By employing the latest research results to the full, potato growers could cut their nitrogen, phosphate and potash fertiliser needs by 100, 130 and 50kg/ha, respectively. But inputs must be tailored to individual crops, stresses Mr Allen.

“Potato prices haven’t tracked the recent cereals and oilseeds rises, so without decisive action growers will see margins fall. Reducing inputs will improve margins and benefit the environment by lowering the risk of water contamination. New research shows fertiliser applications can be cut without loss in yield or quality.”

So just how much may growers trim? Optimum fertilisation should be calculated from crop and field data, advises Mr Allen.

Variety, growing season length, crop end use, pest problems, soil type and its phosphate, potash and magnesium status will all affect what should be applied.

Soil testing a year before planting has long been advocated and many farms have data going back 20-30 years, he notes. “This provides an ideal opportunity to look at long-term P, K and Mg trends. If levels have risen for 10-20 years it shows crop off-take is less than fertiliser input, so economies may be possible.

“Having established average soil nutrient levels, it’s time to decide which recommendation system to follow. Cambridge University Farm research shows potatoes don’t need as much fertiliser as traditionally thought.”

While the accepted standard reference RB209 is being revised Mr Allen suggests comparing its recommendations with those on the Potato Council website* where field data can be entered and a possibly more accurate recommendation generated.

Nitrogen

Surveys of soil N residues after potatoes often point to carry-over, proving that crops are often over-supplied.

Too much N delays tuberisation, lodges canopies and delays skin set. CUF has shown that the N required for 100% leaf cover and to maintain the canopy should be 0-175kg/ha.That differs from RB209 figures of up to 270kg/ha, which suggests potentially large savings could be made.”

Most N should go into the seed-bed. Later applications can scorch leaves to let in blight, he warns.

CUF also found that dressings beyond 55 days from emergence offered no yield benefit whatever the state of the crop. “All that happens is more leaching, with potential harm to the environment and inevitably reduced gross margin.”

Leached nitrate can increase growers’ chances of being in an NVZ in the future, warns lead south-west CSF officer Louise Webb.

Phosphate

Phosphate levels in surface waters have been highlighted as causing river biodiversity problems.

“The amount of phosphates entering watercourses needs to be reduced by all sectors to help comply with the Water Framework Directive,” says Mrs Webb. “So growers and advisers must decide whether phosphate is really required for soils with more than adequate reserves.”

Phosphate can be applied at indices of 0-1 with reasonable confidence that crops will benefit and none will be leached. “Triple super phosphate is the most water-soluble material and the most appropriate if the soil is low in P.

“At index 1 RB209 recommends 230kg/ha, but current thinking is that 100kg/ha should be enough. Applications above this help to raise soil indices, but at over £600/t for TSP it may be better to put off trying to increase them until prices fall.”

On soils with P indices above 2 there is no response to applied phosphate, so even the often used 50kg/ha insurance dressing may be a cost that can be saved, she suggests.

“Certainly at soil indices above 3 no applied phosphate is needed and dressings should be shunned to avoid water pollution.”

Potash

Potassium is essential for other nutrients to be used efficiently. But crop removal is quite modest – just 5.8 kg/t so only very high yielding crops need significant amounts of applied fertiliser.

Even then, CUF trials show that above soil index 2 there is little response. The evidence that large amounts increase tuber dry matter is very weak, says Mr Allen.

Magnesium

Magnesium deficiency must be avoided. To prevent it, kieserite (25% Mg), is the most available source, at index 0-1, he says. “A cheaper option is good quality calcined magnesite, which should suffice where the soil index is higher.”

For more advice in CSF priority catchments visit http://www.defra.gov.uk/farm/environment/water/csf/index.htm