Are P and K Deficiencies Holding Back Your Crop Yield?

In the quest for higher yields, some farmers may be missing a critical step. Nearly half of North American soils (46%) tested below critical levels of phosphorus (P) and a similar number (44%) tested below critical levels of potassium (K), according to a 2020 survey from The Fertilizer Institute.

Keith Byerly, commercial sustainability lead with Mosaic, attributes the long-term trend of declining soil nutrient levels to increased crop removal over the last 25 years: more than 25% for corn and soybeans, and about 20% for wheat. "When we see nutrient deficiencies in the field, it's because the crop removal of nutrients increases and yield goals haven't been managed accordingly," Byerly says.

The soil environment only adds to the challenge, says Jason Gregory, brand manager with Helena. "The complexity of the soil environment can lead to inefficiency in soil-applied nutrients," he says. "They get tied up, run off, leach, and can be counteracted by other nutrients in the soil. Soil pH is another big player."

As farmers strive for higher yields, effectively managing P and K is critical to achieving maximum yields and profitability.

Roles of P and K

Gregory explains that phosphorus and potassium are two of the 12 essential plant growth nutrients: phosphorus for early-stage root growth and photosynthesis, potassium for overall plant health and standability. "Many times, in fields struggling with disease issues, if you were to have taken a tissue sample, you'll see it was probably a little low in potassium," he says.

Byerly points out that potassium plays a significant role in nutrient interactions as well. "We did some research last year showing a 10% nitrogen efficiency boost when potassium is sufficient versus deficient. Those interactions with the macronutrients, as well as micronutrients, [are huge building blocks] of having strong, well-rooted, healthy crops," he says.

Managing P and K

Wade Kent, an agronomist with Nutrien Ag Solutions, compares soil to a nutrient bank, from which farmers can withdraw when revenues are tight. However, in many cases, increasing yield levels have reduced reserves. "Deciding how to invest in those nutrients starts with something as simple as a realistic yield goal," he says.

Kent advises knowing your soil test levels and not reducing application rates in low-testing soils. "If your K levels are good, maybe you can lower the rate on that," he agrees. "But if your P values are low, you can kind of adjust where your spending is."

Given the current grain market, the ability to use precision technologies and accurately replacing what has been removed bring the most value, Kent says. "We have a point for yield, so now we can accurately measure removal," he notes. "We can layer yield and soil sample data together and write variable-rate scripts."

Byerly recommends that when growers think about optimizing nutrients in their cropping system that "it's the balance of the appropriate yield goal, executing on your plans, having accurate soil tests, and using technologies to achieve balanced crop nutrition in both your challenging and your optimal areas that really gives the farmer the advantage over Mother Nature."