This article is a summary of the article "No-Tillers May Want to Re-examine Soil Test 'K' Application Habits, " written by John Dobberstein on Saeed Khan's research into K use and need. The original article appeared in Residue Management, Nutrient Management, Soil Health.
At Growers we have long held that farmers are overusing fertilizers, especially potash. In Khan's research, we have third-party verification that most farmers do not need to be applying K at current levels, or even at all. This article targets no-tillers, but after reading it I believe many of its conclusions can be applied to operations of all kinds.
Saeed Khan has been sampling different research plots and reviewing international peer-reviewed journals and has completed his PhD thesis paper, titled Potassium Paradox: Implications for Soil Fertility, Crop Production, and Human Health. His research found out many things, including little to no connection between K application levels and increased yield. Khan claims this has been known by agronomists for decades, but conflicts with the establishment narrative and so is not widely publicized. Sound familiar?
Khan, with co-authors Richard Mulvaney and Timothy Ellsworth, found a paper from 1915 stating that all the K needs of a crop in Illinois can be drawn from the natural supply in the soil profile. Why, then, did Khan find no correlation between K application and yield? His conclusion is that there is enough available K in the soil profile and plant residue that we do not need to be applying it. Soil tests only test the top 6-8 inches of the soil profile, and even with that limitation, there is 30,000 lbs/acre of K in a typical Illinois soil, enough to last 650 years. Furthermore, go down to a depth of 6 feet and you increase that amount to 360,000 lbs/acre, a near inexhaustible supply. Cyril Hopkins, who wrote the 1915 paper, was a promoter of the Illinois System of Permanent Fertility, which focused on legume rotations, rock phosphate, and limestone. Interesting that even 100 years ago the link between limestone and farming the farm underneath your current one existed.
Surely, even with "new" K replacing that removed by the crop, our modern intensive farming methods will remove K faster than it can naturally be replaced. Khan contends this is not true either. A big difference between K and its sibling macro-nutrients, N and P, is that K is not primarily released from crop residue by microbe action or decomposition, but rather simply by water. This means that it is returned, in an available form, to the soil after only a few rains have fallen on the field. Khan notes that with a 200-bushel corn crop, 46 lbs/acre of K is removed in the grain, but 260 lbs/acre is returned from the stover. This is from a 1984 study, so not a terribly recent finding.
Underscoring this finding, Khan sampled a check plot in Illinois as part of his PhD work where there was no K applied for more than 15 years. He limited sampling to the plow layer and took samples every two weeks. With a 205 lbs/acre starting point, he observed that K levels fluctuated tremendously over a four year period, but trended upwards to 300 lbs/acre. Not only did this prove that subsoil sinks of K were replenishing lower levels, but that the accepted soil test practices were woefully inadequate. As if to add salt to the wound, he analyzed an unfertilized continuous corn plot in the U.S.'s oldest research fields, the Morrow Plots, where the K level has increased by 144 lbs/acre despite 796 lbs/acre of K removed in grain over a 50 year period.
Khan also addressed the assumption that a critical level of 300 lbs/acre of K (as per soil test results) is vital to good yield in corn. According to a 1993 study in a field in Illinois where no K had been applied for 6 years, soil test K results grouping soils into "low-yield" (183-300 lbs/acre of K) and "high-yield" (303-380 lbs/acre of K) had no predictive value on yield. There was an average of 167 bushels across both low and high-yield areas of the field. This calls into question how much K corn needs as well as the value of soil tests for nutrient use planning. Lastly, the authors reviewed 2100 peer-reviewed trials from the entire world, and only 7% showed a significant increase in yield with K application. These increases were mostly in shallow and sandy soils.
Another drum that Growers has consistently beaten is that excessive K application will harm the soil structure and microbes. Khan cited numerous studies going back to 1937 stating that too much K will collapse clay layers, causing compaction and all the problems that brings. Furthermore, K has been proven to cause calcium (and therefore nitrogen) deficiency in soybeans, corn, alfalfa, cotton, and potatoes.
This article is focused mainly on Illinois soils and no-till operations, but I believe it is relevant beyond those spheres. Though parts of Illinois are known for their fertile soil, work done by Jim Halbeisen has shown significant K levels in many soil types across North America. Further, though no-till is generally known for returning more crop residue to the soil than conventional practices, Khan's insight about subsoil K sources as well as the shear amount of K that can be returned in a single crop leads me to believe that growers who practice tillage are not missing out on sufficient natural K.
This is an excerpt from the Early Fall Growers Solution (2018) written by Zach Smith.
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