Soil Acidification – A Growing Problem In Western US Soils
Alan Blaylock, Ph.D.
Dr. Alan Blaylock brings extensive North American and international experience in nutrient management to the agronomy team. University studies and service as a university extension soils specialist prepared him for a long career in the fertilizer industry. Having managed both domestic and global research and education programs, Dr. Blaylock has a wealth of experience in applying science-based nutrient management principles and products to solving practical questions. Dr. Blaylock earned Bachelor of Science and Master of Science degrees in agronomy and horticulture from Brigham Young University and a Ph.D. in soil science from Iowa State University. He has been in agriculture his entire life — from his childhood on an irrigated farm in eastern Oregon to teaching soil science at Iowa State University to his current role as an agronomist at Nutrien. These diverse experiences helped Dr. Blaylock develop the skills to excel in translating complex scientific principles into practical solutions. Although early in his university studies he explored computer science as a profession, deep family roots in agriculture brought him back to the people and values of his heritage. His career satisfaction comes from helping others improve the performance of nutrients and cropping systems.
In recent years, a growing number of reports have emerged documenting the increasing acidity of western U.S. soils. Reports have been published on acid soils in Montana, the High Plains of eastern Colorado, and several areas of the Pacific Northwest. The recent 2020 TFI soil test inventory indicates that in most western U.S. states and Canadian provinces, the number of samples testing below pH 6.0 has increased since 2001 while the number of samples testing above 7.5 has decreased in the same time. This general trend shows only small changes in median values over all samples, but this does not tell the whole story, as some areas are declining to pH levels that severely affect productivity if not properly limed. Soil pH is usually highly variable across the landscape, which may necessitate variable management or amendments across a field or farm. Soils should be sampled to identify and account for this variability.
Soil scientists in the Pacific Northwest have been noting soil pH declines since the 1980s, noting that soils in the Palouse region of eastern Washington and northern Idaho had declined by an average of a full pH unit from the native soil pH near neutral in pre-cultivation times. At that time, some areas had declined by more than two pH units to below 5.0 in the top foot of soil. In almost 40 years since then, soil acidification has continued to be a growing problem.
Soil pH of about 6.0 to 7.0 is optimum for many crops and for nutrient availability. Low soil pH can have many negative effects on crops. At soil pH below about 5.5 aluminum solubility increases, impairs root growth, and can become toxic. Manganese can also become toxic in acid soils with high manganese levels. Phosphorus availability decreases as aluminum solubility increases. Acid conditions also impair soil biological activity and reduce soil health. As soil pH drops below 5.0 to 5.5, crop productivity can become severely impaired except in the most acid-loving crops, like blueberries and some others. In areas of some fields in Montana where soils with pH below 4.0 have been observed, little if any crop growth is possible. Another result of soil acidification that has been in focus more recently is the liberation of carbon dioxide from soil carbonates, or lime, that is in the soil from native parent materials and dissolves in acid conditions.
Acidification of western soils that were formerly alkaline or near-neutral pH has been attributed to long-term use of ammonium-nitrogen fertilizers, including urea. Nitrification of ammonium to nitrate-nitrogen releases hydrogen ions into soil solution and lowers soil pH. This pH decline is gradual and may not be noticed in the short term, but over the long term can be significant.
The best solution to soil acidification is liming—applying calcium carbonate or related materials to neutralize acidity in the soil—but other practices are also recommended. There is much literature available discussing liming, determination of lime rates, and lime management. A few are cited at the end of this article. Some areas in the western U.S. do not have readily available, economic lime sources. If lime is not available, management will need to focus on other practices. Planting acid-tolerant varieties or crop species is advised. Some state land-grant universities have published aluminum tolerance ratings of local crops. Nitrate nitrogen fertilizers are preferred where soil acidity is a problem and cannot be limed. Ammonium sulfate is the most acidifying nitrogen fertilizer because it liberates the most hydrogen per unit of nitrogen. Calcium nitrate has no acidifying effect on soil pH, plus the soluble calcium can partially mitigate aluminum toxicity. Gypsum is a neutral calcium salt, is not a liming material, and does not provide pH correction, but increasing calcium availability can partially mitigate aluminum toxicity if lime is not available. Avoid excess nitrogen; apply at proper rates to minimize nitrogen’s impact.
Soil pH is one of the soil’s most important chemical properties. It affects almost every chemical and biological process in soil. Soil pH should be monitored with regular soil testing to establish long-term trends and alert a grower if soil is becoming more acid. Growers can manage pH and its effect through a variety of practices, but liming is ultimately the best long-term solution. If you have questions about soil acidification or its management, consult some of the references below or contact your Nutrien agronomist.
C. Jones, R. Engel, S. Ewing, P. Miller, and K. Olson-Rutz. 2019. Soil Acidification: Identification, Prevention, Adaptation and Restoration. In Proc. Western Nutrient Management Conf. pg 18. https://www.westernnutrientmanagement.org/files/WNMC-2019-Proceedings.pdf
Visit http://landresources.montana.edu/soilfertility and click on Soil Scoops where there are two documents on soil acidification, or click on Presentations.
C. McFarland and D.R. Huggins. 2015. Acidification in the Inland Pacific Northwest. Crops and Soils 48:4-12. https://doi.org/10.2134/cs2015-48-2-1
R. Koenig, K. Schroeder, A. Carter, M. Pumphrey, T. Paulitz, K. Campbell, and D. Huggins. 2011. Soil Acidity and Aluminum Toxicity in the Palouse Region of the Pacific Northwest. Washington State University Extension Fact Sheet FS050E. https://s3.wp.wsu.edu/uploads/sites/2076/2013/08/FS050E-Soil-Acidity-AL-Toxicity.pdf
S.H. Chien, M.M. Gearhart, D.J. Collamer. 2008. The Effect of Different Ammonical Nitrogen Sources on Soil Acidification. Soil Science 173:544-551.
W.H. Thompson, C. McFarland, T. Brown, D.R. Huggins. 2016. Agricultural Lime and Liming, Part 1: Introduction to Agricultural Lime and Liming. WSU Extension FS212E. http://pubs.cahnrs.wsu.edu/publications/wp-content/uploads/sites/2/publications/fs212e.pdf
D.M. Sullivan, D.A. Horneck, and D.J. Wysocki. 2013. Eastern Oregon Liming Guide. Oregon State University Extension EM 9060. https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/em9060.pdf