Get the most from your ag lime application

Get the most from your ag lime application

Crop profit margins are tight, but if your soil test shows a low pH it will pay to apply lime on that field.

EDITOR'S NOTE: Ahlers is an ISU Extension and Outreach field agronomist who covers south central/southeast Iowa. Contact her at [email protected].

As harvest wraps up, attention turns to evaluating how things went this year and planning for 2016. Lots of decisions will be made in coming months and with tight profit margins it's important to be sure your decisions are best for your bottom line.

One factor having a big impact on crop production is proper soil pH. Soil pH affects the availability of essential nutrients as well as activity of microorganisms, aluminum and other metal toxicities, and performance or carryover of some herbicides.

SOIL TEST IS BEST: How much ag lime should you apply per acre? It depends on how acidic the soil is in the field. Also, different crops have different pH requirements. Best bet is to sample and test soil this fall and apply recommended amount.

Sample soil this fall, have it tested, apply lime if needed
Typically, we have issues with soils becoming acidic -- having a lower pH which can be attributed to some of our management practices. For instance nitrogen fertilization causes soil to become acidic. Thus, to increase and maintain soil pH we apply lime. However, before simply applying lime to your fields you need to know what your soil pH levels are to see if the field really does need lime and if so, how much. This requires sampling the soil and having it tested at a lab.

The soil test pH reading will only tell you if your field needs lime not how much lime is needed. That's where buffer pH comes in. Buffer pH estimates the reserve acidity in soil and is actually what is used to determine the amount of lime needed to raise soil pH to the desired level. Methods used to determine buffer pH have been calibrated for Iowa soils and include the SMP and Sikora buffer pH methods.

Determine your application rate field-by-field
How much lime should you apply per acre? It depends on how acidic the soil is in the field. Crops have different pH requirements as explained in publication PM-1688, "A General Guide for Crop Nutrient and Limestone Recommendations in Iowa", available from ISU Extension and Outreach.

The publication says a soil pH of 6.9 is recommended for alfalfa and 6.0 for grass pastures or haylands. Recommended soil pH for corn and soybeans depends on whether the subsoil has a high or low pH. A pH of 6.0 is sufficient for soils with a high subsoil pH. Soils that fall in the high pH subsoil category include soil association areas Clarion-Nicollet-Webster, Galva-Primghar-Sac, Moody, Monona-Ida-Hamburg, Marshall, and Luton-Onawa-Salix. These regions are mainly in north-central and western Iowa (see map in PM-1688).

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Otherwise, with other soils (soils with low subsoil pH) it's recommended to raise soil pH to 6.5 for corn and soybean production. The optimum pH values suggested in PM-1688 for corn and soybeans were updated in 2013 based on new on-farm research conducted in many Iowa fields from 2007 until 2012.

Tillage type makes difference in how much lime is needed
Tillage practices used on a field will also affect how much lime is needed, as tillage determines the volume of soil that will be neutralized by the lime. A 6-inch sampling depth is recommended if you till the soil, and the amount of lime to apply is calculated to increase the pH in this soil volume.

When collecting soil samples to be tested to find out the pH and lime requirement in no-till, strip till, haylands or pastures a sampling depth of 2 to 3 inches is recommended. That's because lime seldom is effective below a soil depth of about 2 to 3 inches, and applying the amount of lime calculated for a 6-inch depth would be too much for a no-till field, for example. Take note: This shallow sampling depth is not recommended if you are sampling and testing the soil for phosphorus, potassium or micronutrients.

However, if you take and submit 6-inch samples to a lab for P and K analysis, you can still use these same soil samples to determine lime requirements by making a simple adjustment that approximately provides the correct amount of lime to apply. You should apply about one-half of the amounts recommend for the 6-inch depth to raise the soil pH to the target level.

Knowing the buffer pH, and also the target soil pH and what soil depth you want to neutralize, you can then use Table 16 in PM-1688 to help determine how much lime to apply per acre. 

The recommendations in PM-1688 are given in pounds of pure finely ground calcium carbonate (CaCO3). But that's not typically the actual rate of lime that will be applied to the field because not all lime products are created the same. Different products come in different particle sizes and vary in fineness. To determine the actual rate of limestone to be applied to a field, you need to know the effective calcium carbonate equivalent or ECCE. All limestone quarries are required to determine the ECCE for agricultural lime they sell, according to methods and standards set by the Iowa Department of Agriculture.

As you make decisions for the upcoming growing season think about soil pH in your fields. How might the pH affect the productivity of a field? If lime is needed, how much should you apply? Lime is not a one-year investment, but rather a long-term investment. Soils that have an adequate pH or higher (based on the crop's needs) will likely not show higher yields, and you will see less of an economic return if you apply lime compared to soils that have a lower pH.

For more information on lime and soil fertility visit agronext.iastate.edu/soilfertility/NutrientTopics. Publication PM-1688 can be found on the ISU Extension & Outreach online store store.extension.iastate.edu/Product/A-General-Guide-for-Crop-Nutrient-and-Limestone-Recommendations-in-Iowa.

Doing the math
Here's how to calculate the lime requirement. This example walks you through how to figure the amount of lime to apply per acre to a corn-soybean rotation field that is tilled. Use the table accompanying this article, which is Table 16 from ISU publication PM-1688.

Soil test pH: 5.7
Buffer pH:       6.5
Target pH:       6.5
Limestone ECCE: 1,500 (1,500/2,000 = 0.75 or 75%)

The CaCO3 or limestone rate from PM-1688 Table 16 (assume 6-inch soil sample depth) is 2,800 lbs. per acre. The correction for limestone quality is (2,800 pounds per acre) divided by 0.75. Thus, the recommended ag lime application rate is 3,700 pounds per acre.

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