or more samples of drain water at 30 farms in the sea-clay regions of the Netherlands from 1992 to 1995. The drain-water composition was used to describe the significant effects of the weather (precipitation minus evapotranspiration) and the distribution of travel times in the soil on the drain-water composition. The effect of nitrate leaching from arable land (and orchards) provided with tile drains assumed linear relationships with the N load, depending on the texture of the soil (sand, loam, clay) and groundwater depth (Gt). The N leaching was 10% to 25% of the load for deep-water levels, after taking into account a possible reduction by the oxidation of sulfide compounds, or the occurrence of seepage water in drain discharge. The leaching of nitrate from grassland is generally less than from arable land. Reduced leaching was also observed after growing grass seedlings on arable land in the year before sampling. The leaching from grassland is a function of the load (after subtracting uptake by vegetation), of the soil type and of groundwater depth. The leaching of phosphate in clay regions has no direct relationship to surface loading, yet the crops grown differed. Phosphate concentrations in drain water discharged from arable land had relatively constant values of 0.10 mg.l-1 (as ortho-P). Leaching from recent grassland was roughly 0.20 mg.l-1 (as ortho-P), while leaching from older permanent meadows had a relatively high value of 0.40 mg.l-1 (as ortho-P) and greater. The number of cases investigated was too small to make significant conclusions on phosphate leaching from grassland.