Crop Nutrition

Boosting Yields Starts with Rearranging Priorities In Many Cases, N K P Trumps N P K

eKonomics News Team

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Pop quiz: When fields lack both phosphorus (P) and potassium (K), which nutrient should take priority?

“In our area, the answer is definitely to apply K,” says Nathan Slaton, Director of Soil Testing and Assistant Director of Agriculture Experiment Station at the University of Arkansas with the Crop, Soil, and Environmental Sciences Department.

Increasingly, researchers like Slaton recommend applying K ahead of P in a new nitrogen-potassium-phosphorus (N-K-P) prioritization ranking. That’s a major departure from the historical N-P-K ranking, but the approach can help growers who are facing tighter budgets, thinner margins, and balancing crop yield goals with the bottom-line realities of how much they have available to invest in fertilization.

This change is significant for farmers. In Arkansas, for example, soybeans have been considered the “other crop” grown in rotation systems that feature corn, rice, or cotton with little to no fertilizer applications to soybean. However, not properly fertilizing soybean in the rotation can lead to decreased yields and potentially decreases in soil test levels with limited nutrients for future crops.

Farmers typically apply fertilizer for crops in the rotations that require N, and, if their budgets allow, they also apply P, following the well-known N-P-K prioritization, says Slaton. Whenever farmers need to cut costs, they reduce or eliminate applications of K first, then P. Yet, Slaton’s field experiments show that soybean yield responses to K fertilization are larger and more frequent than yield responses to P fertilization.

“While I like the traditional sound of N-P-K, the change to N-K-P makes sense when it comes to plant nutrient uptake and frequency of response that we observe in Arkansas,” Slaton says.

Three Factors That Favor K over P

  1. Frequency of fertilizer responses
  2. Magnitude of the fertilizer response
  3. Maximizing profits with fertilizer application

The frequency of response to fertilization is the first factor for farmers to consider. According to Slaton’s research, the chance of a yield response is greater for K than it is for P in the following scenarios:

  • When K levels are very low (less than 60 ppm), soybeans responded to fertilization in 100% of the test sites.
  • When P levels are very low (less than 8 ppm), soybeans responded to fertilization in only 40% of the test sites.

The next two factors are the magnitude of the response (the change in yield with fertilizer application) and how much fertilizer to apply to maximize both yield and Return on Investment (ROI). In Slaton’s research, applying P fertilizer when soil test levels are low resulted in a 10 percent increase in yield. However, studies where soybean were fertilized with K resulted in twice as much of a yield increase at 20% above the control.

Source: Nathan Slaton, unpublished data, University of Arkansas Division of Agriculture Research and Extension, 2018.

*Soil K and P levels considered “low” based on University of Arkansas recommendations

**Average yield on research sites where soybeans responded to fertilization; does not include yield change on sites where soybeans did not respond to fertilization

A third consideration is how much fertilizer to apply to maximize the potential yield and also meet business goals for the farm. Farmers should make investment decisions based on the results of soil test levels, but they may not always need to apply the full amount of fertilizer recommended in state guidelines.

When growers consider how much to spend on inputs, Slaton again argues that they should prioritize K over P. He bases that view on preliminary research results from three locations, where Arkansas scientists have discovered that K can be applied later into the growth cycle of irrigated soybeans and still maximize yield targets. Check out how you could increase your ROI with our very own eKonomics ROI Calculator.

Research Echoing Other’s Recommendations

Slaton’s findings echo a growing chorus of researchers from across the country. For example, Carrie Laboski, a professor of soil science at the University of Wisconsin-Madison, found that if both P and K levels are testing low, prioritizing K offers the bigger bang for the fertilizer buck.

T. Scott Murrell, potassium program director for the International Plant Nutrition Institute (IPNI), agrees, adding that an emphasis on K over P is more accurate in terms of the sheer volume of these nutrients that crops consume: plants utilize N and K in similarly large quantities, but they use less P because they don’t need as much for their metabolism.

Researchers do, however, express a word of caution before changing the shorthand for fertilizer across the board: “The potential for yield loss from P deficiency is a bit scary because, when severe, it has greater effect on plant growth and is harder to correct than K deficiency,” says Slaton.

Liebig’s Law of the Minimum states that yields will be limited by the most limiting factor. “So, it really comes down to how low your P and K levels are,” Slaton says. Check out this video on Liebig’s Law of the Minimum.

Is K a Victim of Tradition?

Too often, growers base their nutrient strategies on what’s worked in the past versus today’s data, says Daniel Kaiser, associate professor and extension specialist at the University of Minnesota. Kaiser is concerned that growers sometimes don’t rely on clear soil test results to inform fertilization decisions. “Instead, they simply do what they’ve always done,” he says.

The fact that K is not as well researched as other primary macronutrients (N and P) compounds the problem. “It can be difficult to get research funds for K work, as the industry has often placed priorities on other nutrients because of their demonstrated returns,” Kaiser says. However, as more researchers focus on K, its importance is likely to increase — and he is convinced that such a shift is coming.