Most farmers know the importance of nutrients such as nitrogen and phosphorus in the soil and the vital role their presence plays in crop development.
But what about potassium?
Potassium (K) is critically important in helping provide water balance within a plant. The only way a plant can move sugar and nutrients is by moving water or sap within itself and it does that by pumping potassium from one part of the plant to another. Symptoms of deficiency vary among crop species, but they often mimic drought stress, and will show most severely on the oldest leaves.
With so much riding on it, potassium should be top of mind for most growers in Western Canada, right? Not exactly, says a leading agronomist.
Lyle Cowell, Nutrien’s senior agronomist for Canada, says the problem is that soil in Western Canada tended to be fairly high in potassium and farmers didn’t typically think about it. As a result, many didn’t replace the potassium their crops drew from the soil over time.
While the amount of potassium lost in a field in a single year is relatively low, Cowell says it can be substantial over decades and can lead to significantly reduced yields if left untreated.
Comfort zone
“We kind of got into a comfort zone of not applying potassium to many of our crops in Western Canada,” says Cowell.
“If a crop removes 15 to 20 pounds of potassium per year, it doesn’t sound like much. But over the course of a lot of farmers’ careers, they’ve probably removed 400 or 500 pounds of the most available potassium and haven’t replaced it.
“Farmers recognize that whole scenario with nitrogen and phosphorus: you’ve got to put some on if you’re going to take some off. That seems to be embedded into the management thinking of farmers.
“We seem to be missing that with potassium, and we have to start thinking about that a little bit harder.”
Higher crop yields in the past 20 years have compounded the issue, boosting the amount of potassium withdrawn from the soil. Newer crops like soybeans require far more potassium than others.
Cowell says he is seeing more cases of potassium-deficient soils compared with a few decades ago. That is likely due in part to rising potash prices, which prompted producers to trim costs and stop using it.
“Unfortunately, that habit has hung on a little too long now. We’ve removed 300 or 400 or 500 pounds of potassium per acre from our crops without applying anything and now we’re in trouble in what were the marginally deficient soils.”
A hit to the bottom line
Once soil has become potassium deficient, it can have an enormous effect on yields, according to Cowell. In rare cases, it can lead to complete crop loss. A more common scenario is a yield loss of 10 to 20 per cent, which can be a significant hit to the bottom line.
While crops, such as soybeans, remove large amounts of potassium, much of it eventually cycles back into the soil. Unlike nitrogen, phosphorus or sulphur, very little potassium is removed with harvest but remains in crop residue.
Don Flaten, a professor emeritus in the soil science department at the University of Manitoba, says one of the most common symptoms of potassium deficiency is when older parts of the plant appear chlorotic and leaves begin to yellow, especially at the tips and margins.
Cowell says another common sign appears within a canola crop that is swathed and then sits in the field for three or four weeks. The potassium leaches back into the soil before harvest and the following year’s crop grows better where the previous year’s swathes were laid.
“Nine times out of 10, you’re looking at a potassium deficiency (in that spot),” he says.
Flaten says forages, and particularly alfalfa, are large consumers of potassium and are particularly sensitive to potassium deficiency. With a forage crop, the entire plant is removed, along with its potassium.
In Western Canada, barley is sensitive to potassium deficiency and is one of the crops most likely to show problems, he added.
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Pay attention to potassium levels
Soil testing is the best way to measure potassium levels in a field, establish any deficiency problem and determine severity, says Cowell. He suggests more testing should be done and the information farmers receive from those tests should be analyzed more closely.
“When farmers do soil sampling, they usually do measure potassium levels in the soil,” he says. “The problem is that we don’t look at that number. Our eyes are always glued on the nitrogen level, maybe the phosphorus level. Sometimes we just don’t pay attention to the potassium number.”
Cowell cautions that soil testing only provides averages for a field.
“I think farmers would do well to start thinking about dividing their fields into segments. That way, if you’re in a situation where there is marginal potassium deficiency based on a whole field sample, you probably should be thinking about sampling the areas of that field that are most likely to be potassium deficient and checking to see if those areas of the field are severely deficient.
“Potassium can present a great opportunity for variable rate fertilizer application.”
In addition to soil testing, Cowell suggests that farmers take a closer look at the nutrient balance of their soil and crop rotations. This will allow them to compare how much of each nutrient is removed versus how much they apply and address any shortcomings.
He says determining those numbers has become much easier thanks to the recent release of the Prairie Nutrient Removal Calculator, which was developed by soil scientists at the University of Saskatchewan.
Growers don’t have to choose from a myriad of products to address potassium deficiency, he adds. Fertilizer potassium, or potash, is the only common product available.
How much and how to apply potash largely depends on a farmer’s management strategies, he says. For some, that could mean applying small amounts each year and for others it could mean spreading larger loads every few years.
