When Liming Soils Pays

The world of soil fertility revolves around nitrogen (N), phosphorus (P), and potassium (K). But soil pH is important, too.

The soil pH scale runs from 1 (very acidic) to 14 (very basic). For most crops, a slightly acidic soil proof of 6.2 to 6.8 is considered ideal.

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Over time, agricultural soils can slide further down the acidic scale. A number of reasons exist. The primary way soils become increasingly acidic, though, is through the lifeblood of crop production — nitrogen fertilizer, says Dave Mengel, a retired Kansas State University soil scientist. Mengel spoke at last week's North Central Extension-Industry Soil Fertility Conference in Des Moines, Iowa.

Mengel cited the effect of 20 years of N application on soil pH in a north-central Kansas experiment field. When no commercial N was added, soil pH stayed constant at 6.5 in both a continuous corn and corn-soybean rotation. When 200 pounds per acre of N was added to corn, though, soil pH for the corn-soybean rotation was 5.8. Under continuous corn, it was even lower at 4.8.

What To Do

Of course, you can't nix N from your crop production strategy. Fortunately, there's a solution for alleviating overly acidic soils — liming them.

Agricultural lime can:

• Reduce aluminum (Al) and manganese (Mn) toxicity that can develop in acidic soils.
• Replace the supply of essential nutrients like calcium and magnesium that become depleted as soils become acidic.
• Increase survival of microbial activity.
• Enhance activity of some herbicides.
• Restore yield potential.

Mengel cited research from the late Stanley Barber, a Purdue soil fertility specialist, that corn at 100% of yield at a 6.5 pH would garner just 89% of that yield at a 5.0 pH. Meanwhile, soybeans that yielded 100% at a 6.5 pH would drop to 82% of that yield at a 5.0 pH.

The good news is some crops outside of corn and soybeans aren't so sensitive to soil pH. "Wheat with a soil pH of 5 still may have 95% of yield (at 6.5 pH)," says Mengel.

Lime Forms

Lime comes in a number of forms, says Mengel. They include:

• Ground aglime. This crushed and ground limestone rock is the most widely used liming material in the Midwest. It's easy to transport and apply, and is normally the lowest-priced liming material.
• Fluid lime (liquid lime). This is finely ground aglime with water along with a suspending agent. Its main claim is that it raises soil pH faster than dry aglime and that less material is needed because it's finely ground. That's true, for the first few months after application. Within one year, though, soil pH changes normally will be equal for liquid and dry sources. Its main disadvantage is a higher cost than for ground aglime.
• Pelletized lime (pel lime) It nixes dust problems associated with spreading fine dry lime because it's compressed into pellets or granulated using a binding agent. It's generally one of the more expensive liming materials.
• Marl. This is soft, unconsolidated lime material made up of marine shell fragments and calcium carbonate. Marl is difficult to uniformly spread unless it's dried and ground.

Lime is hard to get in some areas like the southern Great Plains. In those areas, starter P in the row at seeding can help seedlings root through an acidic soil surface into more favorable conditions deeper in the soil profile.

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Variety selection is another way to deal with acidic soils. Mengel notes that plant breeders have developed acid-tolerant wheat varieties. He says combining banded 20 to 40 pounds per acre of P2O5 with an acid-tolerant variety has helped Great Plains wheat farmers produce high wheat yields in soils with a surface pH as low as 4.5 to 4.7.