Foliar urea fertilization of cereals: A review

Abstract

It has been suggested that there are several potential benefits of providing nitrogen to cereals via the foliage as urea solution. These include: reduced nitrogen losses through denitrification and leaching compared with nitrogen fertilizer applications to the soil; the ability to provide nitrogen when root activity is impaired e.g., in saline or dry conditions, and uptake late in the season to increase grain nitrogen concentration. Factors that influence the degree of foliar absorption in field conditions have not, however, been clearly defined and losses to the atmosphere and soil can occur. Foliar urea applications may also hinder crop productivity although the explanations for this vary, and include desiccation of leaf cells, aqueous ammonia and urea toxicity, biuret contamination and the disruption of carbohydrate metabolism. It has not yet been determined which one, or combinations, of these mechanisms are most important in field situations. When damage has not been severe, foliar urea applications have increased grain yield, particularly when applied before flag leaf emergence and when nitrogen availability is limiting. Increases in grain nitrogen content are often larger when applications of nitrogen fertilizers to the soil are reduced, and when the urea solution is sprayed either at anthesis or during the following two weeks. It is during this period that foliar urea sprays can be of greater benefit than soil applications with regard to nitrogen utilization by the crop. Increases in wheat grain nitrogen concentration following urea application can improve breadmaking quality. Responses in loaf quality may, however, be variable particularly when increases in grain nitrogen content have been large, and/or when the nitrogen: sulphur ratio in the grain is increased. These circumstances have lead to alterations in the proportions of the different protein fractions which influence breadmaking potential. To exploit the full potential benefits of foliar urea application to cereals, more needs to be known about the mechanisms, and thus how to prevent losses of nitrogen from the foliage, and to reduce the phytotoxic influences of sprays. More information is also required to exploit the reported effects that urea may have on limiting the development of cereal diseases

Effects of nitrogen supply on wheat and on soil nitrate

“…These results suggest that increases in grain yield following late-season urea applications were found only when previous N applications were insufficient to obtain maximum yield (Gooding and Davies, 1992;Gooding et al, 1997). No significant response of specific weight was found in these experiments, suggesting that the lowest topdressed N rate of 100 kg ha -1 was high enough to obtain a high specific weight.…”

“…To decrease these aleatory variations of yield and grain N content farmers often use higher rates of N fertilizer than those required to satisfy the demand, and as a consequence there is an increase in the risk of leaching. To reduce N losses several reports recommended applications of N to be postponed to later stages of the growing season, which favors protein build-up in the grain over yield (Sarandón and Gianibelli, 1992), and enhances the bread-making quality of the flour (Altman et al, 1983;Gooding et al, 1991;Gooding and Davies, 1992). However, the effect is a function of the crop N nutrition status and of the climate conditions, and so this effect is difficult to forecast in Mediterranean areas with high variation in climate (Lloveras et al, 2001).…”

“…However, the effect is a function of the crop N nutrition status and of the climate conditions, and so this effect is difficult to forecast in Mediterranean areas with high variation in climate (Lloveras et al, 2001). Applications of N as foliar urea to simultaneously increase the protein content of wheat and reduce risks of nitrate leaching or denitrification have been proposed in regions with little or no water stress during grain filling (Gooding et al, 1991;Gooding and Davies, 1992;Kettlewell and Juggies, 1992;Readman et al, 1997). However, crop responses to urea sprays have been highly variable and no clear benefits have been obtained in Mediterranean 440 A. Abad et al…”

Late‐Season Nitrogen Increases Improver Common and Durum Wheat Quality

“…Moreover, this late-season N fertilization strategy could be more easily adapted by farmers as it does not require a dedicated passage in the field, if combined and tank mixed with a fungicide treatment, although there is a limit to how much foliar N can be applied without incurring leaf burn (Jones and Olson-Rutz, 2012;Krogmeier et al, 1989). The risk of foliage burning is higher when the N source is ammonium nitrate or ammonium sulfate rather than urea (Gooding and Davies, 1992). Moreover, foliar application needs to be avoided early in the morning when dew is still on the crop or when there are high temperatures during the day (Arnall et al, 2012;Woolfolk et al, 2002).…”

“…Gooding and Davies (1992) suggested that benefits include reduced denitrification and leaching, improved fertilizer efficiency and reinforce the late seasonal N uptake. Tea et al (2004) demonstrated that N provided by foliar fertilization is mainly incorporated in storage proteins, which are responsible for bread-making quality.…”