Why maintain soil potash reserves?

Effects of omitting potash

Saving money on potash applications might be appropriate if the soil happens to contain large reserves of K in the less-readily available pool or a large amount of clay that releases potassium. But what are the risks and consequences of getting it wrong?

Total use of potash fertilisers in the UK in the 2005/06 growing season was estimated at 325,000 t K₂O. Calculations indicate that about 68% of this K was used on arable crops but that the overall application rate of potash on arable land has decreased by about 22% over the last 15 years. In fact, over these last 15 years the actual field rate of potash applied to winter wheat has remained constant at 75 kg K₂O/ha, with the overall reduction resulting from the increased number of crops not receiving this annual dressing. It is important to remember that for cereals straw contains more K than does grain, so that if straw is removed from the field large amounts of K will also be removed. However, as mentioned, the overall use of K on cereals has shown a marked decline because of a decrease in the percentage of the crop area that receives K in any one year: 55% of the winter wheat area received K in 2006 compared with 69% in 1998. This decline could be the result of a response to recent grain prices and the need on some farms to improve gross margins. However, if this trend continues and is repeated for other arable crops, levels of readily plant-available K in the soil will decline.

Rate of soil K run-down

The rate of decline in the readily plant-available soil K (determined in the laboratory as exchangeable K) will depend on the initial value, the amount of K removed in the harvested crop, and the size and rate of transfer of K reserves from the less-readily available pool. Experiments suggest that a good 'rule of thumb' is that the exchangeable K will halve in 10 years when no potash fertiliser is applied. To put this into context, the K Index scale is:

* May be shown as mg/l, mg/kg or ppm all of which are roughly the same for most mineral soils for advisory purposes

Thus it could take 10 years to go from the top of K Index 1 to the top of K Index 0.

Effects of declining levels on yields

Farmers are unlikely to see a decline in yields in the first few years of not applying potash to arable land at or above the critical index of 2-. This is because of:

1. replenishment from reserves;
2. seasonal variation in yields, caused by differences in the weather and the incidence of pests and diseases that change the amount of K taken off in the harvested produce;
3. effects of soil cultivation and uptake of K from the subsoil by deep-rooted crops like winter wheat and sugar beet.

Uptake of K from the subsoil is of particular interest. There is some evidence that crops traditionally considered very responsive to potash are not responding even on soils with a low K Index. For example, no response by sugar beet to K fertiliser has been reported on some soils at low K Indices. This might be caused by past enrichment of subsoils below light textured topsoils to which large amounts of K have been applied. Deep-rooted sugar 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 well delay the eventual decline in yields as a result of omitting potash, but eventually the 'crunch point' will come and however well roots develop and grow in K-impoverished soil there will be too little K available for the crop to achieve its optimum economic yield.

In the longer-term declining soil K will inevitably result in declining yields

This has been demonstrated many times in long-term field experiments at Rothamsted and elsewhere.

Trials show that the decline in yield results in a significant loss of profit, even for cereals at low market prices. The optimum index of 2- (120-180 mg K/kg soil) is clearly identified by the shoulder of the blue line.

Biggest effects are with those crops that need most potash or have poor root systems which are unable to exploit soil reserves effectively.