In the PNW, the average pH in the top 12 inches of soil declined from near neutral to lower than 5.7 by 1984 after years of nitrogen fertilizer applications. Soil pH further declined to lower than 5.2 by 1995, after another 10 years nitrogen fertilizer application.
A soil survey conducted by Paul Carter (WSU) in 2014 found that 97% of surveyed minimum and no-till wheat fields in the Columbia County had a soil pH lower than 5.0 at 3-6 inch (seed/rooting zone) soil depths. Similar survey numbers have been observed in many areas across the eastern Washington dryland crop production areas.
Soil pH will continue to decline as a result of continued nitrogen fertilizer applications if no mitigation actions are taken to reverse the acidification process. In fact, our recent survey found that many grower fields or areas of fields have pH values as low as 4.0. Some farmers in Montana have reported extremely low soil pH on their farms and those fields are no longer suitable for some crop production.
When soil pH is low, soil chemistry, fertility, and soil microbes are shifted to the direction that is unfavorable to soil and crop health. This includes reduced nutrient availability, increased incidence of plant disease, reduced herbicide, and pesticide efficacy, and reduced choices for crops and varieties you can grow.
For example, Paul Carter has found increasing soil pH, through liming acidified soils, significantly increased micronutrient uptake and improved winter and spring wheat yields by 7 to 17% respectively. Tim Murray found that increasing soil pH of the can reduce some crop disease incidences.
Liming to counteract soil acidification hasn't been a common practice in the PNW due to the lack of documented crop response to liming, economically feasible methods, and locally available low-cost liming materials. For example, traditional coarse ag lime materials are less effective for reduced tillage soils because they are slow to break down.
Without tillage, it may take many years for some of these lime materials to change the acidified soil layers. More finely ground lime product applications can accelerate the rate of change, but applications of these more highly refined lime products may be less economically viable in the PNW.
Researchers at Washington State University have been conducting studies on PNW soils to improve soil and crop health and to evaluate using locally available industrial materials as alternative liming sources. The results of these projects will show you why liming is necessary and where lower cost materials might be found that have high liming values and are currently disposed of as waste.
These studies may also help in understanding product and application methods for the most economically effective results in various cropping systems.
For further information, view the Soil & Water Publications page.