Dean White, Water & Soil Resources Technician, Lincoln County CD
150+ growers and consultants concerned about soil acidification in the Palouse attended a recent workshop hosted by Washington State University. Speakers included scientists from WSU, the University of Idaho, the U.S. Department of Agriculture, and the Latah Conservation District, along with faculty and Extension agents from WSU and University of Idaho. The workshop on soil acidity focused on providing the most current research on low pH soils of the Palouse and engaging participants, and to support area farmers and industry professionals in navigating the management of low pH soils. The agenda included: how low pH affects soil health; managing to reduce acidification; a panel discussion focused on soil microbes, herbicides, and general agronomy; the economics of soil acidity on the Palouse; several presentations on liming - determining lime requirement, lime materials, how much to apply, and results of several local field trials
As the pH of the Palouse’s soil decreases, the concern of farmers and researchers has increased. Soil acidification in the top 0-6” of cropland soil is widespread across the Washington and Idaho Palouse.
Soil Acidity - WSU Update
Below are some of the takeaways of the workshop -
The primary cause of soil acidification is the application of ammonia based fertilizers to the top 0-6” of soil to increase crop yields. Soil bacteria convert ammonia fertilizer to ammonium and nitrate through the process of nitrification. Hydrogen H+ ions, which are produced as a byproduct, lower the soil pH and acidify the soil. Soil pH is known as a Master Variable because it affects soil chemistry, soil structure and soil biology. For wheat, 50% of roots are in the top 8-10” where soil is most acidic; 70% of roots are in the top 24”.
The good news: The application of finely ground agricultural lime (calcium carbonate) will raise the surface soil pH and will convert toxic aluminum to non-harmful forms. The bad news: Applying ag lime is expensive. Soil pH indicates if ag liming is needed. Buffer tests are needed to determine how much ag lime is needed. Native soil pH in the Palouse is 6 to 7. Cropped fields in the Palouse now have pH as low as 5.5 or lower. Surface soil pH < 6.2 will extend the plant back restriction periods for some commonly used herbicides. Surface soil pH < 5.8 will inhibit the beneficial soil bacterial that help to break down crop residue. For growing wheat, the soil pH should be > 5.5. Below pH 5.5, nutrient availability drops rapidly. At pH 5.0, only ½ of the nitrogen and potassium and only 1/3 of the phosphorous is available to wheat. Below pH 5.2, aluminum Al3+ content in the soil increases rapidly and is highly toxic to wheat roots. Some wheat varieties tolerate acidic soils better; their roots exude malate and citrate, which ties up Al3+.
Previously forested soils have lower % base saturation and are more susceptible to soil acidification. Once % base saturation falls below 50%, aluminum Al3+ content in the soil rapidly increases. Cropland soils should have > 50% base saturation with calcium, magnesium, potassium, and sodium dominating the cation exchange capacity (CEC) sites in soil. Acid forming cations including hydrogen, aluminum, iron (Fe3+), and manganese on CEC sites should be kept to low levels if possible.
Wheat producers need to find out how soil pH varies across their fields and how it varies with soil depth. In the future, producers using field maps of surface soil pH can precision apply ag lime to help reduce the cost.
The Washington State University Wheat & Small Grains website at smallgrains.wsu.edu contains information on soil pH and soil acidification within the Soil & Water Resources webpage under the Overview (Soil pH Video Series), Calculators (Ag Lime Label Comparison Calculator), and Publications (Soil pH & Liming) sections.