22 December 2000

Thinking shift for spring N…

By Andrew Swallow

YIELD should not be used as a basis for calculating spring nitrogen requirements in cereals, according to the new edition of the RB209 Fertiliser Recommendations book – marking a fundamental shift in thinking.

Speaking at the first of a series of conferences introducing the seventh edition, ADASs Roger Sylvester-Bradley said most yield differences are accounted for by factors which cannot be forecast, such as drought and disease.

Yield itself is not a good basis for assessing nitrogen fertiliser requirement.

Instead, fundamental drivers such as length of growing season and output quality of the crop should be used, he said.

Soil nitrogen supply (SNS) is then subtracted and a fertiliser requirement derived allowing for efficiency of fertiliser uptake (see formula).

All three factors in the calculation can vary a lot, but SNS is the most variable. "Growers must look at this on a field by field basis," stressed Dr Sylvester-Bradley.

Defined as "the soil nitrogen available for uptake by the crop from establishment to the end of the growing season", SNS is most affected by previous crop and soil type.

Growers are encouraged to use soil mineral N tests on their own land, or refer to results of tests in similar situations.

"Testing is expensive, so you really do need to think about when you are going to use it," says Rothamsted soil scientist Keith Goulding. Sampling to 60cm, or preferably 90cm, with rapid delivery for analysis is essential for accurate results, he added.

However nitrogen within a field can be extremely variable, and soil mineral N sampling offers only a snapshot of a dynamic system. So tests should be used to avoid serious errors rather than fine tune supplies.

Soil type clearly affects the efficiency of nitrogen fertiliser uptake. Typically, on sands, 70% of N applied as fertiliser is taken up by the crop. On a medium loam that falls to 60%, and for chalk 55% is the norm.

Larger root systems make early sown crops more efficient at uptake. But that is offset by their greater demand for N. So sowing date is not considered a key factor.

Despite the detail behind the new recommendations, Dr Sylvester-Bradley says they will be within 40kg/ha (32 units/acre) of the optimum only half the time.

"We are not exactly pleased with that. But it is a measure of the issue that faces us as an industry and shows how difficult nitrogen requirements are to predict."

Economic optimum

Nitrogen to grain price ratio affects the economic optimum rate of fertiliser, but its influence is relatively small, says Dr Sylvester-Bradley. RB209 recommendations are based on a ratio of 3:1. But at this seasons price of £112/t for 34.5% N, and a grain price of £65/t the ratio is 5:1. How much that affects the economic optimum depends on soil type, shallow or medium textured soils being most sensitive, sands least. At a ratio of 5:1 the optimum on shallow or medium soil is 24kg/ha (19 units/acre) less than in RB209 tables, but on a sandy soil the shift is only 13kg/ha (10 units/acre). Clays and silts are somewhere between.

Yield alone has little effect on optimum N for cereals, so adjustments to account for it have been dropped from MAFF recommendations.

N fertiliser req = (Crop N req – SNS) x 100/fertiliser efficiency.


&#8226 Yield not a factor.

&#8226 Previous crop and soil type key.

&#8226 Use or refer to soil mineral nitrogen tests.

&#8226 Allow for price shifts.