Potassium is the element which is responsible for making moisture metabolism in both animals and plants function properly. In hot weather, cattle and horses drink a lot of water, and for them sodium (Na), mostly in the form of regular salt, is more critical than potassium (K). Both Na and K are electrolytes (minerals that carry an electrical charge); Na and K are positively charged. However, for plants, K is the more important electrolyte. Although magnesium – also a positively charged electrolyte – is the cornerstone of the green-colored chlorophyll, present at the cellular level in all plants, it’s K that oversees chlorophyll’s synthesis. Plants need K before they can extract free nutrients from air, namely nitrogen, carbon, hydrogen and oxygen. Plants can’t make starches, sugars, proteins, enzymes or cellulose without K. Plants can’t survive summer dry stretches without ample K. When too much warmth combines with too little precipitation, moisture management in plants becomes a critical concern.
Stomates are the microscopic holes on leaf surfaces where moisture and oxygen escape, and K is the element most responsible for regulating how the stomates open and close. When moisture is limited, like in droughty conditions, the stomates constrict, a self-preservation mechanism. This is why corn leaves curl and get “spikey.” When precipitation finally does arrive – assuming its absence hasn’t been too long – the corn leaves unfurl and growth resumes. Understand that the maturing process continued during the droughty period, often resulting in mature, undersized plants. K deficiency increases the odds of plants not recovering optimally post-drought.
Another prominent K deficiency symptom is reduced standability, commonly referred to as lodging. There are K deficiency symptoms that are very visible in grasses like corn. They include mottling, brown top, purplish spotting and streaking. This purpling is not to be confused with corn leaf tip die-back of that color indicative of a phosphorus deficiency. In grains and grasses, tan-colored firing starts at the tip of the leaves and proceeds down the edges, usually leaving the midrib green. This K deficiency symptom is basically the opposite of the primary symptom of nitrogen (N) shortage. The most classic N deficiency symptom resembles a tan arrowhead which starts at the leaf-tip and follows down the midrib – almost as if attempting to split the leaf symmetrically. In almost all cultivated plant species a K deficiency can result in premature leaf loss and reduced resistance to cold.
According to Wikipedia, “Potash is the name used for various mined and manufactured salts that contain potassium in water-soluble form. The name derives from pot ash, which refers to plant ashes or wood ash soaked in water in a pot, the primary means of manufacturing the product before the Industrial Era. The word potassium is derived from potash.” My addition here is that the chemical abbreviation K comes from the New Latin word kalium. The Old Latin alphabet lacked a “K.” Modern German uses Kalium for potash. Most likely it’s German scientists who made K the official chemical symbol for potassium. And while we’re on the subject of K, specifically vitamin K – the one responsible for effective clotting of blood – that “K” come from the German word Koagulation. The German language has very few words starting with the letter C.
In addition to affecting crops, K deficiency affects livestock. In hot weather many livestock nutritionists recommend increasing K intake to ruminants as well as horses and hogs. Whether in the form of sweat or fluid moved through the kidneys, the extra water consumed due to heat results in the loss of four electrolytes: Na and K, plus calcium and magnesium. Unlike soil phosphorus, which is usually quite limited and not considered a positively charged electrolyte, most soils have abundant K. The K supply in the ocean – though much less than the Na supply – is considered to be inexhaustible.
The actual availability of K in the plant is a separate matter. This is a subject addressed by Charles Walters in his Acres USA Primer. Let me quote him: “Unless microbes in the soil give a mighty assist, and pH is managed at the proper acid level, K availability is a massive problem. It is not helped by the kind of fertilizers available either. When corn is starved for want of available potassium, it gets plugged up with iron at the nodes. Roots starve, and stalk rot becomes the observed result.” Walters stressed that K has a character of its own, much like Na, except that it is found in many rock formations, usually in small amounts. Let me point out that larger soil particles – like sands and “big” silts – tend to offer scarcer K resources, largely due to their lower surface-to-mass ratios.
Thus, gravelly silt loams tend to be a chintzy source of K compared to clayey soils. The normally desirable permeable soil, with its lower K reserve, proves to be a two-sided coin, confronting crop growers. It’s harder to build organic matter on sands than on silts and clays. A few years ago, I examined a grass hay stand peppered with K deficiency symptoms. Soil analysis showed K base saturation percentage (BSP) at 1%. In his Primer, Walters recommended 3% BSP in addition to a generous K level in pounds/acre. BSP is critical for determining if soil nutrients are in balance. So, for what it’s worth, I only work with labs that perform BSP tests. Not having that information is like powering a rowboat with only one oar.