Farm Progress

Cropping Systems: Recent ISU studies provide a better understanding of soil pH and lime application.

Terry Basol

November 20, 2018

6 Min Read
APPLYING LIME: Liming will make the nitrogen, phosphorus and potash already present in the soil more available for crop plants.

Hard to believe it’s that time again. Corn and soybean harvest is pretty well wrapped up throughout Iowa. As we contemplate everything that needs to be accomplished before planting next spring, one of the most important tasks is to soil sample.

This is done to determine current fertility levels in each field, assess soil pH, and determine if and how much fertilizer to apply. It also helps us decide if a lime application is warranted to optimize crop production in upcoming growing seasons.

Due to continued crop production practices and the inherent properties of the parent soils of Iowa, liming is a necessary practice for most soils to obtain optimum yields. The ideal soil pH range is anywhere from about 6.5 to 7.2 for Iowa crop production. In this range, all the necessary nutrients are available for best plant growth.

Soil pH
Understand that pH is a measurement of the concentration of hydrogen (hence “pH”) or H+ ions. It is measured on a 0 (extremely acid) to 14 (extremely alkaline) scale, with 7 (the concentration of H+ ions at room temperature in pure water) being neutral. Both extremes are damaging to plants.

Also, pH is expressed on a base 10 logarithmic (log) scale, which means there’s a tenfold difference between numbers. So, pH 4 is 10 times more acidic than pH 5; 100 times more acidic than pH 6; and 1,000 times more than pH7. Therefore, as we get below a soil pH of 6.5, the hydrogen concentration logarithmically increases as we approach a soil pH of 5 or lower, thus becoming much more acidic.

Effect on plant growth
Acidic soils can affect plant growth both directly and indirectly. These affects include:

 decreasing the availability of nitrogen, phosphorus, potassium, sulfur, molybdenum, magnesium, boron and calcium to plants

 increasing aluminum and manganese toxicity to plants

 reducing nitrogen fixation by legumes

 decreasing biological activity of soil microbes and thus reducing recycling of nutrients

 suppressing root growth and the plant’s ability to take up water and nutrients

Causes of high soil acidity
Some of the causes of increased soil acidity:

 natural rainfall, and the leaching and uptake of cations, as well as acid rain

 application of ammonium or ammonium-forming fertilizers, such as anhydrous ammonia, urea, UAN, DAP, MAP and manures, particularly swine manure due to high ammonium content (The process of changing ammonium to nitrate by microorganisms produces hydrogen cations.)

 application of elemental sulfur and ammonium sulfate, as sulfuric acid is produced in the process

New research answers questions
Antonio Mallarino, Iowa State University Extension soil fertility specialist, continues to conduct lime-related research across the state to help answer questions posed by the ag industry and producers. Mallarino spoke at the fall 2018 field day at the Northeast Iowa Research and Demonstration Farm near Nashua. He shared some of the new field research findings regarding lime.

One study was comprised of 14 fields (in 10 Iowa counties) with 10- to 20-acre plots (replicated strip-trials) in a corn-soybean rotation, from 2007 to 2012. In this study, Mallarino looked at the lime effect on soil pH over time, corn and soybean yield response to lime application, and the lime effect with depth in no-till (comparing 0 to 3 inches vs. 3 to 6 inches), with an application of 3-ton-ECCE (effective calcium carbonate equivalent) lime per acre.

His research findings show the speed at which soil pH is increased after lime application is much faster than expected. Soil samples were taken one, two, three and four years after initial lime application. Using the 14-strip trial average, the initial soil pH before application was just over 5.6 and right at 6.4 a year after. The second year after lime application, it slightly rose again before starting to decrease in the third year after application. This is also true of soil pH decline (it decreases quicker than we think), especially on continuous corn, as nitrogen is applied every year.

When to expect a yield response
Research also shows a significant yield response to lime application for both corn and soybeans when soil is less than 6.0 pH. Small yield increases also were observed when soil pH was between 6.0 and 6.4 but only in fields with soil having high-pH calcareous subsoil, which is common in north-central and western Iowa.

It’s important to note, when lime was applied to calcareous soils, yield reductions were observed in both corn and soybeans. Another finding from this research was that a lime application is equally important for both corn and soybeans, not just soybeans.

In no-till, the major lime effect is in the top 3 inches of soil with much smaller and shorter lasting effects in deeper depths. Because of this, it’s recommended to only sample the top 2 to 3 inches for no-till, and for forages used for hay or pasture.

If a sample is taken from the 6-inch sample depth, the lime rate can be roughly approximated by applying one-half of the recommended amount. For those managing the field with tillage, soil-sampling depth should be from a depth of 6 inches.

For more information on sampling soil, refer to the publication available from the ISU Extension store titled Take a Good Soil Sample to Help Make Good Fertilization Decisions.

Types of liming products
Does the type of liming product make a difference? In another study, Mallarino conducted six two-year trials (2015-16) on pelleted lime in corn-soybean rotations. Products compared were pure calcium carbonate, calcitic aglime and pelleted lime (all incorporated in the fall). Soil samples were taken 4.5 and 12 months after lime application.

The soil samples taken 4.5 months after application showed that the pelleted lime and calcium carbonate took less time to increase soil pH than the aglime did, but after 12 months, they were all essentially the same, with the aglime adjusting the soil to the same pH as the other two products. In this study, there was no difference in corn and soybean yield with any of the products.

Just as soil sampling is an important procedure for checking on fertility levels in your field, it’s equally important to assess your soil’s pH to determine if liming is needed. For most soils in Iowa, soil pH is reduced over time to levels that inhibit optimum crop growth by natural and chemical soil reactions due to rainfall, leaching and application of nitrogen fertilizers (manure and man-made).

It has been proven that applying lime is just as important for corn as it is for soybeans, and the rate of soil pH increase (adjustment) after lime application occurs faster than originally understood, as well as the rate of soil pH decrease. Because of this, it’s important to check soil pH on timely intervals to ensure crop production is at its optimal yield potential.

Mallarino will be sharing his latest liming research at our upcoming winter meetings; we look forward to seeing many of you there.

Basol is an ISU Extension field agronomist based at Nashua in northeast Iowa. Contact him at [email protected].

About the Author(s)

Terry Basol

Terry Basol is an Iowa State University Extension field agronomist at Nashua in northeast Iowa.

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