An integrated approach to nitrogen nutrition for wheat

Abstract

This project developed a new approach to using fertiliser N on winter wheat, and it showed this approach to have economic and environmental advantages over conventional N use. The new approach, which has been dubbed 'Canopy Management', is more sophisticated than the conventional approach to N use, in that it entails field by field measurement of mineral N in the soil, and field by field assessments of the size of the crop's photosynthetic canopy, as opposed to simple identification of previous crop, soil type and expected yield.

The aim of this project was to augment developments from the understanding of soil N supply with developments based upon the understanding of crop N demand. This was possible through (i) accepting that a crop's performance must relate closely to the amount of sunlight its green canopy can use for photosynthesis, and (ii) applying our finding that the size of a crop's canopy relates directly to the amount of N it contains. Thus a 'Canopy Management' approach could be envisaged in which the growers' aim would be to ensure an optimum 'photosynthetic machine', and accepting that the grower has a limited capacity to predict or control grain yield. A theoretical framework was developed to link N application to grain yield through a series of quantitative steps: (i) recovery of fertiliser N by the crop, (ii) use of this N to form the green canopy, (iii) interception of sunlight by the canopy, (iv) conversion of sunlight to crop dry matter, and (v) partitioning of dry matter to grain. From this framework, we estimated the optimum canopy size for light capture to be between 5 and 6 units of green area index; smaller than many conventionally fertilised crops. A series of 'rules' was then developed to help judge the amounts and timings of fertiliser N applications necessary to supplement soil supply and provide for the expansion of the canopy to this optimum size. Additional N was then applied late, at flowering, to prolong the canopy's life during grain filling.

Experiments were designed to compare Canopy Management with conventional N management across the range of conditions in which winter wheat is likely to be grown. Soil mineral N in February varied from 20 to 150 kg/ha N and crop N uptake in February ranged from nil with very late sowings to almost 60 kg/ha N where sowing was early and soil N supplies were large. Soil mineral N in February was often near-uniformly distributed through the 90 cm depth of soil measured, particularly after large applications of N had been made to the preceding crops.

The relationship between soil mineral N in February and subsequent uptake by crops not receiving fertiliser N was remarkably (and conveniently) direct, demonstrating for a wide range of crop conditions that the recovery of soil mineral N can be predicted confidently. Fertiliser N required to make good the shortfall in uptake from soil supply was best estimated by assuming 60% recovery, and applications should be timed before maximum canopy size, allowing for a minimum N uptake rate of 2 kg/ha/day. The recovery of N applied at flowering was generally poor, averaging only 25%, and there was no significant improvement in recovery from using foliar urea compared to granular ammonium nitrate because a significant proportion of sprayed N either did not reach the canopy or passed through the canopy to the soil.

Canopy expansion rates and maximum canopy sizes were strongly related to the amounts of N taken up. There was direct evidence that the 'canopy nitrogen requirement' for Mercia should be revised from 30 to 28 kg/ha if a green area index of 6 is the target maximum canopy size. The results demonstrated that Beer's Law (which predicts a diminishing increase in light interception from successive increases in canopy size) holds for Mercia wheat grown over a wide range of conditions, and there was little benefit in terms of light interception from increasing canopy size to more than 6 units of green area index. During the early phase of grain filling, the coefficient (k) describing attenuation of sunlight by Mercia is best taken to be between 0.45 and 0.50 when considering all solar radiation, or between 0.50 and 0.55 when considering photosynthetically active radiation. There was more variation than expected in the efficiency with which crops converted solar energy into biomass. Significant variation in this efficiency was attributable to seasonal effects, to a change in sowing date and to use of fertiliser N. Notwithstanding these effects, Canopy Managed crops converted sunlight into biomass with similar efficiency to conventionally fertilised crops.

The partitioning of biomass to grain was significantly improved by Canopy Management. In 17 out of 24 comparisons, Canopy Management produced either the same or larger yields of grain when compared with the conventional approach. Analyses from milling and baking tests on samples taken in all years, showed that Canopy Management resulted in similar concentrations of grain protein and an equivalent quality of grain for loaf production.

Canopy Management resulted in average savings in N of 35 and 10 kg/ha in 1993 and 1994 respectively. However, slightly more N (16 kg/ha) was used in 1995 compared with conventional N management. Comparing the value of the yield improvements and the changes in N use, and taking into account the cost of analysing the soil for mineral N, Canopy Management resulted in an overall economic benefit of almost £10/ha.

These results are sufficiently promising for Canopy Management to be introduced into commercial practice on farms. Probably this could best be achieved in a step by step manner, perhaps through first adopting wider use of soil mineral N analyses, then by changing from a yield target to a canopy target, or alternatively the complete system might be adopted, but on a few selected fields, so that the demand for more intensive management is small at first, allowing the operator to gain familiarity with and confidence in the system. But, Canopy Management is not yet at a stage for wholesale adoption by the industry. Further work is needed to show how growers can best accommodate the requirement of Canopy Management for more detailed field by field assessments of both soil and crop on a whole farm scale.

Finally, irrespective of the economic or environmental advantages found for the Canopy Management approach, this project provides strong support of the justification that the industry makes for its dependence on fertiliser N.