NPK – nitrogen, phosphorus and potassium – is the standard abbreviation for the major crop nutrients, but when it comes to soil tests in Western Canada, it’s traditionally been just NP. It’s been assumed that the soil supplies most if not all of the potassium needed, especially for mainstay crops such as wheat and barley. But while potash mines hundreds of feet underground may be home to some of the world’s largest potassium supplies, that doesn’t mean it’s universally available everywhere in Prairie topsoil. Recent soil sampling is showing that levels are dropping, and growers should pay attention.
The 2015 Soil Test Summary from the International Plant Nutrition Institute shows declining potassium levels since the last summary in 2010. Nutrien senior agronomist Alan Blaylock says the results vary across the Prairies. “In previous soil tests in Manitoba and Alberta, 30 to 45 percent of the samples were below critical levels for potassium. When I look at the median soil test K level for Alberta, it is 173 parts per million (ppm) which is adequate. But 45 percent (of the samples) were considered deficient, so there is a lot of variability.”
Blaylock says it isn’t a drastic decrease but rather a long-term trend, the result of traditional farming practices that rarely include potassium applications. “In arid and semi-arid areas, potassium is generally very high because the soil is less weathered. The older and more weathered soil becomes, the higher the rainfall, the more potassium is lost.”
Potassium plays a vital role in managing the movement of water from the soil, through the plant and out through the leaves. It regulates the plant’s cooling. While nitrogen and phosphorus become incorporated into components of the plant, potassium works differently.
“It kind of floats around in the plant’s sap… it doesn’t get integrated into the organic molecules of the plant,” Blaylock says. “This means that much of the K taken up by the crop, will remain in the straw and not the grain, and if left in the field will go back into the soil.”
Forages are potassium-hungry
While that means much of the potassium is retained by grain crops, forages are another story. “Forage crops are particularly notorious for depleting the soil potassium levels,” Blaylock says. “They take up a lot and we remove it all when we harvest or graze the crop.” Alfalfa or other forages are often shipped miles away and so the potassium is removed and usually not replaced.
There’s a significant difference in the amount of potassium taken up by cereals and forages. A grain crop may remove 30 or 40 pounds of potassium per acre, but a forage crop can take up 300 to 400 pounds per acre. Grazing forages in the field where they are grown will help retain the nutrient, but not uniformly. And replacement is not keeping pace with removal throughout the Prairies.
Certain crops have a higher demand for potassium than others, and that translates into higher removal at harvest. The increase in corn and soybean acres has contributed to the downward trend of potassium soil levels, especially in Manitoba. Blaylock says soybeans in particular use a lot of potassium. “There’s quite a bit of data in the United States to show that K removal is high for soybeans. We need to be paying more attention to this, especially as yields increase.”
In Alberta, sugar beets and potatoes are also big users of potassium, but they tend to be grown in sandy soils and are well fertilized, so those regions are less likely to have potassium deficiency.
Tillage practices are also playing a role. “Roots are extracting potassium and phosphorus from the subsoil, and then crop residue is deposited at the surface. Phosphorus and potassium are particularly immobile in the soil and can’t move back down into the subsoil,” Blaylock says. The one-pass direct-seeding methods now routinely used means the nutrients aren’t returned to the subsoil for the roots to access. However, Canadian trends toward banding and subsurface applications help place potassium and other nutrients in the crop root zone.
Identifying the “hidden hunger”
Because the depletion of potassium in the soil happens slowly, identifying the effects may be difficult.
“There may be nothing outwardly visible, and yet if tested, the crop is deficient. This is known as ‘hidden hunger.’ Symptoms like lodging or reduced yields can be blamed on other factors, but may also be a sign of deficient potassium levels,” Blaylock says. Visual signs include “firing” (turning brown or yellow) along leaf edges, primarily in older portions of the plant. Weak stem strength is another. “If we are growing high yields with high nitrogen rates, we can create an imbalance. Too much nitrogen and not enough potassium can weaken straw strength and lead to lodging.”
Soil condition and weather may affect a crop’s access to the potassium. “If you are at 120 parts per million, you might be alright most years. But if you have dry conditions, potassium, which requires moisture to move through the soil, will have restricted movement and difficulty getting to the roots,” Blaylock says. Compaction of the soil hinders root growth and can also cause potassium to be “stranded” in the soil.
On the other end of the spectrum, extremely wet conditions saturate and create oxygen deficiencies in the soil. Oxygen is required for potassium uptake so even short periods of saturation disrupt a plant’s access. Finally, the nature of very sandy soils tends toward low potassium levels and lower moisture content, so they must be managed accordingly.
Sometimes the grower needs to become a detective to identify the cause of an under-performing field. “If one of the nutrients is deficient, you have a hidden hunger. You might not get the yield you expected, and it isn’t explained by other factors, so you need to do some detective work,” Blaylock says. Bringing all available information together will help to identify any deficiencies.
Pay attention to crop removal
North American soil data has been collected for many years and results are showing a downward trend in potassium levels. Tools are now available that will help farmers understand the trends in their region. One of these tools, eKonomics (www.nutrien-ekonomics.com), includes calculators for nutrient removal by crop, and for return on investment by nutrient applied. As well, there is geographical data on North American soils that highlight regional issues related to various nutrients.
Soil and tissue testing are important first steps if a deficiency is suspected. There are a number of laboratories that will run the tests, and that can be facilitated by a grower’s crop adviser. For example, Nutrien Ag Solutions offers to pick up and submit samples for testing and then work with the grower to analyze and build a plan to address any deficiencies.
“The best thing a grower can do is soil test,” Blaylock says. He suggests testing every other year is usually sufficient for phosphorus and potassium, and always test at the same time of year to ensure a clear picture of the nutrient levels in the field. “Nutrient levels shift between seasons so by sampling at the same time every year, the trends will be easier to see.”
He also advises growers to pay attention to crop yields and crop removal. “If you remove 50 bushels of wheat, how much potassium have you removed? Tie that to your soil test results and trends to determine if it is time to replace what was taken out at harvest.”