Phosphate (P) and potash (K) do not follow the same principle as nitrogen (N) when determining the appropriate application rates.
According to the Potash Development Association (PDA), the correct rates are driven by the crop demand, irrespective of price.
The offtake by a crop, which is affected by yield, is also important in this regard.
If N rates are reduced significantly due to cost, then this may lead to a lower yield, which in turn would reduce the offtake of P and K, but this reduction is not driven by the price of these inputs directly.
Importance of potash
All plants, except legumes, require a larger supply of K than any other nutrient – even N.
If the total requirement is not available, or if the rate of supply at periods of peak growth is limiting, plant performance will be impaired leading to lower yields and poorer crop quality.
This becomes even more important in a situation where there is greater levels of risk and cash tied up in growing a crop.
With the variable costs of growing a winter wheat crop increasing from around €750/ha to potentially over €1,250/ha for 2023, it is more important than ever to ensure that costs are managed as efficiently as possible.
Focusing entirely on N inputs will not deliver the anticipated returns if other major nutrient decisions, such as phosphate and potash, have been overlooked where these are required, according to the PDA.
These adverse effects will be worse if nitrogen and potash are out of balance and in difficult growing seasons where there is more stress – especially drought.
Saving money on potash applications might be appropriate if the soil contains large reserves of K in the less-readily available pool or a large amount of clay that releases K. But what are the risks and consequences of getting it wrong?
The effects of omitting K applications to tillage ground at, or above, the critical index of 2- are unlikely to be noticeable in the first few years because of: Replenishment from reserves; seasonal variation in yields; effects of soil cultivation; and uptake of K from the subsoil.
There is some evidence that crops traditionally considered very responsive to potash do not respond even on soils with a low K Index.
E.g., no response by beet crops to K fertiliser has been reported on some soils at low K indices. This might be caused by past enrichment of subsoil below light textured topsoil to which large amounts of K have been applied.
Deep rooted beet can probably access this subsoil K. In addition to exploiting potash reserves in the subsoil, greater attention to soil cultivation and improved soil structure will allow plant roots to explore a larger volume of soil for nutrient acquisition.
Better cultivations and soil structure may delay the eventual decline in yields as a result of omitting potash.
But eventually the ‘crunch point’ will come and there will be too little K available for the crop to achieve its optimum economic yield.
Loss of yield is not the only result of K deficiency in soil. Lack of K also results in: Inefficient use of other nutrients; enhanced susceptibility to crop diseases; weaker straw; and reduced grain quality, according to the PDA.