Seasonal K Considerations: How to Maximize Uptake and ROI

Introduction

Varied soil types and weather conditions can all impact potassium (K) availability, which in turn affects fertilization needs. Seasonal considerations are particularly important for farmers in the Corn Belt and southern region of the United States.

To determine the optimum time of year for K application, we asked experts to explain how growers in these areas can achieve maximum uptake and return on investment (ROI) with K fertilizer applications.

Spring or Fall: What’s the Best Time in the Corn Belt?

More often than not, weather drives fertilizer applications. The dramatic shift in temperature from season to season — hot, dry summers, cold winters and wet springs — can make it difficult to time K application just right.

The first step to determine how much K to apply is testing the soil to reveal existing nutrient levels. The second step is fertilizing when conditions are less favorable for losses. Soil samples should be collected in the spring, according to Dan Schaefer, Director of Nutrient Stewardship at the Illinois Fertilizer and Chemical Association.

“My recommendation has always been to conduct soil testing in the spring or late spring — around planting time,” Schaefer says. “This way, you have your recommendations ready to go for fall, when you can get your K and P [phosphorus] applied as the harvest goes on.”

Schaefer works extensively with retailers to help them implement the 4R Nutrient Stewardship principles of Right Source, Right Rate, Right Time and Right Place.

Why Fall May Be the Best Time to Apply K

Other experts insist fall is the best time to sample and apply K and P fertilizer. In fact, Dr. Emerson Nafziger, Professor Emeritus of the Crop Sciences department at the University of Illinois, says soils are the driest and fertilizer application is most easily accomplished with less compaction in the fall. “Plus, the freeze-thaw action caused by temperature fluctuations between fall and spring can help move surface-applied K into the soil and root area,” Nafziger says.

Fall application also offers reduced workloads, better fertilizer product availability, and the opportunity to spread K before fall tillage. In addition, according to Mike Staton, Senior Soybean Educator at the Allegan County Extension Office with Michigan State University, it’s tailor-made for two conditions:

• No-till soybeans in the spring
• Minimal and Conventional Tillage soybean

Applications of K in the fall in no-till soybean allows for the K from fertilizer to move deeper into the root zone when soybeans are planted in the spring.

When fertilizer is applied in the fall before minimal or conventional tillage practices, then K can be incorporated deeper in the root profile. A good example, Staton says, is when a chisel plow is used on corn stalks following harvest in a corn-soybean rotation.

“The chisel plow will incorporate the fertilizer uniformly throughout the soil to a depth equal to one half the chisel plow’s operating depth,” Staton says. “The deeper placement could prove beneficial if surface soils become dry, thus reducing potassium diffusion and uptake.

While fall application of K works well in most scenarios, Staton recommends avoiding it on some soil types due to an increased risk of leaching. These types include organic soils and coarse-textured soils that have a cation exchange capacity (CEC) of less than 6 meq/100 grams.

Spring Application Recommended for Sandy and Organic Soils

“Growers in this situation should plan to apply K fertilizer in the spring, prior to planting, and incorporate it with their primary or secondary tillage,” Staton says. In addition, surface applications are effective in no-till situations.

Other researchers, like Dr. Antonio Mallarino of Iowa State University, also recommend fall soil sampling. However, most agree that sampling should occur at the same time of year (spring or fall), and nutrients should be applied with highest potential of crop uptake based on the results of the soil analysis and by following state guidelines for nutrient management.

In the South, Skip Fall Application

In the southern states, fertilization timing can vary by state. “Across the Coastal Plains and Southeast, we do things a little differently than other areas, even the mid-South,” says Glen Harris, Professor and Extension Agronomist at the University of Georgia.

For the most part, southern growers do not apply fertilizer in the fall because they would lose much of it before spring. In fact, Georgia has sandy soils low in organic matter and high in acidity, which results in low fertility overall. Due to these conditions, growers in the South feel the need to conduct a soil test every year — with priority given to pH and K, because levels can drop quickly between growing seasons, Harris says.

In an effort to determine the optimum application time in this part of the country, Harris has considerable work experience with K in cotton, crunching numbers on fertility recommendations for best ROI. Harris says they indicate that investing in fertilizer to achieve a higher yield target is a smart strategy.

Harris is working on pushing the yield boundaries in Georgia cotton with higher K applications. The results of his research? Yields were increased enough to cover the cost of the additional fertilizer applied; that is increased ROI.

Can Higher Rates Ensure Potassium Isn’t the Limiting Factor?

As yields are pushed to 3.5 and 4 bales per acre with higher K applications, the potential for returns above the cost of fertilizer decrease. However, higher rates could ensure that K is not the limiting factor in cotton production. Harris’s research showing higher cotton yields could lead to changes in the fertilizer recommendations set forth by the university.

While the cotton research is ongoing, Harris and the University of Georgia team have developed yield goal plans for corn that include recommendations for nitrogen (N), as well as P and K. “We think we know what kind of N-P-K applications growers need for different yield goals,” he says.

Harris’s research is discovering that in order to hit those yields, growers cannot reduce K rates, even when budgets may be tight. Simply put, to achieve yield targets, farmers need good soil fertility as determined by regular soil testing. In general, farmers need to remember that as yield goals increase so do nutrient requirements.

“If you apply at that rate, the goal is to make sure fertility is not going to be your limiting factor,” Harris says.

For farmers in the Corn Belt and southern states, where weather and soil conditions vary dramatically from season to season, fertilization is one of the factors that growers can control.

To achieve maximum nutrient uptake and ROI from fertilization, it’s critical for farmers to follow a consistent soil sampling process to determine nutrient needs unique to their fields and crops. Then growers can follow recommendations from crop advisers and state guidelines to determine the fertilizer investment and application rates that best meet each farm’s business goals.