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eKonomics News Team

Nearly half of soil types in Minnesota are below the critical level for two primary nutrients, potassium (K) and phosphorus (P), due to more than 40 years of declining nutrient balances.

Table 1. Percentage of Minnesota soils below the critical level.

A Nutrient Balance analysis — the first study to assess state-by-state nutrient-removal records, fertilizer-consumption information and manure data on a yearly basis — revealed that soils across North America are becoming depleted of nutrients at an alarming rate. In Minnesota, potassium removal has increased 36 percent and consumption of potassium is down 21 percent since 1975. As a result, the potassium balance has decreased from 6 pounds to -20 pounds per acre.

Indeed, producers have experienced record yields of corn, soybeans, and other crops in recent years. Nevertheless, the wide variety of soil and climate conditions across the state require growers to invest in fertilization programs to ensure an adequate supply of nutrients.

That’s especially true for K. Minnesota soils can supply some K for crop production natively however, that supply may prove to limit crop growth. Potassium is associated with the movement of water, nutrients and carbohydrates in plants and can regulate photosynthesis. According to Dr. Daniel Kaiser, Assistant Professor at the University of Minnesota, who also advises farmers through the University’s Extension Nutrient Management service, if K is not supplied in adequate amounts, plant growth can be stunted and yield can be reduced.

Marketplace changes are impacting soil nutrient levels. In 2016, soybeans surpassed corn as Minnesota’s largest crop. That trend continued in 2017 as 8.2 million acres of soybeans and 7.6 million acres of corn were harvested (NASS, 2017). This year, grain prices and production input costs are again expected to lead farmers to plant more soybeans than corn, a switch which concerns Kaiser, because soybeans remove more than four times as much K from soil as corn or small grains.1

“We’re starting to see situations where we’re drawing down soil levels of potassium as a result of the increase in soybean acreage,” Kaiser says. “Growers haven’t been concerned with K levels because they’ve been involved with small grains, corn or other production systems. They didn’t have to apply K, but now we’re starting to see with the soybeans that the K levels are dropping.”

For example, what is the difference in nutrient removal between a 230 bushel corn crop and a 60 bushel soybean crop? Our eKonomics™ Nutrient Removal calculator shows that corn would remove 88 and 62 pounds of P and K per bushel, while soybean would remove 48 and 78 pounds of P and K per bushel.

One Soil Nutrient Level Does Not Fit All

Soils in Minnesota are comprised of different clay materials, which affects how much K the soils hold and release. Potassium can be trapped between layers of certain types of clays (2:1 expanding clays specifically), limiting its availability to crops. Minnesota also has sandy soils, which can experience K leaching with high rainfall. This wide variety of conditions can lead to the over- or underestimation of nutrient levels, especially in the southeastern part of the state. Historically, such miscalculations happen because Minnesota had one set of recommended soil nutrient levels that were used statewide.

Therefore, Kaiser and other Minnesota soil scientists wondered why the response of crops to fertilization varies so much across the state. So, they began developing the following recommendations (also see how North Dakota recommendations are being updated), which recognize regional differences:

  • Central counties, where native K levels can be 300 to 400 ppm, are generally considered adequate for crop production.
  • Northwestern counties have unclear K levels, but Kaiser says he suspects the research is going to show declines as a result of the change in crop rotations from corn and small grains to include soybeans.
  • Western counties have some native K levels of 200 ppm, but the introduction of soybeans is quickly removing K in this area.
  • The Red River Valley has some native K levels as high as 500 ppm.

“Minnesota growers historically didn’t have to worry about applying K routinely, but with the growth in acreage planted with soybeans and the trend toward more rented ground, we’re starting to see soil test levels drop,” Kaiser says. “There’s a threshold being reached, and some growers might not be aware of it. They had crops that didn’t remove a lot of K annually and now they’re putting in a crop that does.”

Table 2. Current soil test values for K in Minnesota soils.2

Soil testing is the best management tool for predicting the amount of K in the soil and how much to invest in your fertilizer program. Check out the eKonomics™ ROI Calculator to see how much your investment could impact your return.

That’s especially true as more land is farmed on a cash-rent basis. For example, approximately two-thirds of the corn and soybean acres in southern Minnesota are under some type of cash rental agreement.3 “Growers typically do not invest in fertilization on rented land”, Kaiser says.

“One of the things that worries me with more rented ground is growers neglecting soil test levels, particularly neglecting potash [potassium] applications,” Kaiser says. “Growers should be mindful of where soil test levels are going and make sure they’re not getting into situations where they’re losing yield by under-fertilizing.”

Test results represent an estimate of the soil’s ability to supply all of the K needed for a crop. Yields can increase if potassium is applied when soil test values for K are in the “low “or “very low” ranges. Yield may or may not increase if potassium is added when soil test values are in the “medium” range or greater. A response to potassium fertilization should not be expected if soil test values for K are in the “high” or “very high” range.

How Potassium Improves Yield

Kaiser and the other soil scientists are studying the impact of potassium on soybeans in a variety of Minnesota soil conditions. These new results are not yet published, however, recommendations for K application for other crops are available.

For example, potassium needed for corn or small grain production can be applied in a band near the seed at planting, or broadcast before planting.

Until they release new recommendations, growers should conduct regular soil tests using the existing statewide standards to better inform their nutrient management programs.

“The question is, for a particular field, what’s the optimum nutrient management?” Kaiser asks. “With soil testing, we can give a probability of response to fertilization. The soil test results give growers confidence that they’re making the right decision about where to invest their money.”

To summarize, although soil nutrient levels in Minnesota have historically contributed to record crop yields, the adoption of increased soybean acres is removing more K. At the same time, other production costs are increasing, leading growers to cut back on fertilizers — especially potassium fertilizers. This combination of factors makes soil testing well worth the price. By determining soil nutrient status, growers are able to better manage their fertilization program costs and improve their yields.

Sources

  1. George Silva, “Nutrient removal rates by grain crops,” Michigan State University Extension, October 10, 2017.
  2. Daniel E. Kaiser, Carl J. Rosen and John A. Lamb, “Potassium for Crop Production,” University of Minnesota Extension Nutrient Management, 2016.
  3. Kent Thiese, “Tight profit margins likely to continue in 2018,” The Farmer, January 4, 2018.