Soil Management

Missouri Growers Urged to Check K and P in Soils

eKonomics News Team

Share This:

Missouri’s recent run of outstanding corn and soybean yields is reason to celebrate. For instance, in 2016, yields hit 163 bushels per acre; the second highest level in state history (186 in 2014). Soybean yields averaged 49 bushels – a new record high.

Experts predict the successes will only continue for growers who, mindful of the increasing prevalence of soil deficiencies, are committed to applying adequate potassium and phosphorus fertilizer.

Soil Test Levels are Revealing Inadequacies

Dr. Manjula Nathan, Director of the University of Missouri Soil and Plant Testing Lab, shares some interesting trends her staff is seeing from field data gathered over the past decade. The lab handles about 25,000 to 30,000 soils samples each year.

Nearly 65 percent of samples tested at the Missouri lab are of concern due to low K levels.

The soil test summary for K shows that about 20% of the K tests fall within the ‘low’ range (below 55 ppm or 110 lbs. of K per acre), while 45% fall in the ‘medium’ range (56 to 110 ppm of K or 112 to 220 lb. of K per acre).

These percentages show that, from both a sufficiency and a ‘build-and-maintain’ approach, K fertilizer would be required to avoid profit loss by major crops, Nathan explains.

The summary for P shows that about 40% to 50% of the samples received have ‘low’ levels of P (less than 11 ppm P or 22 lbs. of P per acre), indicating P fertilizer is essential to avoid profit loss by major crops because they test below the critical level. Another 25% test at the ‘medium’ range (12-22 ppm P or 24 to 44 lbs. of P per acre), suggesting P fertilizer may be required for economic production, in both the sufficiency and built-and-maintain approach.

The latest findings by the International Plant Nutrition Institute (IPNI) that show more than one quarter (27%) of Missouri soils are testing below critical levels of K, and 54% are testing below critical levels of P. Details on this work, as well as a first-of-its-kind nutrient balance analysis exclusive to eKonomics.

What to Do

Nathan says that during these economically challenging times, her staff has received calls from growers about the merits of cutting K and P fertilizer rates. “Our fertilizer recommendations for P and K have a maintenance component and build-up component,” she says. “Cutting down the build-up component can be a possible option for growers during financially tough times, but in the long run it will have negative effects. Producers will be depleting the soil P and K pool eventually, resulting in bigger yield losses.”

Applying fertilizer now is especially practical, says Ray Massey, Extension Professor at the University of Missouri Department of Agricultural & Applied Economics. Fertilizer prices are at almost 10-year lows.

“I’m looking at farmers today and telling them, ‘if you want to put on a build-up, now may be a very good time because you’ve got low P and K prices. They’re not always going to be so good,’” he says.

Growers often respond by admitting that because of low crop prices, cash flows prevent increasing applications. “I find farmers frequently decide whether or not to do a build-up based on cash reserves rather than how efficient it would be in helping their yields,” says Massey. “That’s understandable but dangerous.”

Soil Testing

Massey also advises producers that when they decide not to fertilize, they’re taking a risk. They may actually have adequate nutrient levels in their soil and only need to apply removal rates (as in a build-and-maintain approach).

“But you don’t want to take that risk out of ignorance,” he says. “I would suggest before you put yourself in danger of losing one bushel of corn, you do the soil test and then you’re making a decision based on knowledge rather than saying, ‘I just don’t have the money right now.’”

Indeed, Nathan’s advice to growers is to follow the soil test-based recommendations – rather than guesswork – to optimize production.

Of Cotton and Targets

Corn and soybeans aren’t the only Missouri crops at risk for losses due to soil K and P deficiencies. This summer, Dr. Gene Stevens, Extension Professor at the University of Missouri, saw several cotton fields in the southeast part of the state with leaf chlorosis symptoms indicating potassium deficiency – yellowing of the leaf tip following the edge back to the base in lower leaves on the plant.

Ultimately, potassium deficiency causes cotton to defoliate prematurely, reducing yields. Some research has shown that rescue K applications at the boll growth stage can increase yield. However, K fertilizer application is needed to insure no deficiencies in the future. Therefore, farmers are studying past soil test results from those fields to determine where insufficient K levels are occurring in these fields.

Growers need to know the critical target level for soil test K, says Stevens. “If the soil drops below the target, fertilizer is needed to avoid yield losses.

Our critical level for most row crops is:
Critical level (lb K per acre) = 220 +5CEC (Cation Exchange Capacity).

“That’s pounds of K, not K2O,” he notes. (Note that some labs report in elemental K and P vs. K2O and P2O5. Be sure your soil test is clear about what it’s reporting. If you’re ever unsure, do not hesitate to confirm with the lab). Stevens adds that soil type plays a role in K recommendations. Target soil K is higher for clay than sandy soils.