EFFECT OF NITROGEN INPUTS TO CEREALS ON NITRATE LEACHING FROM SANDY SOILS

The effect of nitrogen fertilizer inputs to cereal crops on nitrate le aching after harvest was tested on 21 experiments on sandy soils in En gland. At small nitrogen fertilizer rates leaching increased very litt le with increasing inputs, while at high rates more than half of any a dditional nitrogen could be accounted for as increase in nitrate leach ed. In many cases the response fitted two straight lines. Nitrogen off take in grain also fitted two straight lines, with a form which comple mented the leaching response. The gradient averaged 0.52 kg N in grain for every additional 1 kg N applied below the break point, but only 0 .05 kg/kg above. The break points were generally close to or above the economic optimum N input. The effect of inputs on leaching could be q uantitatively related to nitrogen offtake in grain, assuming a constan t ratio of nitrogen in grain to total nitrogen uptake. The results sho w that fields receiving N inputs in excess of the economic optimum cau se a disproportionately large nitrate loss. However because of uncerta inty in predicting the break point in advance, modest further reductio n in leaching will occur by reducing inputs to somewhat below the expe cted economic optimum.