Evaluation of soil potassium testing

Soil testing is widely regarded as the best approach for making economical fertilizer recommendations or monitoring soil fertility changes in relation to management practices. If these objectives are to be properly met, soil sampling must be adequate to represent the field area under investigation, and test values should be calibrated to crop response and sufficiently stable that interpretations are unaffected by the time of sampling or the method of sample processing.

In the modern era of input-intensive agriculture, soil testing is often done to measure exchangeable K, following the concept originated by BrayReference Bray ¹³ . This prevalence reflects the major emphasis given to rapid methods of sample processing and analysis in commercial soil testing, despite an inherent complication that has long been recognized: K test values can change markedly when samples are air dried to expedite processingReference Attoe ^{37 ,} Reference Hanway, Barber, Bray, Caldwell, Fried, Kurtz, Lawton, Pesek, Pretty, Reed and Smith ³⁸ (see also supplemental references [4–8] for the online version of the paper). A further complication arises because of variability associated with the time of samplingReference Luebs, Stanford and Scott ^{18 ,} Reference Skogley, Havlin and Jacobsen ²¹ (see also supplemental references [4] and [9] for the online version of the paper), which becomes a major source of error in soil testing for site-specific managementReference Hoskinson, Hess, Alessi and Stafford ³⁹ .

Both factors clearly contributed to the variability documented in Fig. 1 for a 4-year field study that involved biweekly sampling to monitor changes in exchangeable K test values with and without air drying. Any such changes would have been minimized because the work involved a single investigator utilizing a fixed protocol and experimental design, and a study site devoid of fertilizer K inputs for at least 14 years. By repeatedly imposing a dense sampling design on a single small research plot subject to minimal compaction from mechanical traffic, spatial variability was minimized so as to better quantify any occurrence of temporal variability.

Figure 1. Biweekly changes in exchangeable K for field-moist (FM) and air-dried (AD) samples (0–18 cm) collected on 96 dates between March 13, 1986 and March 15, 1990, from a Drummer soil under a corn (Zea mays L.)–soybean (Glycine max L. Merr.) rotation. No fertilizer P or K had been applied since 1970. Data points are a mean of nine determinations representing triplicate analyses performed on five-core composites collected from each of three subplots. Trendlines were generated by linear regression. SD, standard deviation.

Despite these precautions, Fig. 1 shows that soil K test levels, with or without air drying, varied drastically with the biweekly sampling strategy adopted throughout the study period, and the variation became more extensive as test values began to increase after harvest, during the usual sampling period for commercial soil testing in a temperate region such as the Midwestern USA. Statistical analyses were highly significant (P<0.001) in detecting a seasonality effect on exchangeable soil K, which is consistent with several previous field studiesReference Blakemore ^{40 ,} Reference Childs and Jencks ⁴¹ (see also supplemental references [10–13] for the online version of the paper) that document greater test values during the winter months. This increase can be attributed to a high soil moisture content that promotes the leaching of K from crop residues and the conversion of non-exchangeable to exchangeable K through valence dilution. After seasonality adjustment, a highly significant (P<0.001) linear increase in exchangeable K was observed over the 4-year study period.

Drying was expected to decrease temporal variability by homogenizing soil samples; however, the data in Fig. 1 reveal no such effect as the coefficient of variation was nearly the same before and after air drying that escalated K test values by 55% on average. A major complication can also be found in the numerous incoherencies between field-moist and air-dried test values documented by Fig. 1, which precludes the use of a simple correction factor to compensate for air drying or the timing of sample collection. Both findings raise a serious concern that soil K testing by the usual approach is of no practical value, in contrast to the advocacy that is common in extension publications and presentations, trade magazines and popular articlesReference Fernández, Hoeft and Nafziger ^{28 , 42} .

The same concern emerged when sample drying was evaluated by a one-time sampling of soils that ranged widely in K fertility, so as to eliminate the confounding effect of seasonality. The results (Fig. 2) follow the usual trend toward higher test values upon air drying; however, the increase tended to be greater as the fertility level decreased, further demonstrating the difficulties inherent in correcting air-dried test values to a field-moist basis. This finding reveals that air-dried K test values can easily be misinterpreted when utilized for predicting K fertilizer requirement under low fertility management or for monitoring buildup/depletion.

Figure 2. Potassium release curves for a Drummer soil cropped to a corn (Zea mays L.)–soybean (Glycine max L. Merr.) rotation, with or without annual application of 46–139 kg K ha⁻¹ as KCl. Field-moist samples (0–18 cm) were air dried to approximately 40 g kg⁻¹ in a forced-air oven at 40 °C, followed by oven-drying at 105 °C. Data points are a mean of triplicate determinations performed on five-core composites collected from each of three subplots. Error bars representing one standard deviation are shown, when they exceed the size of the data marker, for field-moist (FM) and air-dried (AD) test values. The percentage increase upon air drying was calculated as 100×(K_AD−K_FM)/K_FM. **, significant at P<0.01.

A more rigorous evaluation becomes possible when soil K test data span a prolonged period documented with a complete record of K inputs and outputs. The Morrow Plots serve as an ideal resource for this purpose, providing three cropping systems that include the corn–soybean rotation common throughout the Midwestern USA. With this rotation, and also with continuous corn, K removal from the Morrow Plots occurs only in grain, whereas the intensifying effect of forage production is present with a 3-year rotation of corn, oats and alfalfa hay.

As shown by Table 1, K test levels for the surface soil were invariably increased by 51 years of cropping. Remarkably, the largest increases, highly significant at P<0.001, occurred in the absence of NPK fertilization, when the K balance was decidedly negative for continuous corn or the corn–oats–hay rotation. Equally remarkable is the substantial soil test increase observed for the unfertilized subplot in the two-crop rotation, despite K removal that totaled 1.4 Mg ha⁻¹. These observations are consistent with the seasonality study discussed previously (Fig. 1), which showed an upward trend over time in air-dried and field-moist K test levels in the absence of fertilizer K inputs. No less troubling are the similar net changes documented by this soil test for the three subplots receiving NPK, regardless of whether fertilizer K inputs were above (continuous corn or corn-oats-hay) or below (corn-soybean) crop removal. The implication is that the K test cannot differentiate soil K buildup from depletion.

Table 1. Soil potassium test changes and potassium balance for the Morrow Plots, 1955–2005.

*** Significant at α=0.001 by the step-down Bonferroni procedure.

¹ C, corn (Zea mays L.); H, alfalfa (Medicago sativa L.) hay; O, oats (Avena sativa L.); S, soybean (Glycine max L. Merr.). Since 1967, the two-crop rotation has involved soybean instead of oats.

² NPK, nitrogen–phosphorus–potassium fertilization using urea (168 [1955–1966] or 224 [since 1967] kg N ha⁻¹ yr⁻¹ for corn, 28 kg N ha⁻¹ yr⁻¹ for oats), triple superphosphate (0–96 kg P ha⁻¹ yr⁻¹) and KCl (0–186 kg K ha⁻¹ yr⁻¹). No amendment was applied before 1955.

³ Determined by NH₄C₂H₃O₂ extractionReference Knudsen, Peterson, Pratt, Page, Miller and Keeney ²⁴ of air-dried samples.

⁴ Mean values reported from triplicate determinations of a single archived five-core composite soil sample (0–15 cm). Standard deviations shown in parentheses.

⁵ Mean values reported from triplicate determinations performed on each of three replicate soil samples (0–15 cm). Standard deviations shown in parentheses.

⁶ Estimated as 44% by corn, 7% by oats and 49% by soybean.

⁷ Estimated as 49% by corn, 3% by oats and 48% by soybean.

⁸ Estimated as 54% by corn, 25% by oats and 21% by alfalfa hay.

⁹ Estimated as 26% by corn, 65% by oats and 9% by alfalfa hay.

The latter flaw is also apparent from the sheer magnitude of the K test levels reported in Table 1 for the three unfertilized subplots, particularly those under continuous corn and the corn–oats–hay rotation. By 2005, following 130 years of K removal, test values for these subplots were within the range of critical levels calibrated for North AmericaReference Fixen ⁴³ , which would normally be interpreted as evidence of successful fertilizer K management.

To ascertain whether soil K testing is generally subject to the aforementioned difficulties, baseline changes in exchangeable K were compared with cumulative K balance compiled for numerous field experiments with and without K fertilization, encompassing a global range of soil orders, cropping systems and management practices. The resulting database (Table 2), consisting of 68 trials, provides no convincing evidence that soil test K reflects the net balance of K addition and removal. 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 incongruities, paralleling what was found for the Morrow Plots (Table 1), leave little alternative but to question the validity of soil testing for exchangeable K, and this concern is especially relevant to the developing world, where soil K removal has been intensified by many centuries of grain and biomass harvest. To make matters worse, Table 2 shows that K test values sometimes decreased with a positive K balance, which according to the usual interpretation would justify unnecessary K fertilization. The implication is that soil K testing does not provide a scientific basis for fertilizer K management.

Table 2. Soil potassium test changes in relation to potassium balance, as reported for static plot experiments.

¹ Surface texture designated parenthetically as c (clay), cl (clay loam), fsl (fine sandy loam), l (loam), ls (loamy sand), s (sand), sc (sandy clay), scl (sandy clay loam), sic (silty clay), sicl (silty clay loam), sil (silt loam) and sl (sandy loam). Subscripts indicate the number of soils studied with a given texture.

² A, alfalfa (Medicago sativa L.); B, barley (Hordeum vulgare L.); C, corn (Zea mays L.); Cb, coastal bermudagrass (Cynodon dactylon L. Pers.); Ce, cereal; Cp, cowpea (Vigna unguiculata L.); C_s, silage corn (Zea mays L.); Ct, cotton (Gossypium hirsutum L.); Gcl, grass-clover; L, ley; P, pea (Pisum L.); Pm, pearl millet (Pennisetum glaucum L. R. Br.); Po, potato (Solanum tuberosum L.); Pr, perennial ryegrass (Lolium perenne L.); Rc, root crop; Rcl, red clover (Trifolium pratense L.); Ri, rice (Oryza sativa L.); Ry, rye (Secale cereale L.); S, soybean (Glycine max L. Merr.); Sb, sugarbeet (Beta vulgaris L.); Sp, sweet potato (Ipomoea batatas); W_s, spring wheat (Triticum aestivum L.); W_w, winter wheat (Triticum aestivum L.). Values in parentheses indicate the number of separate trials summarized.

³ Parentheses indicate digital uncertainty in estimating data reported in figures.

⁴ All crops were grown annually.

⁵ Prior to 1901, the cropping system was W_w–W_w (1856–1875) or Po–Po (1876–1901).

⁶ Assumed as the depth for sampling surface soil.

⁷ Rice grain was assumed to contain 0.04 g K kg⁻¹ at a moisture content of 140 g kg⁻¹ Reference Tisdale, Nelson and Beaton ⁷⁹ .

⁸ Cropped to cereals between 1982 and 1987, with 4 years of barley, 1 year of oats (Avena sativa L.) and 1 year of winter wheat.

⁹ Includes atmospheric K deposition.

This dilemma is inherent to a one-time measurement of exchangeable K, which can never represent the highly dynamic effect of soil moisture on the interchange with vastly larger soil pools that occur in the form of non-exchangeable and mineral KReference Cassman, Olk, Brouder and Roberts ⁸⁰ . The dynamic nature of soil K is no less relevant in refuting the central assumption implicit to soil K testing, namely, that plant K availability is directly related to exchangeable K in the surface soil. Table 2 leaves no doubt that plant K uptake must originate from other sources, and is consistent with previous evidence that soil K reserves contribute considerable quantities of plant-available KReference Reitemeier ^{10 ,} Reference Srinivasa Rao, Rupa, Rao and Bansal ^{11 ,} Reference Mengel and Uhlenbecker ^{20 ,} Reference Lee and Metson ^{75 ,} Reference Öborn, Edwards and Hillier ^{78 ,} Reference Abel and Magistad ^{81 –} Reference Mengel, Rahmatullah and Dou ⁸³ (see also supplemental references [14–26] for the online version of the paper).

These reserves were once recognized to be a major source of K-supplying power for the highly productive soils that dominate the Corn Belt, as well as for most other areas of the continental USAReference Hopkins and Aumer ^{8 ,} Reference Blair ^{9 ,} Reference Rouse and Bertramson ⁸⁴ . The importance of the non-exchangeable fraction as a storehouse for exchangeable and soluble K is documented by Table 3, which shows the magnitude of this fraction by six successive determinations performed on surface samples of a soil cropped for 17–20 years with no K input. Initial recoveries were three- to fivefold higher for non-exchangeable than exchangeable K, followed by a gradual decline toward a stable level of 240–300 kg ha⁻¹, with approximately 3000 kg ha⁻¹ as the cumulative recovery of non-exchangeable K. The resilient behavior of soil K is further revealed, in that 4 years of crop K removal had no consistent effect on soil concentrations of exchangeable, non-exchangeable or total K, implicating the mineral fraction as an important source of buffering. These findings are to be expected, considering what has long been known about the availability and dynamics of non-exchangeable and mineral K, based on chemical extractionReference Martin and Sparks ^{19 ,} Reference Wood and DeTurk ^{85 –} Reference Ganeshamurthy ⁸⁷ (see also supplemental references [22] and [27–30] for the online version of the paper), exhaustive croppingReference Abel and Magistad ^{81 ,} Reference Oliveira, Ludwick and Beatty ^{86 ,} Reference Badraoui, Bloom and Delmaki ^{88 ,} Reference Zubillaga and Conti ⁸⁹ (see supplemental references [8], [15] and [16] for the online version of the paper) and electrodialysisReference Mengel, Rahmatullah and Dou ^{83 ,} Reference Mortland, Lawton and Uehara ⁹⁰ .

Table 3. Speciation of potassium in surface (0–18 cm) samples collected in August and September between 1986 and 1989 from a Drummer soil under a corn–soybean rotation with no P or K fertilization since 1970. ¹

¹ 160–180 kg N ha⁻¹ applied for corn production in 1986 and 1988. All values reported as a mean of triplicate determinations performed on a sample composited from 45 to 75 cores collected in August and September of each year. Standard deviations shown in parentheses.

² Determined by extraction with 1 M NH₄C₂H₃O₂ (pH 7)Reference Knudsen, Peterson, Pratt, Page, Miller and Keeney ²⁴ after air drying at 40 °C.

³ Determined after extraction of exchangeable K, by boiling with 1 M HNO₃ Reference Knudsen, Peterson, Pratt, Page, Miller and Keeney ²⁴ .

⁴ Determined by fusion with LiBO₂·8H₂O, followed by dissolution in 1 M HNO₃ Reference Suhr and Ingamells ²⁵ . ND, not determined.

The importance of non-exchangeable and mineral K for plant uptake is by no means unique to micaceous clays, which are subject to biological weathering in the rhizosphereReference Hinsinger, Dufey, Jaillard, McMichael and Persson ^{91 –} Reference Arocena, Velde and Robertson ⁹⁴ (see also supplemental references [28] and [31] for the online version of the paper). Sand- and silt-sized muscovite and biotite can also be a major source of plant-available K, as demonstrated conclusively by Mengel et al.Reference Mengel, Rahmatullah and Dou ⁸³ in studies with 14 loess-derived Alfisols. These primary minerals, and also K feldspars, are believed to account for the high K-supplying power of sandy Ultisols of the Atlantic Coastal PlainReference Parker, Hendricks and Sparks ^{82 ,} Reference Yuan, Zelazny and Ratanaprasatporn ^{95 –} Reference Woodruff and Parks ⁹⁷ , while non-exchangeable K reserves can even be important for growing a shallow-rooted crop such as pineapple on the basaltic Andisols of HawaiiReference Abel and Magistad ⁸¹ .

Soil K reserves are much greater for the profile than the plow layerReference Bradfield ^{12 ,} Reference Lutz ^{62 ,} Reference Yuan, Zelazny and Ratanaprasatporn ^{95 ,} Reference Woodruff and Parks ^{97 ,} Reference Bray ⁹⁸ (see also supplemental references [32–34] for the online version of the paper), and can be more fully exploited by many agricultural crops with a well-developed rooting system. The importance of subsoil K is clearly apparent from K/Rb isotope dilution studies demonstrating that the extent of uptake is directly proportional to rooting depthReference Kuhlmann, Claassen and Wehrmann ^{99 –} Reference Witter and Johansson ¹⁰¹ . This uptake is largely concentrated in the vegetative biomassReference Mengel, Barker and Pilbeam ^{33 ,} Reference Jouany, Colomb and Bosc ¹⁰² , and enriches the surface soil when inorganic K leaches from plant shoots or residuesReference Hopkins and Aumer ^{8 ,} Reference Blair ^{9 ,} Reference Abel and Magistad ^{81 ,} Reference Jouany, Colomb and Bosc ^{102 –} Reference Rosolem, Calonego and Foloni ¹⁰⁶ (see supplemental references [35–45] for the online version of the paper).

Fertilizer value of potassium chloride

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. Uptake occurs much more readily for K⁺ than for Ca²⁺ or Mg²⁺ because of greater membrane permeability further enhanced at low concentrations by active diffusion, while specialized transport systems selective for K rapidly distribute this nutrient in the plantReference Mengel, Barker and Pilbeam ^{33 ,} Reference Glass ¹⁰⁷ . Under these circumstances, and considering the ubiquitous nature of K, it becomes clear why many years can be required to induce fertilizer K response in long-term static plot experimentsReference Davis, Patton, Teal, Tang, Humphreys, Mosali, Girma, Lawles, Moges, Malapati, Si, Zhang, Deng, Johnson, Mullen and Raun ^{59 ,} Reference Girma, Holtz, Arnall, Tubaña and Raun ^{60 ,} Reference Amberger and Gutser ^{71 ,} Reference De Datta, Gomez and Descalsota ^{77 ,} Reference Saha, Chaudhury, Saha, Ray, Swarup, Damodar Reddy and Prasad ¹⁰⁸ (see also supplemental references [46–52] for the online version of the paper). On the basis of such findings, there is little reason to expect economically viable crop response to shorter periods of K fertilization that limit check-plot depletion, and a far lower likelihood of response in a production setting where the entire field shares the same history of regular fertilizer K inputs.

This view was indeed confirmed by compiling a global database of crop response to KCl from 2121 short-term field trials conducted by public universities or experiment stations. The resulting database is too extensive to be accommodated herein but is available in the online supplement as Table S4 (for the online version of the paper). 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. The inherent capability for plant uptake of soil K is manifestly evident from studies in which shallow-rooted crops growing on sandy soils were non-responsive to K fertilizationReference Walker, Flowers, Henning, Keisling and Mullinix ^{109 –} Reference Davenport and Bentley ¹¹² , and often made fertilizer K superfluous when deeper rooted crops had access to profile K supplies in more productive soils. Unfortunately, KCl usage in the latter case has long been exacerbated by a buildup-maintenance philosophy that promotes intensive fertilizer K inputs without regard to huge soil K reserves or their recycling through crop residues.

To ascertain the value of K fertilization, those trials showing a statistically significant response to KCl fertilization, accounting for 24% of the total database, are presented herein in Table 4. Most of the responses were 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.

Table 4. Significant yield responses to KCl fertilization in field studies documented by a survey of 211 peer-reviewed and university publications involving ≤10 years of KCl application at a fixed rate. ¹

*^, **^, ***Significant at α=0.05, 0.01 and 0.001, respectively; ns, not significant.

¹ A complete listing of all studies surveyed can be found in Table S4, for the online version of the paper.

² Surface texture designated parenthetically as c (clay), cl (clay loam), fs (fine sand), fsl (fine sandy loam), gl (gravelly loam), l (loam), lfs (loamy fine sand), ls (loamy sand), s (sand), scl (sandy clay loam), si (silt), sicl (silty clay loam), sil (silt loam) and sl (sandy loam). Subscripted values indicate the number of soils studied that represent a specific order.

³ Air-dried data reported. Values in parentheses indicate sampling depth in cm. NR, not reported.

⁴ Values take into account the number of genotypes studied and/or the use of multiple tillage systems.

⁵ A, alfalfa (Medicago sativa L.); B, barley (Hordeum vulgare L.); Bg, bermudagrass (Cynodon dactylon L. Pers.); Bt, birdsfoot trefoil (Lotus corniculatus L.); C, corn (Zea mays L.); Cn, canola (Brassica napus and campestris); Cp, cowpea (Vigna unguiculata L.); Cs, cassava (Manihot esculenta); Ct, cotton (Gossypium hirsutum L.); F, fallow; Gn, groundnut (Arachis hypogoea L.); O, oat (Avena sativa L.); Og, orchardgrass (Dactylis glomerata L.); P, pea (Pisum sativum L.); Pn, peanut (Arachis hypogaea L.); Po, potato (Solanum tuberosum L.); R, rape (Brassica napus L.); Ra, radish (Raphanus sativus L.); Rc, reed canarygrass (Phalaris arundinacea L.); Rcl, red clover (Trifolium pratense L.); Ri, rice (Oryza sativa L.); S, soybean (Glycine max L. Merr.); Sa, safflower (Carthamus tinctorius L.); Sb, sugarbeet (Beta vulgaris L.); Sbg, smooth bromegrass (Bromus inermis Leyss.); Sc, sugarcane (Saccharum officinarum L.); Sg, small grain; Tf, tall fescue (Festuca arundinacea Schreb.); Vg, vegetables; Wc, white clover (Trifolium repens L).; W_w, winter wheat (Triticum aestivum L.). Underscoring indicates the crop(s) studied for K response.

⁶ Data for legumes were obtained from fertilization with P but not K, whereas data for non-legumes were obtained from fertilization with N or N and P but not K. Parentheses indicate digital uncertainty in estimating data reported in figures. When otherwise reported, grain yields were adjusted to market-standard moisture content.

⁷ Estimated as 100×(fertilized yield – unfertilized yield)/unfertilized yield. Values in parentheses indicate the rate of K application in kg ha⁻¹ yr⁻¹. Application of N or N and P was constant throughout the range of K rates cited.

⁸ Values expressed as mg kg⁻¹.

⁹ Above-ground residues removed.

¹⁰ Yield when the lowest K rate was applied with N or N and P.

The input-intensive approach to fertilizer K management can have negative economic consequences for producers, which are apt to go unnoticed unless fertilizer response is determined relative to yield in the absence of fertilization. These consequences are to be expected considering the ample evidence in Table S4 (for the online version of the paper) that KCl inputs are often ineffective for increasing yield, but will be more serious when yield is depressed. The latter effect has indeed been observed, and was significant in field studies with cornReference Hanway, Barber, Bray, Caldwell, Fried, Kurtz, Lawton, Pesek, Pretty, Reed and Smith ^{38 ,} Reference Wortmann, Dobermann, Ferguson, Hergert, Shapiro, Tarkalson and Walters ^{144 ,} Reference Lueking, Johnson and Himes ^{148 ,} Reference Rehm, Sorensen and Wiese ¹⁹² , soybeanReference Rupe, Widick, Sabbe, Robbins and Becton ^{120 ,} Reference Svec, Andrews and Crittenden ^{125 ,} Reference Parker, Gascho and Gaines ^{131 ,} Reference Hymowitz, Jethmalani, Tiwari and Walker ¹⁷⁵ , wheat (Triticum aestivum L.)Reference Bakhsh, Khattak and Bhatti ¹⁸⁶ , sugarbeet (Beta vulgaris L.)Reference Gascho, Davis, Fogg and Frakes ¹³⁹ , sugarcane (Saccharum officinarum L.)Reference Elwali and Gascho ¹²⁷ , alfalfaReference Malm ¹⁴⁵ , peanut (Arachis hypogaea L.)Reference Walker, Flowers, Henning, Keisling and Mullinix ^{109 ,} Reference Hall ¹⁹¹ , rape (Brassica napus L.)Reference Gupta, Pathak, Bhan and Singh ¹⁷⁶ and cowpea (Vigna unguiculata L.)Reference Cox and Uribe ¹⁶⁶ . In several of these studies, the loss of yield was intensified by increasing the rate of KCl applicationReference Hanway, Barber, Bray, Caldwell, Fried, Kurtz, Lawton, Pesek, Pretty, Reed and Smith ^{38 ,} Reference Walker, Flowers, Henning, Keisling and Mullinix ^{109 ,} Reference Rupe, Widick, Sabbe, Robbins and Becton ^{120 ,} Reference Svec, Andrews and Crittenden ^{125 ,} Reference Hymowitz, Jethmalani, Tiwari and Walker ^{175 ,} Reference Rehm, Sorensen and Wiese ¹⁹² , and in some cases the higher rate transformed significant yield gain to lossReference Elwali and Gascho ^{127 ,} Reference Bakhsh, Khattak and Bhatti ¹⁸⁶ .

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 growthReference Hoeft, Walsh and Liegel ^{193 –} Reference Sangoi, Ernani, Bianchet, Vargas and Picoli ¹⁹⁵ and microbial processesReference Curtin, Steppuhn, Campbell and Biederbeck ^{196 ,} Reference Roseberg, Christensen and Jackson ¹⁹⁷ . More serious consequences can occur because of the anion supplied by KCl. Many leguminous crops are sensitive to Cl⁻ toxicity, including soybeanReference Rupe, Widick, Sabbe, Robbins and Becton ^{120 ,} Reference Parker, Gascho and Gaines ¹³¹ and alfalfaReference Smith ^{159 ,} Reference Rominger, Smith and Peterson ^{160 ,} Reference Smith and Peterson ¹⁹⁸ , and Cl⁻ can reduce soil N availability by inhibiting nitrification in soilsReference Sangoi, Ernani, Bianchet, Vargas and Picoli ^{195 ,} Reference Curtin, Steppuhn, Campbell and Biederbeck ^{196 ,} Reference Hahn, Olson and Roberts ^{199 ,} Reference Golden, Sivasubramaniam, Sandanam and Wijedasa ²⁰⁰ (see also supplementary references [53–55] for the online version of the paper) and by acting as a competitive anion that suppresses plant uptake of NO₃ ⁻ Reference Goos, Johnson and Holmes ^{147 ,} Reference Younts and Musgrave ^{201 ,} Reference Timm, Goos, Johnson, Sobolik and Stack ²⁰² (see also supplementary references [38] and [56–59] for the online version of the paper). A further difficulty arises from the mobility of Cl⁻ in soils, which intensifies profile leaching of Ca²⁺ as a counterionReference Johnston, Goulding and Mercer ^{203 –} Reference Brye and Norman ²⁰⁵ .

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 increaseReference Smith ^{159 ,} Reference Rominger, Smith and Peterson ¹⁶⁰ .

Table 5. Condensed version of a survey of peer-reviewed and university publications concerning the effect of KCl fertilization on the quality of selected food, feed and fiber crops. ¹

¹ A complete listing of all studies surveyed can be found in Table S5 for the online version of the paper.

² Latin names for crops not included in Table 4: ryegrass, Lolium perenne L. and Lolium rigidum Gaudin; subterranean clover, Trifolium subterranean L.

³ NR, not reported.

⁴ Values take into account number of genotypes studied.

⁵ Abbreviations: c (clay), cl (clay loam), fs (fine sand), fsl (fine sandy loam), gl (gravelly loam), l (loam), lfs (loamy fine sand), ls (loamy sand), m (muck), p (peat), ps (predominately sandy), s (sand), scl (sandy clay loam), sic (silty clay), sicl (silty clay loam), sil (silt loam), sl (sandy loam). NR, not reported.

One of the better known consequences of KCl fertilization, reported in Table 5 for studies with cornReference Wittels and Seatz ^{153 ,} Reference Younts and Musgrave ^{201 ,} Reference Heckman ^{212 ,} Reference Liebhardt and Murdock ²¹⁶ , wheatReference Wahhab and Ali ²²¹ and sorghum (Sorghum bicolor L. Moench)Reference Sweeney, Moyer, Jardine and Whitney ¹³⁷ , arises from the antagonistic effect of Cl⁻ on NO₃ ⁻ uptake that reduces lodging or stalk rot when susceptible cereal varieties are grown on a soil with high N-supplying power and/or with heavy inputs of fertilizer N, but at the expense of promoting NO₃ ⁻ loss through leaching or denitrification. The use of KCl can also have a beneficial effect by increasing the fiber strength (micronaire) of cotton (Gossypium hirsutum L.) grown on soils inherently limited in K reservesReference Bennett, Rouse, Ashley and Doss ^{222 –} Reference Cassman, Kerby, Roberts, Bryant and Higashi ^{224 ,} Reference Pettigrew, Heitholt and Meredith ²²⁶ , which would be expected considering the lack of a dense rooting system or residue K inputs.

Other qualitative benefits are often claimed for KCl fertilization of grain, fiber, oilseed or sugar crops, but these are difficult to reconcile with the listings in Table 5. This fertilizer, for example, has usually been ineffective for reducing disease severity in barley (Hordeum vulgare L.), wheat, soybean or cotton, and even with a positive effect, there was no significant yield increaseReference Timm, Goos, Johnson, Sobolik and Stack ^{202 ,} Reference Tinline, Ukrainetz and Spurr ^{210 ,} Reference Sharma, Duveiller and Sharma ^{219 ,} Reference Minton and Ebelhar ²²⁵ except where root rot was associated with a high content of NO₃ ⁻ but not K⁺ or Cl⁻ Reference Goos, Johnson and Holmes ¹⁴⁷ or when a susceptible variety was grown on an infertile sandReference Brennan and Jayasena ²⁰⁷ . There was likewise little evidence that KCl enhances sucrose content in sugar crops or oil, protein or fiber content or composition. Instead, these constituents were more likely to be adversely affected, as reported by Fintel and QuickeReference Fintel and Quicke ²¹⁴ for corn protein that declined in niacin and tryptophan. Such a decline has serious implications for human nutrition in the developing world, where grain is the staple food source.

Tuber crops are no less important to human nutrition, and particularly potato (Solanum tuberosum L.) that has a much higher K requirement than grain crops. In many parts of the world, KCl fertilization is a normal practice for potato production; however, there is a hidden cost from lowering starch content, which in turn reduces specific gravity. These effects, clearly documented in Table 5, have adverse consequences for human health such as obesity and cardiovascular disease, arising from greater oil retention in processed products such as potato chips and french fries. The anion supplied by KCl intensifies the decline in starch or dry matter content and specific gravityReference Panique, Kelling, Schulte, Hero, Stevenson and James ^{163 ,} Reference Maier, Dahlenburg and Frensham ^{238 ,} Reference Allison, Fowler and Allen ^{241 –} Reference Murphy and Goven ²⁴⁴ , and becomes a toxicological concern by enhancing the mobility and plant uptake of soil CdReference Sparrow, Salardini and Johnstone ^{239 ,} Reference McLaughlin, Maier, Freeman, Tiller, Williams and Smart ^{240 ,} Reference McLaughlin, Palmer, Tiller, Beech and Smart ²⁴⁷ . The latter problem has also been reported for cereal cropsReference Grant, Bailey and Therrien ^{208 ,} Reference Norvell, Wu, Hopkins and Welch ²²⁰ , and may thus be of broader interest for contamination of the food chain, particularly in view of recent clinical evidence linking breast cancer to dietary Cd exposureReference Julin, Wolk, Bergkvist, Bottai and Åkesson ²⁴⁸ .

Forage crops remove large amounts of K because their aboveground biomass is repeatedly harvested, and intensive usage of fertilizer KCl is widely promoted to not only sustain productivity, but more importantly, to ensure high forage quality that is critical to animal nutrition. Unfortunately, Table 5 provides no evidence that KCl is of any qualitative benefit to forage crops; rather, the effect was invariably negative, often involving a low content of Ca and/or MgReference Smith ^{159 –} Reference Smith and Smith ^{161 ,} Reference Rogers, Porter, Jolley and Leaver ^{229 ,} Reference Ketterings, Godwin, Cherney and Kilcer ²³⁰ that predisposes livestock to milk feverReference Goff and Horst ^{249 ,} Reference Horst, Goff, Reinhardt and Buxton ²⁵⁰ or grass tetanyReference Rogers, Porter, Jolley and Leaver ^{229 ,} Reference Allcroft and Burns ²⁵¹ . To prevent these disorders while improving forage quality, Cherney et al.Reference Cherney, Cherney, Bruulsema, Cherney and Cherney ²⁵² have recommended that the use of fertilizer K or manure be avoided on dairy farms in the northern USA, unless K deficiency is detected by plant analysis.

Elevated K supplies have long been recognized to have an antagonistic impact on the bioavailability of Ca and Mg, with adverse consequences for crop yield and quality. For example, McCallaReference McCalla ²⁵³ found that raising the K:Ca ratio in culture studies led to a progressive decrease in the growth of legume bacteria and in the nodulation of alfalfa and soybean roots. This imbalance would normally be ameliorated by liming, but even a neutral Ca salt can be utilized successfully to enhance soybean nodulation on an acidic soilReference Albrecht and Davis ²⁵⁴ . There are broader ramifications from the antagonistic effect of K on the Ca content of food crops, because low dietary Ca intake has been linked to several human diseases, including osteoporosis, rickets and colon cancerReference Heaney and Barger-Lux ^{255 –} Reference Centeno, de Barboza, Marchionatti, Rodríguez and de Talamoni ²⁵⁹ .

The problems associated with KCl usage have been thoroughly documented for arid and semi-arid regions by Krauss and SauratReference Krauss and Saurat ²⁶⁰ , and arise in part from the presence of a bioactive anion that has detrimental consequences for crop yield and quality. Some of these consequences can be avoided by applying K₂SO₄ instead of KCl. The benefits of this strategy for increasing crop yield and/or quality have been documented in numerous studies with potatoReference McDole, Stallknecht, Dwelle and Pavek ^{133 ,} Reference Panique, Kelling, Schulte, Hero, Stevenson and James ^{163 ,} Reference Jekić, Trpeski and Raimi ^{168 ,} Reference Lucas, Wheeler and Davis ^{261 –} Reference Højmark ²⁶⁵ and to a lesser extent with several other crops, including alfalfaReference Rominger, Smith and Peterson ¹⁶⁰ , soybeanReference Rosolem, Nakagawa, Machado and Yamada ²⁶⁶ and wheatReference Bakhsh, Khattak and Bhatti ^{186 ,} Reference Abd El-Hadi, Khadr, El-Kholy, Zahran and Negm ²⁶⁷ .

Tables 4 and 5 (see also Tables S4 and S5 for the online version of the paper) provide little agronomic justification for the buildup-maintenance philosophy that for several decades has intensified the input of fertilizer K, often in conjunction with direct or indirect subsidies at considerable public expenseReference Bansil, Dorin and Jullien ^{268 , 269} (see also supplemental references [60–62] for the online version of the paper). The result has been massive KCl consumption, which for Illinois has surpassed 21 Tg since 1950, equivalent on average to approximately 2200 kg ha⁻¹ Reference Brown ^{270 – 272} . 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 illiteReference Velde and Peck ^{26 ,} Reference Tributh, Boguslawski, Lieres, Steffens and Mengel ²⁷³ . The stabilizing value of KCl has long been recognized in the construction of impervious pavement and foundationsReference Frydman, Ravina and Ehrenreich ^{274 –} Reference Venkara Muthyalu, Ramu and Prasada Raju ²⁷⁷ . Unfortunately, the agronomic consequences include a loss of CECReference Amberger and Gutser ^{71 ,} Reference De Datta, Gomez and Descalsota ^{77 ,} Reference Deist and Talibudeen ^{278 –} Reference Cassman, Roberts, Kerby, Bryant and Higashi ²⁸⁰ and lower water-holding capacity, which is not conducive to crop growth and productivity.