Foliar Urea Fertilisation and the Management of Yield and Quality in Wheat

The effects of urea solutions targeted at the foliage of cereals are reviewed with particular reference to effects on the yield and quality of wheat. Nitrogen from foliar urea can enter through the leaves of cereals, and if the majority does so, the adoption of foliar urea may help reduce the nitrogen losses due to denitrification, leaching and immobilisation, often associated with application of nitrogen fertilisers to the soil. However, much nitrogen from foliar urea applied at timings targeted at increasing grain yield can reach the soil, either because of poor spray interception, or because urea is washed from the leaves by rain. Foliar urea-N is at risk of loss via volatilisation, particularly if it reaches the soil, and large application of urea-N to the foliage can suppress yield if it also causes large amounts of leaf damage. Leaf damage appears more likely in conditions conducive to rapid nitrogen uptake and is associated with high urea concentrations within leaves. Urea applied to the foliage may be particularly useful at improving nitrogen uptake when the primary aim is to increase grain protein concentration. Such applications are most effective at anthesis or during milk development, and can sometimes, but not always, lead to higher recoveries of fertiliser N than applications to the soil at these times. However, although actual recoveries of urea-N in the grain may be above 50%, apparent fertiliser recoveries are often lower than 20%, when previous nitrogen applications are sufficient to optimise yield. The implication is that late foliar urea reduces the recovery of N from other sources. Nitrogen from late foliar urea-N appears as effective at improving loaf quality as N from earlier applications applied to the soil, but are more reliable if N:S ratios are maintained.