Farmers are likely to spend less money on fertilizer this spring than they did in 2008. But growers may still be tempted to reduce the rates of some nutrients and increase the rates of others as they begin planting this spring.

That could be a mistake, according to agronomists who say providing the proper balance of nutrients is critical to maintaining economical yields. “Even with the lower N prices, failing to supply the recommended amounts of P and K means you could waste some very expensive nitrogen,” says one.

No matter the price outlook, farmers should not just indiscriminately reduce use of phosphorus and potassium because those nutrients will help make sure the higher-priced nitrogen is diffused efficiently, says another.

“All three nutrients are equally important,” said Terri Roberts, president of the International Plant Nutrient Institute. “As a matter of fact, cutting back on fertilizer below ideal rates does not reduce seed, pesticide, fuel, rent or taxes, but it will reduce yields and decrease profits.”

The facts are plants need a proper balance of nitrogen, phosphorus and potassium. While nitrogen and phosphorus are essential for converting energy into new plant material, potassium plays a key role in regulating plant growth.

The critical level for phosphorus in the soil for many plants is 20 parts per million. If soil P drops below 20 ppm, yield losses can be severe, according to research by IPNI scientists. Example: A corn field testing 10 ppm that does not receive added phosphorus-containing fertilizer could yield as low as 80 percent of a field that tested above 20 ppm.

The critical level for potassium for most soils is 165 ppm. A field testing 100 ppm that did not receive potassium fertilizer would be expected to yield 85 percent of a field that was above the critical level, IPNI scientists say.

Using corn as an example again, a 180-bushel corn crop requires 240 pounds of K2O or other potassium-containing fertilizers per acre. More than 50 percent of the total potassium is taken up by the plants in the first 50 days. (Potassium requirements vary widely, ranging from 240 pounds for corn to 85 pounds for wheat.)

The higher uptake early in the season occurs because potassium is needed for the formation of a larger, deeper root system, reduction of water loss and wilting, regulation of nitrogen uptake, increased protein content and reduction of lodging from weak stalks.

Potassium deficiencies often show up as slow plant growth or poor response to nitrogen. The leaf edges of the plants turn brown, but the midrib stays green. Growers also see thin stands and poor vigor in their plants.

A study by agronomists at Ohio State University indicates corn yields were reduced by an average of 44 bushels per acre when high levels of nitrogen were used on fields with inadequate K levels (from 211 bushels with an N rate of 180 pounds per acre to 167 bushels with an N rate of 280 pounds per acre. Soil test K was at 139 ppm for the higher yield and 80 ppm for the lower.)

What about soils testing high in K? Can growers simply “coast” for a year or two and begin rebuilding K levels when prices moderate? It may be possible for farmers to get by for a year on soils high in potassium, but results may vary by crop and by environmental conditions.

Plants can take up astounding amount of K in a season. As they near pollination, corn plants can remove 15 pounds of K2O per acre per day. Only a small portion of this amount winds up in the grain, and if farmers are removing stalks or foliage for silage or hay or cellulosic ethanol production, they can deplete the potassium from their soils rather quickly.

“If the soil test indicates a soil K level of 100 ppm, and the desired K level for no yield reduction is 165 ppm, most universities will recommend about 180 pounds of K2O or the equivalent be applied,” says one agronomist. “If the grower only applies a maintenance level of 30 pounds, for example, the soil K level could drop by 3 ppm per year.

“On a field testing at a soil K level of 300 ppm, the absence of a K application might not result in a yield loss, but you could still see the soil K levels drop by 6 ppm per year. Deficiencies can also occur in cool, wet years before the soil K becomes available. It may be better to apply a minimum of 30 pounds of K2O to ensure adequate levels of K to the plant.”