‘Mind your P and K,’ says Yara Head of Agronomy, Mark Tucker

Every farmer knows that soil fertility is the bedrock for profitable farming – and that nitrogen, phosphate, potash and sulphur are the essential nutrients.

How many realise, however, that phosphate and potash levels are not only difficult to manipulate, but on the decline?  Mark Tucker, Head of Agronomy for Yara UK, believes it’s a fact that every grower in the country should be aware of.  “It can take years of careful fertilizer application to build phosphate and potash levels to the desired fertility.  Many farmers now have a real problem they are quite unaware of;

67% of soils are at or below the maintenance level for potash and 82% of soils for phosphate. Such a deficiency can lead to a huge drop off in yield and quality.”


Monitored by The British Survey of Fertilizer Practise, falls in phosphate use over the last six years meant that 2006’s application rate of 35 kg/ha was the lowest on record since 1969, with only 57% of tillage crops receiving phosphate.
Similarly, the average rate of potash use is now just 49 kg/ha.

Taking winter wheat as an example, the average phosphate and potash application rate is 34 and 41 kg/ha respectively – and crop removal data really puts this figure into perspective. A typical crop, where straw is removed, takes off 73 kg/ha and 100 kg/ha of phosphate and potash respectively.

In such circumstances, where soil reserves are clearly being mined, it is essential to use soil analysis to ensure that critical levels are not breached.

Evidence suggests that many farms are putting their crops at risk of deficiency and are thus less able to benefit from new variety potential and increasing crop values.

The yield decrease caused by phosphate deficiency is very large with a soil index of less than one causing a dramatic reduction. (see example graph) However, it’s a problem of which many farmers remain unaware.