Faulty soil tests blamed | Study found that inconsistent results were prompting producers to over-apply the nutrient
University of Illinois researchers are questioning farmers’ reliance on soil tests and the use of potash fertilizer.
Unreliable potassium soil tests mean growers are fertilizing when they don’t have to, say the researchers, who published their paper, The Potassium Paradox, in the journal Renewable Agriculture and Food Systems last month.
They took biweekly soil samples for potassium during a four-year field study and found inconsistent results.
“Just like a doctor who tests your cholesterol: one day it’s 150, the next day its 250. So how can he prescribe medicine?” said Saeed Khan, one of the researchers.
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“That’s exactly like our problem was. How can we prescribe fertilizer if we don’t have a reliable number?”
The researchers say soil tests miss potassium in the profile.
“The test targets a fraction that’s called exchangeable,” said Richard Mulvaney, another paper author.
“It makes the assumption that that is the fraction that feeds the plant, but that assumption is simply not valid. You have other forms of potassium in our soils and especially in soils that have what are called 2:1 clay minerals, and those other forms which are not recovered by the standard soil test do supply much of the K that ultimately feeds the plant.… That’s exactly the source of the problem. The test is not specific for what does supply the plant, and available K is not synonymous with exchangeable K.”
As a result of their findings, the researchers argue that many fields in the U.S. corn belt can do without an application.
Khan said a 200 bushel per acre corn crop will remove 46 pounds of potassium, but residues return 180 lb. of the mineral to the soil, which is more than the next crop needs.
According to Saskatchewan’s agriculture ministry, potassium content in the province’s soil varies from 100 to 50,000 lb. per acre with one million acres deficient in the mineral. Sandy and peaty soil is the worst.
Both the ministry and the Canola Council of Canada note that most crops won’t show a response to potassium.
Canola growers will see the greatest response in soil that shows less than 150 lb. per acre.
“Canola rarely responds to applied K, even under conditions where cereals normally respond,” says council literature.
In their studies on potash fertilizer, researchers surveyed more than 2,100 yield response trials, of which more than 700 were under grain production in North America. They found that potassium chloride, or potash, was largely ineffective for boosting yields.
The researchers say use of the fertilizer comes with risks, potentially depressing calcium, nitrogen and magnesium levels and even suppressing yields. Potassium chloride can also harden soil.
The paper recommended that growers abandon the conventional potassium soil test and conduct their own small strip trials.
“Seeing is believing. When the farmers put out their strip trials and find that they do get a yield increase from applying potassium, they should use it,” said Mulvaney.
“But in most cases they will see no such increase.”
