Factors Contributing to the Benefits of Foliar Nitrogen Application for Wheat in a Sub-Tropical Winter Environment

Abstract

Beneficial effects of foliar N application to cereal crops have been widely believed to rely on subsequent rainfall or irrigation in the semi-arid region where in-crop rain is insufficient and unreliable. To verify this, factors contributing to the effects of foliar N application were assessed by implementing foliar N application in trials on three farmers' properties in central Queensland, Australia. Solutions of CRN35 (30 kg N ha−1 at 1.94 N % w v_1), urea and UAN (both 26, 2.25%), and urea (20, 20%; 40, 20%) and UAN (20, 20%; 40, 40%), were applied to wheat at anthesis respectively at the three properties (sites). Adjuvant chemical agents were also applied in mixture form with the N at the latter two sites. At the first site, with foliar N (CRN35) application, grain protein content on average increased by 1.07 percentage points, and by 1.6 in the presence of basal superphosphate. Superphosphate alone also increased grain yield. At the second site, while the applications of UAN and urea increased grain protein content by 1.3 and 0.9 percentage points, respectively, UAN showed superiority to urea in N recovery. At the third site, foliar N applications with or without chemical adjuvant did not affect grain protein content, but due to the higher concentration in the N solution, some of them (excluding UAN40) had negative effects on grain yield, compared to the water only control. Based on our data and supporting literature, to ensure the positive effect on wheat grain protein content, subsequent irrigation or rainfall may not be critical provided that foliar N concentration is lower than 5%. It is also suggested that UAN is superior to urea as an N source for foliar application. For alkaline/sodic soils, while the application of basal superphosphate increased grain yield, the added foliar N ensured an increase of grain protein content.