The relationship between soil mineral nitrogen, applied nitrogen and yields in Scottish soils


Cereals & Oilseeds
Project code:
01 February 2007 - 31 January 2010
AHDB Cereals & Oilseeds.
AHDB sector cost:
£18,000 from HGCA, project (RD-2007-3375)


pr484_summary pr484

About this project

Understanding of the relationship between soil nitrogen, crop residues and crop nitrogen offtake in Scottish arable soils is limited. In this study, an average of 27 fields were soil sampled to measure SMN (soil mineral nitrogen) each year from 2007-2009, covering most of the main arable area of Scotland. The same fields were tested in February before applications of nitrogen and then post-harvest. Mean SMN in the February sampling was 47.9 kg/ha. The mean figure for the post-harvest sampling was 61.1 kg/ha. There were large season to season variations, with very low available nitrogen in February 2008, perhaps due to very heavy January rainfall. The influence of previous cropping was investigated, with oilseed rape crops leaving slightly higher soil nitrogen residues compared to cereals or potatoes. In 2007 the soil nitrogen following potatoes was over 20 kg/ha lower than following cereals. Soil type had some influence on available nitrogen, soils with higher clay content tending to have lower values than more sandy soils. This is possibly due to greater mineralisation of organic nitrogen in the lighter textured soils, as they tend to heat up more rapidly. This relatively limited sample confirms that amounts of inter-crop SMN are relatively low, and that current Scottish Nitrate Vulnerable Zone (NVZ) crop residue groupings may need to be revised.

A series of 39 replicated nitrogen dose response trials on winter and spring barley, and winter wheat was carried out in Scotland over 3 seasons (2007-2009). Trials had incremental doses of nitrogen applications, including nil, and were statistically analysed to determine optimum nitrogen inputs. This was based on a 5:1 break-even ratio of nitrogen to grain. The majority of trials were highly responsive to applied nitrogen and nitrogen optima exceeded the relevant maximum allowance for nitrogen under Scottish NVZ guidelines (Nmax) in 27 of the trials. All trials were also sampled for SMN, both before nitrogen was applied and for all nitrogen treatments post harvest. There was no general trend for any increase in soil nitrogen as applied nitrogen dose increased. As a predictor of likely crop response to nitrogen the early soil samples did not generally provide a good guide. The exceptions were the soil samples with the highest and lowest soil nitrogen levels (91.9 kg/ha and 16.0 kg/ha), which subsequently resulted in the most unresponsive and responsive trials to applied nitrogen respectively. These trials indicate a potential yield limiting situation for Scottish growers. There is no evidence to suggest that nitrogen usage in excess of Nmax limits is resulting in additional leaching of nitrates.