Use of autumn nitrogen in no-till farming systems

Reduced tillage farming systems are increasingly being adopted across the UK, and there is a need to understand their impact on soil nitrogen supply (SNS) for autumn sown crops. The aim of this review is to establish whether autumn applications of manufactured N fertiliser for winter cereals and over-wintering cover crops are required under no-till or shallow min-till conditions. The primary applied N source considered was manufactured N fertiliser. Evidence from peer reviewed publications and relevant industry data was used. The review covers; i) autumn crop N demand, ii) autumn N supply, iii) ability of the crop to acquire N, iv) evidence for the effect of autumn N on crop performance and nitrate leaching, v) conclusions, vi) knowledge gaps and how to fill them.

The difference between autumn crop N demand and the expected N supply (i.e. from the planted seed, atmospheric deposition, soil mineral N (SMN) and mineralisation of soil organic matter and crop residues) under no-till conditions was estimated for winter cereals and winter cover crops. No-till reduced the amount of SMN by 5 to 25 kg N/ha, equalling a 6-29% reduction in the median autumn SNS. There was very little experimental evidence demonstrating the effect of autumn applied manufactured fertiliser N on winter cereal crop performance and nitrate leaching under no-till or shallow min-till conditions, with 15 relevant experiments in total of which only three included a no-till treatment. Under no-till conditions, there was no evidence that autumn applied manufactured fertiliser N increased winter cereal crop yield. One shallow min-till experiment found increased wheat yield following autumn fertiliser N application, but whether the same could have been achieved by altering the spring N application was not tested. Autumn applications of manufactured fertiliser N to cover crops increased crop N uptake, but usually under half of the applied N was taken up. The review concluded that autumn SNS for no-till systems will be sufficient to meet the demand of most winter cereal crops and for winter cover crops to achieve 50% ground cover.

Several factors (soil moisture, quantity and residues, crop type, availability of other nutrients) will also affect crop performance under no-till systems. There was some evidence that autumn applications of manufactured N fertiliser to cover crops increased weed numbers and nitrate leaching. A nitrate leaching model predicted that autumn applications of manufactured fertiliser N (30 kg N/ha) would increase nitrate leaching by 7 to 12 kg N/ha for cover crops or cereal cash crops respectively under low leaching risk scenarios (early crop establishment and low over-winter drainage), whereas all of the applied autumn N could be leached in high leaching risk scenarios.

Overall, the review concludes that ‘there is insufficient evidence to change autumn N guidance for no-tilled crops’. However, further research is required to improve the guidance for no-till crops, in particular; the effect of reduced soil disturbance on the availability of all main plant nutrients, how best to manage crop residues in no-till systems, the impact of no-till systems on nitrate leaching losses and spring fertiliser management in no-till systems.