Soybean School: How soil test P and K levels impact yield

We know that phosphorus (P) and potassium (K) are needed in soybeans to achieve those high yields. But what’s the right soil test level of each of these macronutrients, what happens when there’s too much or too little, and how does that level affect soybean yield?

To answer these questions, RealAg’s Bernard Tobin is joined by Dr. Laura Lindsey of Ohio State University, who recently presented on the subject at Ontario Agricultural Conference in early January.

Lindsey’s fertility research used a Mehlich III extraction to determine P levels in their Ohio soils. Levels of 20 to 40 parts per million (ppm) are a shared guideline with Indiana and Michigan as part of the tri-state soil fertility guidelines.

Those P levels will be different across Canada depending on the soil, but with that being said, Lindsey’s research contains some interesting results comparing P levels below target levels.

“If soil test P falls below 20 ppm, it could be limiting yield and then the lower you get, the more likely you are to be limiting yield,” says Lindsey. “In our research in Ohio, we were able to overlay yield maps to different areas of fields where we tested soil test phosphorus, and when we fell below that 20 parts per million, we saw about a seven bushel per acre yield difference in soybeans, compared to areas that had at least 20 ppm.”

See the full video for graph representation of Lindsey’s results, story continues below:

“When you look at soil test K, you also have to look what that is in relationship to your cation exchange capacity,” Lindsey says. “With our data, we looked at soil test K critical levels, but in relationship to that soil CEC.”

Lindsey’s research compared soil test K at the critical level and below the critical level, like they did with soil test P and found about a four bushel per acre yield difference, when K levels were below the critical level.

In Lindsey’s research, having more soil test P and K isn’t better — the maintenance range guidelines say apply fertilizer for what you want to remove or export off the field in the crop.

“When we exceed that maintenance level and add more fertilizer on anyways, we see very little benefit there,” Lindsey says , adding that there’s no significant difference to yield when excess P and K was applied.

Building and maintaining may make a lot of sense if you’re the landowner, but if you’re renting or short-term leasing, applying only what the crop needs for P and K will be a better approach. Knowing what the limitations are in your fields — whether P is low, or K is low, high weed pressures, or if soybean cyst nematodes are present — and what the particular problems are will help achieve those yield goals.

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