The three primary nutrients fertilizer companies want to sell farmers in large quantities are N-P-K.

While many crops require as much or more more calcium than nitrogen and potassium, and as much or more sulfur than phosphorus, these nutrients are less profitable to sell, and don’t produce the marketing incentives NPK fertilizers do.

We know that we can grow all of the nitrogen our crops require. Biology can sequester and ‘fix’ some of the 35,000 pounds of nitrogen above each acre, more than enough to supply 100% of a crop’s requirements.

We know that mycorrhizal fungi and phosphorus solubilizing bacteria can solubilize abundant phosphorus and supply all of a crop’s phosphorus requirements, as long as the parent soil material contains phosphorus that can be released. (Which most soils do)

Of course, for the biology to supply either adequate N or P, there needs to be an abundant microbial population that is well supported and has a strong food source from living roots systems.

The realization of these possibilities has led many farmers to greatly reduce or completely eliminate N and P applications. This leaves potassium as the remaining nutrient common in mainstream fertilizers that growers are wondering if it is safe to no longer apply.

The answer is dependent on context of course, as it is for N and P, but for most soils, and for most crops, there are no yield benefits from applications of potassium chloride fertilization. There are significant negative effects on crop quality and soil health produced by potassium chloride fertilization.  Arden Andersen wrote about the damaging impacts of potassium chloride in Science in Agriculture.

Richard Mulvaney and colleagues have published two must-read papers on the lack of value from potassium fertilization. They indicate there is no correlation between potassium levels that show up on common soil analysis and the quantity of potassium delivered to a crop, that soil potassium levels continue to increase even with crop removal and no applications, and that there is no yield response from potassium chloride applications.

Here are a few excerpts, and you definitely want to read the full papers linked below.1,2

These evaluations leave no alternative but to question the practical utility of soil K testing because test values cannot account for the highly dynamic interchange between exchangeable and non-exchangeable K, exhibit serious temporal instability with or without air drying and do not differentiate soil K buildup from depletion. The need for routine K fertilization should also be questioned, considering the magnitude and inorganic occurrence of profile reserves, the recycling of K in crop residues and the preferential nature of K uptake. An extensive survey of more than 2100 yield response trials confirmed that KCl fertilization is unlikely to increase crop yield. Contrary to the inculcated perception of KCl as a qualitative commodity, more than 1400 field trials predominately documented a detrimental effect of this fertilizer on the quality of major food, feed and fiber crops, with serious implications for soil productivity and human health. 

By 2005, following 130 years of K removal, test values for these subplots were within the range of critical levels calibrated for North America43, which would normally be inter- preted as evidence of successful fertilizer K management. 

The most disturbing disparities involved 17 trials in which test levels were either constant or increased while crop K removal far exceeded fertilizer inputs or occurred in the complete absence of fertilization. Such obvious incon- gruities, paralleling what was found for the Morrow Plots (Table 1), leave little alternative but to question the validity of soil testing for exchangeable K.

Besides being abundant in soils and plant residues, K is notable as the only macronutrient dominated by inorganic forms in both the soil and plant, and thus availability is not dependent upon microbial transformations. 

As hypothesized, KCl fertilization was often ineffective for increasing productivity, according to non- significant responses that occurred in approximately 76% of the total trials surveyed.

Most of the responses where positive and occurred on coarse-textured, organic or highly weathered soils inherently low in K-supplying power (231 site-years); when the above-ground residues were removed (191 site-years); with crops having a shallow or low-density rooting system (62 site-years); and/or when subsoil rooting was restricted (12 site-years). In the absence of such factors, there is very little reason to expect a significant yield response to KCl fertilization. 

Yield reductions due to KCl fertilization, as documented in Table 4, can be explained by the high salt index of this fertilizer, which has been implicated as a detrimental factor for crop germination and growth and microbial processes.

Many leguminous crops are sensitive to Cl− toxicity, including soybean and alfalfa, and Cl− can reduce soil N availability by inhibiting nitrification in soils and by acting as a competitive anion that suppresses plant uptake of NO3. A further difficulty arises from the mobility of Cl− in soils, which intensifies leaching of Ca2+ as a counterion.

Producers have long been led to believe that KCl fertilization serves an essential role, not only for sustaining crop yield but more importantly, for ensuring a high-quality product that will maximize economic return. To ascertain the credibility of the latter claim, a thorough survey was undertaken of peer-reviewed and university publications that provide the most reliable source of information regarding the agronomic effects of KCl. The findings, summarized in Table 5 for more than 1000 field experiments, altogether contradict the prevailing belief in the value of this fertilizer for improving crop quality, since the frequency of positive responses was only about 8%. On the contrary, the qualitative effect of KCl was negative in 57% of the trials surveyed. In some of these trials, crop quality was reduced despite a significant yield increase.

A cumulative effect on soil physical and chemical properties would be expected since K is prone to interlayer fixation that collapses 2:1 clay minerals and converts an active, swelling smectite to an inactive, non-swelling illite. The stabilizing value of KCl has long been recognized in the construction of impervious pavement and foundations.

Unfortunately, the agronomic consequences include a loss of CEC and lower water-holding capacity, which is not conducive to crop growth and productivity. 

Since the onset of industrialized agriculture more than half a century ago, the view has been inculcated that intensive inputs of fertilizer K are indispensable for maximizing crop yield and quality and for the long-term maintenance of soil productivity. This view cannot be reconciled with the considerable volume of scientific evidence presented herein, encompassing soil testing for plant-available K and the consequences of KCl fertilization for agricultural productivity, food safety, and soil degradation.

If fertilizer K usage is to be profitable in a production setting, current recommendations that rely on soil testing for exchangeable K will no longer suffice. As a more viable alternative, producers should periodically carry out their own strip trials, for comparing yield with and without upward and downward K rate adjustment. Initially, a 3-year period would be appropriate for repeating these trials, but a longer interval could safely be employed with cash-grain cropping that limits K removal. To avoid the adverse consequences of Cl−, K2SO4 would be preferred as a fertilizer source. 

For many years, crop potassium (K) availability has been estimated by soil testing the plow layer for exchangeable K, in conjunction with potassium chloride fertilization widely promoted as an essential prerequisite for ensuring crop yield and quality. As rigorously documented in our paper, both components of chemical-based K management are seriously flawed by the lack of a scientific basis. Under the pretext of providing economic benefit for the producer and a healthy food supply for the public at large, the real purpose is to generate revenue for the fertilizer industry. 

If crop K uptake originates from huge K reserves throughout the soil profile, the question naturally arises as to whether producers can expect a profitable return from annual or biennial applications of KCl. The answer is a resounding NO.

We stand by our two principal contentions that: (1) soil testing for Exch-K is of no use for predicting crop K availability or assessing soil K buildup/depletion; and (2) KCl fertilization is often superfluous for increasing crop yield and quality and can have a detrimental effect on soil productivity and human health. Both these points are strongly supported by the extensive literature citations in our paper, covering peer-reviewed publications from field and laboratory research

The prevailing approach to K management is invariably advocated under the pretext of providing economic benefit for the producer and a healthy food supply for the public at large, but the real purpose is to generate revenue for the fertilizer industry. This is the essence of a paradox, not a dilemma.

Back to John:

In some contexts, potassium applications are indeed needed. When this is the case, we should use potassium sulfate, K-Mag, manure, compost, or sources of K other than potassium chloride. Potassium chloride is the most expensive form of potash you can buy, because it doesn’t work nearly as effectively as others do.

1. Ellsworth, T., Mulvaney, R. L. & Khan, S. A. The potassium paradox: Implications for soil fertility, crop production and human health. Renew. Agric. Food Syst. 29, 3–27 (2014).

2. Khan, S. A., Mulvaney, R. L. & Ellsworth, T. R. Further insights into why potassium fertility is a paradox. Renew. Agric. Food Syst. 30, 120–123 (2015).