Cambridge research focuses on potash rate for spud crops
Cambridge research focuses on potash rate for spud crops
POTATO growers who tailor application rates of potash to soil analysis results could be wasting money applying excessive amounts or using too little for maximum yield.
That is because most soil analyses give no indication of how much potash can become available during the season.
Such is the conclusion of agronomist Marc Allison of Cambridge University Farm, who has just completed a three-year BPC- funded project to improve the precision of potash use.
"A crop needs 250-300kg/ha of potash, which can come from residual reserves in soil, the application of solid or liquid fertilisers, or from organic manures," Dr Allison says. "But as there are sometimes no obvious symptoms of lack of potash, apart from a reduced canopy size, growers often fail to appreciate that crops are being starved. So it is important to match application rates to actual need."
Luxury dressings must be avoided, as excessive amounts close to planting can, particularly in a dry spring, delay or upset emergence. Too much of the nutrient can also depress tuber dry matter.
Both effects are more likely to occur where the cheaper and more widely used chloride form of potash is applied rather than the less risky sulphate form, he says.
In research funded jointly by the Cambridge University Potato Growers Association and BPC, the university team found that the soils ability to make potash reserves available to the crop is highly variable.
"This could be related to soil type, some soils seem to be better able to release potash for the crop than is realised. A better understanding of what happens would allow application rates to be more accurately matched to need."
The team has assessed the value of the "balance sheet" profit-and-loss approach to soil fertility to determine likely residual amounts in the soil. But it concludes that despite shortcomings soil sampling is probably still better.
"Soil sampling methods only provide a general idea of what is there, no account is taken of soil type and we know the clay content can affect potash availability. So there is room for improvement to provide increased precision."
The three-year project, which ended with the harvesting of the 1999 crop, also looked at the yield response to potash fertilisers on a range of soils with K indices ranging from 0 to 6. In most cases the response of the crop to extra K was either small or non-existent, suggesting the soil index figure was misleading, or the rate used was far too high.
"I believe that when calculating fertiliser rates potato growers could be misled by soil indices which do not fully take account of soil type. This could be wasteful and money could be spent on unnecessarily high rates of potash," says Dr Allison.
"On a soil with a K index of 0 the standard recommendation is for 350kg/ha of K2O, but following our work I am now convinced there is scope on many soils to reduce this by at least 50kg without compromising the crop. Bigger reductions may be appropriate on soils with higher K indices.
"A reduction of 50kg/ha provides a cash saving worth only around £10/ha but where a big area is grown this is worth having. It also adds precision to a modern potato production system," Dr Allison points out.
Edward Long
Wider programme
BPCs research director, Mike Storey, says the Cambridge University Farm project is part of a wider programme of work to improve the efficiency of fertiliser and water use on potatoes. "In future the use of any fertiliser must be targeted at actual need to raise production efficiency and we need to know what is available from the soil so application rates can be more precise," Dr Storey comments.
Wider programme
BPCs research director, Mike Storey, says the Cambridge University Farm project is part of a wider programme of work to improve the efficiency of fertiliser and water use on potatoes. "In future the use of any fertiliser must be targeted at actual need to raise production efficiency and we need to know what is available from the soil so application rates can be more precise," Dr Storey comments.
Match potash rate to potato demand, not soil index, urges Marc Allison of Cambridge University Farms. His advice follows three years of BPC-funded work.
Match potash rate to potato demand, not soil index, urges Marc Allison of Cambridge University Farms. His advice follows three years of BPC-funded work.
Match potash rate to potato demand, not soil index, urges Marc Allison of Cambridge University Farms. His advice follows three years of BPC-funded work.