NITROGEN NUTRITION OF CEREALS AND ENVIRONMENT - APPLICATION TO WINTER-WHEAT

One could define optimal ecological N fertilization for winter wheat as that producing maximum return while leaving at harvest a soil devoid of nitrate. To achieve that goal, plant N requirements during its growth, mineral-N production by the soil and of course, regional, or if possible, local climatic and edaphic conditions must be carefully monitored and taken into account. In our loamy soils, by splitting fertilizer-N into 3 dressings applied at defined physiological stages (Feekes 3, 5 and 10), optimal yield and low nitrate levels (10-20 kg N ha-1 for 1.5 m) are generally observed at harvest for winter wheat (preceding crop: sugar beet, leaves turned under) for a total rate of 200 kg N ha-1. For such a situation, mean recoveries of mineral-N fertilizer, measured with N-15, ranging from 60 to 80 % (aerial parts) are typical. Of course, when adverse climatic conditions and/or poor soil conditions impair N uptake, much nitrate may be left in soil at harvest. When organic manures are associated with preceding crop or when crop N residues are significant (after legumes for example), higher but variable amounts for mineral-N residues (50 to 150 kg N ha-1) are frequently measured at winter wheat harvest. It indicates that in such cases the evaluation of adequate N rates for winter wheat is greatly complicated owing to enhanced and unpredictable nitrate production in soil. The root system of winter wheat or barley is able to take up nitrogen from a depth of 1.5 m and sometimes even more if the soil is deep enough. That particularity insures an exhaustive uptake of N mineral-soil and fertilizer of the soil profile on 1.5 m. Excessive dressings seem to depress uptake and increase the risk of nitrate leaching, especially when soil depth or texture or bad weather conditions impair root development. In the near future, winter wheat and winter cereals N fertilization will have to take into account that N in excess of what is needed for optimal yield always induces an important increase of nitrate residues at harvest time.