Don't Forget the Lime

This vital fertilizer shouldn't be neglected despite razor-thin crop margins.

Image by Jim Patrico

Fields could turn into lemons without lime.

Low commodity prices force farmers to scrutinize crop inputs, explains Antonio Mallarino, Iowa State University (ISU) soil fertility specialist. Ag lime is often first on the chopping block, he contends.

That could be a costly mistake.

Crushed or pelleted limestone, or ag lime, keeps soil pH at optimum levels. Allowing pH to wane and acidity to rise will reduce corn and soybean yields and forage tonnage when farmers can least afford it, Mallarino and other experts agree.

“You can see nitrogen and potassium deficiencies, so farmers make sure that’s taken care of first,” Mallarino says. “They get ulcers because they’re so worried about that.

“With lime, unless it’s an extreme case, you can’t see it,” he continues. “That’s one of the first things they cut.”

The crop-nutrient expert provided the latest soil pH and liming research and management strategies at more than a dozen ISU Extension Crop Advantage winter meetings. Mallarino notes it was a well-attended presentation, which indicates producers are concerned.

SOIL pH 101

Soil pH is a measure of acidity or alkalinity. It’s measured on a scale of zero to 14. Seven is considered neutral. Anything below is considered acidic and above alkaline.

Acidity gradually builds in soil. It’s primarily caused by natural nutrient leaching, ammonium or ammonium-forming fertilizers, manure with high ammonium content and some sulfur fertilizers, but not gypsum.

Farmers who use a lot of liquid hog manure and raise corn on corn should pay close attention to soil pH levels, Mallarino says.

Research supplied by the Iowa Limestone Producers Association (ILPA) says elements such as aluminum, manganese and iron become more soluble in the soil at a pH level below 5. These levels are considered toxic to plants. Low pH impairs the function of root cells, decreases nitrogen fixation by legumes and limits nutrient availability.

“A pH level of 5.5 is really bad, and it does happen a lot,” Mallarino says. “I got a call from a farm consultant in southern Iowa that said a test came back with a pH of 4.1. I don’t know how the farmer was growing any crop.”


Corn and soybean yield losses of 6 to 10% or more occur when soil pH is 5.5 and less, ISU research shows. Similar losses can occur in other states, as well, because of lime deficiencies.

Ag lime contains calcium and magnesium carbonates that react with hydrogen ions in the soil, which neutralize acidity. Here’s how it benefits crops in acidic soils:

> reduces hydrogen concentration

> maximizes availability of nutrients

> improves atmospheric nitrogen utilization in nodulated legumes

> reduces aluminum or manganese toxicity, which may become high in extremely acidic soils, usually pH5 or less

> promotes soil microbial activity

> improves soil structure.

Key barriers for crops to reach their full genetic yield potential are removed with ag lime.

“There’s no doubt, regardless of commodity prices, farmers need to apply lime when the situation warrants,” Mallarino says.


The latest ISU research indicates ag lime is needed for corn and soybean fields when high (calcareous) subsoil pH is below 6, and low subsoil pH is below 6.5. For grass/clover mixtures, apply ag lime with a pH below 6. Apply when pH is below 6.9 in alfalfa/alfalfa grass.

Periodic soil tests are needed--grid-sampling is recommended, Mallarino says--to determine pH levels. It will dictate if liming is needed but not the amount.

The buffer pH measurement determines how much ag lime is needed to raise soil pH to desired levels. Soils with clay and high organic matter have more buffering capacity for ag lime to change pH, and sandy soils have less.

Ag lime and other liming materials are tested and sold based on the effective calcium carbonate equivalent (ECCE) in Iowa. There are similar tests with different names in other states.

The ECCE measurement estimates a material’s acid-neutralizing capacity based on its chemical composition and particle size (fineness). The carbonate content of both calcium and magnesium in ag lime raises soil pH, according to the ILPA and agronomists.

Randy Olson, the association’s executive director, says ag lime is a vital component of good soil health so farmers can maximize yields.

“If producers are going to be successful in tough economic conditions, they need to make sure they’re getting value out of every dollar spent,” Olson continues. “Ag lime has proven its worth in dollars and productivity over the years.”

Recent ISU research indicates the following average yield responses in corn and soybeans when applying ag lime:

> More than 8%, soil pH is 5 or less.

> About 4%, soil pH is from 5 to 5.4.

> 2 to 3%, soil pH is 5.5 to 5.9.

> 2% or less, soil pH is 6 or higher.

Applying ag lime doesn’t pencil out when soil pH is at or exceeds optimum levels, research shows.

“If the yield response is 2% or less, with current crop prices, there’s no doubt liming will not pay in most cases,” Mallarino says.


Ag lime application rates vary based on soil pH, buffer pH, product quality and type.

ISU studies indicate that an ECCE rate of 2 to 3 tons of lime per acre can often avert yield loss for two crops even in very acidic soils. That includes if the buffer pH calls for higher amounts to achieve optimum pH levels.

Mallarino says farmers can improve profits by applying lower amounts of lime when crop prices are low and land tenure is unsure. Soil will need to be tested again in two years to see if more lime is needed.

Data shows ILPA members typically apply about 2 million tons of ag lime annually.

Ag lime supplier Lindsey Falk, president of L.R. Falk Construction Co., in St. Ansgar, Iowa, contends keeping pH at optimum levels is critical when commodity prices are down.

“Yields stay up, and chemicals, fertilizer and other nutrients are better utilized,” says Falk, who’s also ILPA president. “The great thing with ag lime: Prices don’t fluctuate much, so it’s easy to budget.”

Granulated ag lime costs about $25 per ton applied using variable-rate technology, which is commonly used, Falk says.

His customers generally apply 2 to 3 tons in the fall, when flat-rate spreading occurs every three to four years. Variable-rate application can be more cost-effective, he adds.

Farmers or agronomists email soil-test data and field maps to L.R. Falk, which are uploaded into spreader computers.

“It helps get product on fields where it’s needed and not spend money where it’s not,” Falk says.

The company says ag lime increases yields, on average, by 4.3 bushels per acre.

Longtime Sac City, Iowa, farmer LaVerne Arndt keeps close tabs on soil pH. He soil-tests every two years.

“It’s a must to maintain productivity,” says Arndt, an Iowa Soybean Association director.

He hasn’t applied ag lime the past five years. But, he will when needed.

The farm economy will play a role in the decision.

“I don’t know what the magic number is, but the last time pH was 5.5,” Arndt says. “It needs to pay.”

For More Information:

> Iowa Limestone

> L.R. Falk Construction Co

> Follow Matthew on Twitter @progressivwilde


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