Potash fertilizer — is there a problem?
In a publication brought to the public’s attention by news release from the University of Illinois several weeks ago, S.A. Khan, Richard Mulvaney and a colleague in the Department of Natural Resource and Environmental Sciences at the University of Illinois challenged a number of basic tenets of soil fertility, especially practices related to use of potassium (K) fertilizer.
Citing hundreds of references and thousands of reported studies, they asserted that: K fertilizer is generally unnecessary in soils like most of those in Illinois; the soil test is not a reliable way to know how much K the soil will supply to a crop; K used as fertilizer can cause crops to have lower nutritional value; using K fertilizer can damage soil structure; and potassium chloride (KCl), which is the most commonly-used (and lowest-cost) K fertilizer material, is harmful to crops.
I won’t try to assess these assertions in detail, but will make some comments here to keep us focused on the larger issues of supplying nutrients to crops to prevent deficiencies and crop yield loss.
Can soils provide enough K without fertilizer?
Clay minerals in soils contain huge amounts of K, measured in tons per acre. Plant-available K – that found in soil solution or bound to exchange sites on soil particles – is typically measured in (low) hundreds of pounds per acre. The K in minerals is part of their chemical structure, and becomes plant-available very slowly – a few pounds per acre per year in many soils – as these minerals break down with weathering.
Very low crop yields and K removal, or no crop removal (as was the case before Illinois soils were farmed) will result in plant-available K levels either not dropping or slowly increasing. Crop roots can pick up K from deeper soil layers, and over time will bring K up to the soil surface, where it will be released when plants die.
One of the items reported in the publication is that soil test K level in continuous corn in the Morrow Plots increased by about 67 percent over 50 years without any fertilizer application. They based this on a soil sample taken in 1955 and another in 2005. Soil test K levels in samples taken frequently from 1967 through 2008 in the Morrow Plots did not show such trends, however (Figure 1). Levels of K increased in both fertilized and unfertilized plots after deep tillage to relieve compaction in the late 1990s, but yields (and K removal) rose after that, and K levels came back down as removal rates rose.