Low soil K is not OK

Mike Rankin

The COVID-19 pandemic shut down world economies for extended periods and continues to be largely responsible for the inflation experienced in all phases of human existence. Ramping back up hasn’t been easy. Then, of course, there is also the war in Ukraine.

Through it all, we have watched the price of most farm inputs and outputs skyrocket. A year ago, many farmers had prepurchased inputs before prices got too out of hand. That wasn’t much of an option in 2022. As of this writing, the retail price of potash is still north of $800 per ton, even though potash supplies are in good shape, according to industry reports.

When will the potash price point come down? I don’t know. But I do know that alfalfa and other forage species don’t care. Without adequate soil potassium levels, forage crops will not perform to expectations. Things may still look somewhat green from the road, but yields and persistence will suffer. I’ve seen too many potassium-deficient alfalfa fields in my life to predict otherwise.

Every crop species has its own unique characteristics. As with people, you have to take the good with the bad. Unfortunately, crops such as alfalfa, orchardgrass, and corn harvested as silage are heavy removers of potassium. For example, alfalfa removes about 55 pounds of potassium (as K2O) per dry matter ton, and corn harvested for silage removes over 8 pounds of K2O per wet ton harvested. Most of this is removed in the stover.

When you break out the abacus and start doing the math for our current situation, the bottom line isn’t particularly heartwarming. If potash is biting at $830 per ton, that equates to alfalfa removing about $37 for each dry matter ton harvested, or $185 per acre for a 5-ton total-season yield. That hurts! Is there any wonder why the price of hay is where it is?

Pumping the brakes on potash applications is a reasonable response, especially if soil tests indicate already high levels. But such a strategy can’t be employed long term, as both soil potassium concentrations and yields will crash.

Although forage species such as alfalfa are heavy potassium users wherever they’re grown, not all regions play by the same set of rules. I talked with an alfalfa grower in Idaho several years ago who put little to no potash on his fields. The parent mineral material of his soils was already rich in potassium and seemed to hold soil levels in a steady state. Lucky him.

With the economic stakes so high, having a good set of soil tests to guide a potassium fertilizer strategy has never been more critical. Both over and under application will be costly in the short term.

If soil test levels are at or below the optimum level, a plant tissue test, which is vastly underutilized in our industry, is a sure-fire approach to confirm the nutrient status of the crop. While working as an extension agronomist, I used plant tissue tests frequently to discern field issues.

Plant tissue tests rarely failed to pinpoint an underlying problem or, in some cases, confirm nutrient adequacy. This old, but valuable, tool is the most reliable way to save hundreds of dollars in fertilizer expense or prevent hundreds (perhaps thousands) of dollars in lost yield.

Potassium deficiency has become a more common occurrence in forage crops. The cost of potassium won’t alleviate that situation. Given the boom in corn silage acres across many regions, potassium removal rates for crop rotations have spiked substantially. If both alfalfa and corn silage comprise the entire rotation, massive amounts of potassium are being taken off fields. Couple an alfalfa-corn silage rotation with inadequate fertilizer or manure application rates, and it doesn’t take long for soils to become potassium deficient.

The price of potash is at historically high levels, but so is the value of forage. Cutting back on potash applications might be a prudent short-term decision on some fields or farms, but that strategy won’t be sustainable in the mid- to long term. Yields of alfalfa and other perennial forage crops will deteriorate rapidly if potassium deficiency persists. That will be true with high- or low-priced potash. Forages don’t recognize fertilizer invoices, but they easily discern when there’s not enough potassium for optimum growth.

Hope your new year is a good one!

This article appeared in the January 2023 issue of Hay & Forage Grower on page 4.

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