Soil Management

Restoring Iowa’s Nutrient-Tapped Soils

eKonomics News Team

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If there was ever a time to think about bumping up potassium (K) and phosphorus (P) fertilizer applications, this is it. There are two very good reasons for this: soils are low on these key nutrients, thanks in part to high yields; and the affordability for a farmer to apply K and P fertilizers is as good now it has been in recent years.

Heavy Feeding

Iowa’s growers have pulled record crops off their fields the past few years, reports Charles Brown, Farm Management Specialist at Iowa State University (ISU). In fact, in 2016, the state had the highest corn and soybean yields ever posted: corn averaged 203 bushels per acre, while soybeans reached 60.5 bushels per acre. “With those high numbers, you have to look at higher levels for replacement,” Brown says.

“I believe growers are aware of their soil test levels, but may not realize the amount of P and K they are removing with the bushels of corn and soybeans they harvest,” says Joe Thelen, Iowa Field Representative for Midwest Laboratories, a leading soil testing company in the region. “At a minimum, growers should at least apply the P and K that was removed with the grain harvest each year.”

One standard way of calculating the minimum replacement rates is using our Nutrient Removal Calculator. For example, one bushel of corn will remove about 0.38 pounds P and 0.27 pounds of K. One bushel of soybeans removes about 0.84 pounds of P and 1.30 pounds K.

Ideally, growers would incorporate a build and maintain approach into their K and P management practices. In this scenario, soil test levels would be built up above the critical level for both K and P – above which we would expect to see a fertilizer response – and crop removal rates would be applied every year. (See Fertilization: Comparing The Maintenance Approach vs. The Sufficiency Approach for more information.)

Soil Numbers

Let us quantify the situation a bit more. Midwest Labs’ Thelen reports that average soil test potassium levels from 2012 to 2016 have ranged from 193 ppm to 231 ppm – with 193 the average value for 2016. “I like to see the K test values stay above 200 ppm, but with good management, a grower can achieve high yields at levels lower than 200 ppm,” he says. “However, keeping K levels above 200 also offers a better chance at a good crop in adverse weather conditions such as drought.”

As for phosphorus, he says from 2012 through 2016 average soil test values ranged from 30 ppm to 34 ppm – with the average value for 2016 landing at 32 ppm. In fact, P test levels have been trending at about this level since 2000.

Especially concerning are the latest findings by the International Plant Nutrition Institute (IPNI) that show 45% of Iowa soils are testing below the critical level for P, and 45% are testing below the critical level for K. This data, along with a first-of-its-kind Nutrient Balance Analysis, is available for most states on the eKonomics website.


ISU’s Brown has seen some growers with high P and K levels cut down on applications. “Back in the good years of 2008-2013, a lot of farmers had money to spend, so they really built up their P and K. They’ve had that bank out there in their soil,” he says. “But they’ve been tapping into it by either cutting back P and K applications or maybe even skipping a year on them.” He says the practice has not been widespread, but some farmers are suffering the consequences.

“Growers with nutrient banks can cut without hurting yields. But farmers who have just been putting on replacement rates every year – or who are showing low P and K soil test levels – can’t afford to skip any applications,” he emphasizes. His team has calculated costs for putting down the maintenance level of P and K: at $3.50/bushel corn, growers need about 13 bushels to pay for applications. At $9 per bushel soybeans, it only takes about 4 bushels.

“You have to weigh whether you’re going to lose yield by not applying, and how much you’re going to lose,” says Brown, “it doesn’t take a lot of bushels to pay for that maintenance application. Producers need to be careful about what they are doing, but the main thing is to know what their soil tests show.”

He recommends this informed approach for farmers who rent as well as those who own property. Indeed, Iowa has “a lot” of rented fields, he says. “The struggle is that if tenants only have a 1-year lease, they certainly don’t want to apply more than what they’re going to get out of it,” he says. “That prohibits renters from trying to build up the soil, if test levels are low. If they’re leasing year-to-year they’re only going to apply enough to get by for that year and let the next guy worry about it.”

Iowa State’s Dr. Antonio Mallarino puts it simply, “Apply fertilizer! It doesn’t make economic sense to apply insufficient P and K to deficient soils, even with short-term land tenure.”

Brown’s advice: “We would prefer that farmers have a 3-year lease. They can build up those soils and get the use out of it.”

He adds a word of caution for owners of rented land. While they may build up soil nutrients themselves, a tenant could come in and mine fields – not apply any P and K – then move on. Brown’s team at ISU encourages landlords to get copies of renters’ soil test results and documentation on fertilizer applications.

Factoring in Corn Stover

Some Iowa producers will need to consider another variable in their P and K planning, says Charles Brown, Farm Management Specialist at Iowa State University: corn stover. Two new cellulosic ethanol plants, set to come online shortly in the state, have been looking for growers to supply stover. But farmers should know that removing it from fields also removes valuable nutrients these plant parts would otherwise be returning to the soil.

“Normally growers just harvest the grain, so that puts the stalk back into the ground,” says Brown. “But stover suppliers take the stalk and everything. The more stover they take off the ground, the more potassium a grower has to put back.”

For example, removing the corn stover would remove an additional 32 pounds of P and 220 pounds of K per acre. This is especially important for the substantial K removed. For more information on how much removing stover from your fields impacts nutrient removal.