Managing P, K for best returns

PEORIA, Ill. — In a time of high input prices, vigilant management of fertilizer inputs is more important than ever to maximize net returns and remain environmentally conscientious.

“Corn and soybean take up relatively large amounts of phosphorus and potassium, and much of this P and K end up in the grain that is taken off the field during harvest. In order to keep soil nutrient levels from dropping over time, the amounts removed need to be replaced by applying fertilizer or manure,” said Emerson Nafziger, University of Illinois professor emeritus in crop sciences and Extension agronomist.

“In order to know how much nutrient a crop removes, we need to know how much there is in a bushel of harvested grain. We’ve been using amounts per bushel that are several decades old and whose origin isn’t clear.”

The P and K crop removal numbers based on a sample survey in Illinois is 0.37 pounds of phosphorus per bushel and 0.24 pounds of potassium per bushel of corn and 0.75 pounds of phosphorus per bushels and 1.17 pounds of potassium per bushel of soybeans.

Using those removal numbers, if a field produced 200 bushels per acre of corn in 2020 followed by 65 bushels per acre of soybeans in 2021 the accumulated crop removal of P and K over two years would cost about $196 per acre using DAP for the phosphorus replacement, according to late 2021 prices.

“If you’ve never written these down or you and your company don’t use them, it’s really time to start using them,” said Nafziger at the recent Illinois Fertilizer and Chemical Association conference.

Delay Or Not

Nafziger gave the following recommendation when deciding whether or not to apply P and/or K:

• If soil test levels in medium- to heavy-textured soils currently are at or above 25 to 30 parts per million for phosphorus and 150 to 175 ppm for potassium, applying none for the 2022 crop is unlikely to lower yields. Root-restricted conditions in no-till or dry soils next season may result in K deficiency symptoms even if soil test levels are adequate.

• If soil test levels are low (less than 10 to 15 ppm phosphorus; less than 100 to 120 ppm phosphate), applying lower rates (15 to 20 pounds each, liquid or dry) near the seed furrow at planting should protect again crop deficiency.

• P and K removed by crops will need to be replaced eventually (maybe not fully if soil test levels are high); total removals will grow; and future supplies and prices (crop and fertilizer) are uncertain.

“You can kick the can down the road, but it keeps getting heavier,” Nafziger said.

“I suppose you could certainly make the case that the different incomes per acre for this last year were about as high as we’ve seen if people didn’t sell too much ahead of time. So, this might be a good time to do this, not a bad time to. We don’t know what the price of corn, phosphorus and potash are going to be five years from now.”