We all know the feeling when someone with a clipboard approaches in the street. But every year some 1500 British farmers take the time to record in detail their use of fertilizers, manures and lime over the previous twelve months. As a result, the UK has a unique record of nutrient use by crop that shows amounts applied by crop and long term trends. Annual reports of the British Survey of Fertiliser Practice can be downloaded free from the Defra web site or hardcopies can be bought via the AIC. The latest is for crop year 2008/2009 and shows the overall nitrogen rate falling over 25 years from 157 to 139 kg N/ha in tillage crops and from 126 to 57 kg N/ha on grass. Over the same period, overall phosphate application fell from 62 to 23 kg P2O5/ha in tillage crops and from 28 to 9 kg P2O5/ha in grass. Potash rates fell from 68 to 33 kg K2O /ha in tillage crops and from 33 to 12 kg K2O/ha in grass.
Avoiding excessive application of nutrients obviously is good practice but this is taking things a bit far. Off take of phosphate and potash in the main arable crops is much greater on average than amounts being applied in fertilizers and has been since the mid-1990s. Soil deficiency can creep up and effects on crops appear suddenly so phosphate and potash applications should be firmly based on regular soil analysis – especially as the typical cost of analysis works out at less than 20p/ha/year.
In 2009, the Survey showed fertilizer sulphur was applied to 60% of the oilseed rape area and 39% of winter wheat. Responses to sulphur can occur in rape anywhere in the country now so 60% looks low. I’d recommend sulphur for all rape crops unless there’s convincing evidence it’s not needed. Sulphur was applied to just 12% of grass cut for silage. This is where yield responses have been reported since the early 1980s so there must be some yellow crops getting extra nitrogen when they really need sulphur. In 2009, around 8% of the tillage area and 4% of grassland was limed. Looks like an average 12 year liming cycle for tillage crops and a 25 year cycle for grass which might not be enough to ensure pH stays adequate.
The impression from the Survey is that nutrient use and liming decisions often are based on, or at least strongly affected by, the cost of the fertilizer or lime. Experience suggests that this can be a false economy. Yield has a big effect on margins and a reduction of, say, 10% due to a nutrient deficiency is really important (do the sums!) but is not visible to the grower. A healthy crop needs a balanced supply of nutrients - a deficiency in one can mean the others are wasted. Apart from the economic cost, this can increase the risk of nitrogen or phosphate loss to water or air.
Ian has over 30 years experience in fertilizer use and crop nutrition. He now runs his own consultancy, Ecopt, involved in all aspects of nutrient management, including provision of the FACTS technical information service and he is also a member of the steering groups for revision of RB209 in 1999 and the Fertiliser Manual from 2007.
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