The role and availability of potassium in crops and soils

This key macronutrient is often misunderstood and misrepresented

Crops such as alfalfa hay (seen here), corn stover or small grain cereal straw contain huge amounts of potassium.

Potassium is a key macronutrient in all plant and animal nutrition. Its role and availability in crops and soils is frequently misunderstood and misrepresented. In plant tissues, potassium (K) is second only to nitrogen (N) in amount found in plants. It may make up one to three per cent by weight of crop plants, from straw to grain. Bananas, by the way, are no higher in potassium than potatoes or carrots.

The letter K that is used to symbolize potassium comes from the German word kalium. In bygone days, people burned organic matter such as straw and brush wood to produce ash, which was washed in water. The water solution was evaporated in a pot or container to produce potash salt. This salt was then mixed and boiled with animal fat to produce soap.

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Crops such as alfalfa hay, corn stover or small grain cereal straw contain huge amounts of potassium. A four-ton crop of alfalfa may contain 175 pounds of nitrogen primarily as protein and 175 pounds of potassium. Soil scientists can mess up the potassium calculation due to the fact that in crop plant tissues they measure potassium as K2O, which means 100 pounds of potash in plants is really 83 per cent by weight of potassium.

Potassium as a fertilizer is usually sold as potassium chloride (KCl) or an older name is muriate of potash. Potassium may also be sold in the more expensive form of potassium sulphate — but that’s two macro fertilizers.

The total potassium content of soil in the upper soil profile may range from 30,000 to 50,000 pounds per acre (i.e. 15,000 to 25,000 parts per million). Most of this potassium, up to 98 per cent, is unavailable for plant growth. The potassium is tied up in the soil particles in the form of insoluble minerals such as mica and feldspar. A second major source of potassium is also unavailable as non-exchangeable potassium by tie-up with clays, ranging from one to 10 per cent of total soil potassium. The soil solution potassium, the available nutrient, is taken up by plant roots. Very little potassium is available in the soil organic or microbial fraction.

Crop removal rates

While the levels of most cropland soils, especially the clays and loams, may initially contain large amounts of available potassium, they can easily — over time and with poor management such as the regular sale of cereal straw or hay — become highly depleted of this most essential nutrient.

Hay, particularly alfalfa hay, at around four tons an acre will remove up to 175 pounds of potassium an acre. A 20-ton crop of potatoes will remove 300 pounds of potassium an acre. The most consistent loss of potassium in Prairie agriculture, though, is by cereal crops, when the straw residue is baled and sold.

A 60-bushel crop of wheat removes about 25 pounds of potassium in the grain and almost 70 pounds in the two tons of straw per acre. Repeated straw removal on cropland of wheat, barley, rye or triticale, will seriously affect Prairie crop yields over just a few years.

Canola crop residue or corn stover is almost never removed so the potassium residue is returned to the soil. Canola residue contains about 70 pounds of potassium per acre (in a 35-bushel crop) and corn stover about 100 pounds an acre (in a 100-bushel corn crop).

I have checked long-term hay fields in many areas of Western Canada and the available potassium in some of these fields is less than 25 pounds per acre. The hay yield of such cropland was barely a ton per acre. On the other hand, I have soil sampled fields associated with frequent applications of cattle manure and found the available potassium exceeded 3,000 pounds per acre.

Where does the chloride go?

Producers who grow quality timothy hay for the export market over many years can run into hay quality problems despite adding lots of potash. What happens is when 100 pounds of potash is applied to hay cropland, almost half that potash is chloride. It’s like adding salt. Where does the chloride go? The hay crop takes up the potassium and the chloride part either forms calcium chloride from the calcium carbonate in the soil, perhaps magnesium chloride and sodium chloride in saline soils.

You now run into a soil build-up of chloride, which causes spotting and burn marks on the timothy leaves. Yields may be reduced as well as hay quality. Also, the added potassium chloride, by taking out the calcium carbonate as calcium chloride, tends to acidify the soil — low-pH soils of less than 6 are important. When leaf burning of the timothy hay occurs, some growers switch crops while others use the more expensive potassium sulphate as the source of potassium.

Potassium in the crop plant has many roles, however, crop standability is not one of them. When small amounts of potassium chloride are added to cropland, growers often notice a growth improvement. This is likely not due to the potassium, but the chloride, which is an essential element needed in small amounts and is often deficient on Prairie cropland.

Potassium itself in the green plant has many roles in the movement of water, other nutrients and sugars within the plant. Potassium stimulates early crop growth, improves protein production and water-use efficiency.

Plants with potassium deficiency are more prone to drought stress, especially in the regulation of water conservation since potassium is a key element in stomatal opening and closing (i.e. the plant’s “breathing” pores).

Potassium has multiple functions in crop growth, yield and maturity, but I will end by saying that a straw chopper is much better for your cropland health and well-being than a straw baler. The latter not only robs you of multiple-use mulch, which has many functions, but also the much-needed potassium for the next crop — and costs you money to replace.

For a little humour, it could be said, when you burn a bale of hemp straw you are left with nothing but pot-ash.

About the author

Contributor

Dr. Ieuan Evans is a forensic plant pathologist based in Edmonton, Alta. He can be reached at [email protected]

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