Potassium truths

Potassium is essential for photosynthesis and the production of starch, sugar, protein, vitamins, enzymes and cellulose. International soil expert Neal Kinsey talks about the impact of this nutrient on production and soil biology, and on how to address deficiencies, reports Glenneis Erasmus.
Issue date : 15 August 2008

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Potassium should ideally represent between 2% to 7,5% of the exchange capacity of soil, depending on the type of crop produced, according to the Albrecht system. Woody plants require higher levels of potassium than most other crops. Sampling wine-producing soil in France, for example, revealed that premium wines flourished in potassium levels of between 7% to 7,5%, says soil expert, Dr Neal Kinsey. But for cash crops and pasture, the desired level is between 3% and 5%.

Yield and crop quality will be sacrificed once the potassium falls below 2% of the exchange capacity. D r Kinsey recalls that a California client never used potassium in his peach orchards because it simply wasn’t done where he was farming. A soil analysis indicated that this farmer’s land was potassium-deficient. By raising the levels, the size of the peaches increased to such an extent that he was able to sell almost all of them. Previously, only a quarter of his peaches satisfied the requirements of the market. The fruit was also less prone to bruising after the treatment

Potassium sulphate vs potassium chloride
Chemical potassium application should be governed by soil pH and the exchange capacity. Both potassium chloride and potassium sulphate are effective in raising the percentage of potassium in the soil at a pH (water) of less than 6,5. Dr Kinsey prefers potassium sulphate over potassium chloride, as chlorine can be detrimental to soil life. T his is confirmed by SA soil expert, John Fair. “Farmers often shy away from using potassium sulphate, because it’s far more expensive than the chloride form,” he says. “The truth is that potassium sulphate is more effective. It’s more stable and less prone to leaching.

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And it doesn’t really cost more.” Potassium and sulphur, unlike potassium to maintain the levels on clay colloids only, thereby ensuring plants have sufficient potassium for optimal production at high pH levels. Anything extra is a waste. “At a potassium deficiency of 300kg/ha in soil with a pH above 6,7, it would be better to only apply 90kg/ha according to the plant’s needs. This is because the plant can only absorb around 90kg of the 300kg applied,” Dr Kinsey says. He adds that it’s better to split potassium applications for plants with high potassium needs than to provide a once-off application in such soil.

Manure and compost = potassium source
The potassium level in all soil, regardless of pH, can also be increased with manure and compost in a high potassium content. Dr Kinsey explains that the microorganisms in these materials help to make the potassium available to the plant, and stimulate microbial activity in the soil and help make potassium previously locked up, available again. But too much potassium has disadvantages. Excess potassium is a huge risk to livestock producers who farm on small pastures. Dr Kinsey had a client who started to lose all his best heifers.

An autopsy indicated the sodium and potassium levels in the bloodstream were higher than their calcium and magnesium levels. He attributed this to the fact that the animals were grazing small areas, resulting in a huge and toxic build-up of potassium in the soil due to all the high-potassium manure. The farmer had also not added other nutrients such as calcium to offset the impact of potassium. As high-producing cattle usually eat more than average animals, they were affected first. The farmer continued to lose animals until he identified the problem and provided calcium supplements and made the pastures safe by correcting the nutrient levels.

A bitter side-effect
Another problem posed by potassium and phosphate excess is that crops get a bitter taste, making them less desirable for human or animal consumption. This is often a problem on organic farms, as producers tend to think that one can’t overdo manure and compost. The higher the percentage of potassium above 7,5%, the more problems there are with weeds.

Dr Kinsey advises farmers to first balance all the other cations before trying to strip potassium from the soil. As mentioned earlier, potassium has a rather weak charge, which means that calcium and magnesium can easily displace it and help bring the percentage in line. Sulphur and sulphates can then be used to strip excess potassium if needed. For more information contact SA Biofarm on (012) 333 4222 or visit www.sabiofarm.co.za. |fw