Potash fertilizer — is there a problem?
The key to whether soils can supply enough K to meet crop needs is whether the crop removes K faster than the soil can free up K through the weathering process. At the low yield levels common 75 to 100 years ago, when return of nutrients to the soil as manure was common, K levels probably dropped slowly if at all even without fertilizer K. Today, a corn-soybean rotation with good yields will remove as much as 100 lb of K over a 2-year period. Most soils in Illinois can supply nowhere near these amounts, and so K levels will drop if no K fertilizer is added.
How long it will take for a deficiency to appear will depend on how much plant-available K is present. But let’s not fool ourselves – K deficiency will appear at some point if removal continuously exceeds replacement. The only reasonable way to replace removed K is with K fertilizer.
There are soils in the world, including in parts of the western Corn Belt, where plant-available K levels are naturally high, due to the mineralogy of the parent materials from which the soils developed and the age of the soils. There are also places where soils have been weathered and leached for so long that K levels are very low. The soils in Illinois, and in most of the eastern Corn Belt US Corn Belt, fall into neither of these categories. Here, soils free up K more slowly than crops remove it, so if we are going to produce crops that remove K, at some point we are going to have to add some K back.
Is the K soil test useful?
There is no question that measuring plant-available K in soils is difficult, and that soil-test K levels vary over time, often not very predictably. Part of this is a sampling issue – soil test K levels often change quickly over short distances, so samples show variability. Soil moisture affects K tieup in clay minerals, and even the way soils are dried before testing can affect the soil test level.
Despite all this, low soil test K values are often predictive of crop deficiencies, and crop K deficiencies are rare (though possible, especially when soils around the roots are dry) when soil test levels are high. Yield increases from adding K fertilizer are much more common when soil test K levels are low than when they are high.
So while it’s easy to find fault with the soil test for K, the test (if samples truly represent the soils in a field) does tell us whether soil K levels are high enough to support full yields or whether deficiency and yield reductions are likely without adding fertilizer K. That’s usually all we need to know, especially when fertilizer K is routinely added back to replace that removed by crops.
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