Keep a Log of Soil Acidity

Applying nitrogen (N) again this year? Chances are you applied N on the same field last year — that is, you’re one of the many farmers planting more corn after corn. In the Midwest, much of the additional corn acres are coming from ground that farmers previously rotated to soybean production every other year. So now, instead of applying N once every two years, many are applying N every year.

Nitrogen acidifies the soil. Whether the source comes from urea or anhydrous ammonia, the acidifying effect is the same. A rule of thumb is that for every 100 pounds of N you apply, enough soil acidity is produced to require 225 pounds of agri­cultural limestone. Does that mean you need to apply 225 pounds? Probably not. But applying N more frequently increas­es the chances that soils will become more acid more quickly.

Soils differ in how they respond to the acidifying effects of N fertilizers. Some soils will be very sensitive, such as sands; others, like silt loams, will not change as much. If you want to find out just how much agricultural limestone your fields need, take soil samples and send them to a reputable laboratory. They’ll run a test that determines how well the soil can buffer changes. This test often appears as “buffer pH” on the soil test report. The labo­ratory compares this test result with calibration data to determine how much agricultural lime you should apply.

A ton is not a ton. Often, when growers see that their report recommends a ton of agricultural lime, they apply a ton of limestone. If only it were that easy. Here’s the twist. Not all limestone has the same chemical makeup. Some lime­stone sources contain more impurities or are not as finely ground as others. Both the purity and the grind of a particular source are used to adjust the rate from the one recommended to the one that actually gets applied. Soil test reports usually contain guidance on how to calculate this adjustment, and university extension publications do, too. The bottom line: If a source has more impurities and is more coarsely ground, you’ll need to apply more than the one ton of lime recommend­ed on the soil test report.

Soil acidity affects how plants respond to nutrient applications. Generally, when soils are too acid, crops grown on them make poorer use of the nutrients applied. So keeping soil acidity in check can improve the economic returns to other applied nutrients.

Monitor changes in soil acidity over time. If you soil-test only occasionally, consider picking out a couple of areas to sample every year. Look at how the soil pH and the recommended lime rates change with time. Consider doing this on fields that have just been limed as well as those that haven’t. Keeping a log of soil acidity will help you gain insight into how quickly your soils become more basic after a lime application as well as how quickly they become more acidic when you apply N more frequently.

Source: Dr. T. Scott Murrell, Northcentral Director, International Plant Nutrition Institute, 2422 Edison Dr., West Lafayette, IN 47906. Email:

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