Uptake of foliar‐applied urea by winter wheat ( Triticum aestivum ): The influence of application time and the use of a new 15 N technique

A new 15 N technique (termed the negative discard method) for measuring recovery of foliar‐applied N by crops in the field is described. 15 N‐labelled fertiliser solution is sprayed on to a small area of crop, using a hand sprayer, while the surrounding area is sprayed with unlabelled N at the same rate. An area considerably larger than that given 15 N is harvested with a small‐plot combine‐harvester, and crop recovery of foliar‐applied N is calculated from the 15 N enrichment of the resulting sample containing a mixture of labelled and unlabelled material. The technique was used to measure recovery of N from 15 N‐labelled urea solution sprayed on to winter wheat ( Triticum aestivum L cv Avalon) at six different times from growth stage 39 (3 weeks before anthesis) to growth stage 73 (2 weeks after anthesis). Each treatment of 40 kg N ha −1 was divided into two equal portions, the second being applied 1–2 days after the first, to minimise the risk of leaf damage. The crop had earlier received 210 kg N ha −1 , as ‘Nitro‐Chalk’, in spring (50 kg ha −1 at growth stage 22 and 160 kg ha −1 at growth stage 31) which was more than sufficient to achieve maximum grain yield. At harvest, 70% of the foliar‐applied N given at anthesis (growth stage 65) was recovered in the above‐ground crop, including 64 % in grain. The proportion of labelled N recovered in the grain (92% of that in the above‐ground crop) was slightly greater than with soil‐applied N given earlier in the growing season. Recovery of foliar‐applied N was slightly less for the earliest (growth stage 39) and latest (growth stage 73) times of spraying: 64% and 58% in above‐ground crop, and 56% and 54% in grain, respectively. All of the foliar applications of 40 kg N ha −1 increased %N in grain to the same extent as an additional 40 kg N ha −1 applied to soil in spring.