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The potassium paradox: Implications for soil fertility, crop production and human health

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Intensive fertilizer usage of KCl has been inculcated as a prerequisite for maximizing crop yield and quality, and relies on a soil test for exchangeable K in the plow layer to ensure that soil productivity will not be limited by nutrient depletion. The interpretive value of this soil test was rigorously evaluated by: (1) field sampling to quantify biweekly changes and seasonal trends, (2) characterizing the variability induced by air drying and the dynamic nature of soil K reserves and (3) calculating the K balance in numerous cropping experiments. 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 ofKin 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.
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... Typically, dose responses show the following patterns: (1) initially Y i increases from Y 0 (control yield) at an increasing rate with increase in X i ; (2) when an inflection point is reached, Y i increases at a decreasing rate until Y max is reached; and (3) after Y max , Y i either remains constant or decreases towards 0 if the nutrient rate is increased further. Decreases in Y i after Y max may occur due to toxicities, nutrient imbalances, salinity, lodging, increased susceptibility to disease, or greater respiration (Agegnehu et al., 2012;Gill et al., 2004;Khan et al., 2014;Kindred et al., 2014). For example, high N rates often cause lodging (Agegnehu et al., 2012), whereas high rates of P induce zinc (Zn) deficiency, resulting in yield reduction in maize (Gill et al., 2004). ...
... For example, high N rates often cause lodging (Agegnehu et al., 2012), whereas high rates of P induce zinc (Zn) deficiency, resulting in yield reduction in maize (Gill et al., 2004). High potassium (K) rates, when applied as KCl, depress crop yields due to its high salt index (Khan et al., 2014). The presence of excessive K in the soil can also result in magnesium (Mg) and calcium (Ca) deficiencies. ...
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... This may be because the potassium content of the fillers was much lower than the nitrogen and phosphorus contents, while the potassium requirements of crops are comparable to or even exceed the demand for nitrogen (Weih et al. 2018). In addition, soil has a fixing effect on potassium, thus reducing its effectiveness (Khan et al. 2014). Therefore, potassium deficiency should be noted when fermentation bed fillers are applied as organic fertilizer and can be addressed by appropriate supplementation with potassium fertilizer. ...
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... Non-exchangeable K is dynamically linked to exchangeable as well as soil solution pools (Bilias and Barbayiannis 2019a), and can replenish both of them; but at much slower rates than those observed between exchangeable and solution K. Non-exchangeable K assumes a significant role in maintaining K supply to plants in soils with low exchangeable K (Ghorban 2007;Dhar and Sanyal 2000), or soils where exchangeable K is nearing minimum exchangeable K (Das et al. 2019c;, or soils having significant amounts of K-fixing minerals, like interstratified illite or vermiculite (Khan et al. 2014;Li et al. 2017b), or soils undergoing intensive cultivation without adequate external K input (Islam et al. 2017;Das et al. 2019c;. In extreme situations, the non-exchangeable pool might contribute to as high as 80-100% of total K availability to plants (Hinsinger 2002). ...
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... Several trials with feldspars have been conducted in Egypt, where conventional K fertilizers such as K 2 SO 4 and KCl are oftentimes unaffordable for farmers (Ali and Taalab, 2008;Hellal et al., 2013). Moreover, KCl can be inefficient and even problematic in arid and semi-arid regions due to salinization of soils, concomitant plant chloride toxicity and inhibition of soil nitrification, among other issues (Khan et al., 2014;Vieira Megda et al., 2014). In a trial with two okra cultivars as test crops, feldspar was compared with phosphate rock, compost and NPK (Abdel-Mouty and El-Greadly, 2008). ...
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Chapter
Plant roots can induce some biological weathering of potassium bearing minerals such as micas. Previous works on this subject did not demonstrate the importance of this phenomenon for plant nutrition. Our approach was based on the choice of the rhizosphere scale of investigation, but this approach is often impaired because of technical limitations. An alternative consists of the simple methodological principle: exacerbating root/mineral interactions for their easier detection through standard analysis. A culture device is described (adapted from Kuchenbuch and Jungk, 1982), providing an experimental simulation of a macroscopic rhizosphere. A root mat was developped at the surface of a net separating plants from minerals. This mat is assumed to act as a macroscopic planar “root surface”. Mica particles, as sole sources of potassium for plants, were diluted in an agar gel. After cropping, this medium was cut into thin slices parallel to the “root surface”, with a simple apparatus. Each slice was analysed through X-Ray Diffractometry for the identification of weathering products, while plants were analysed for potassium.
Book
During his years as a scientist working for the British government in India, Sir Albert Howard conceived of and refined the principles of organic agriculture. Howard's The Soil and Health became a seminal and inspirational text in the organic movement soon after its publication in 1945. The Soil and Health argues that industrial agriculture, emergent in Howard's era and dominant today, disrupts the delicate balance of nature and irrevocably robs the soil of its fertility. Howard's classic treatise links the burgeoning health crises facing crops, livestock, and humanity to this radical degradation of the Earth's soil. His message - that we must respect and restore the health of the soil for the benefit of future generations - still resonates among those who are concerned about the effects of chemically enhanced agriculture.