Soils harbor more diverse microbial populations than any other habitat on earth. Only a very small fraction of those organisms are responsible for any type of plant or animal disease. In fact, the vast majority of these microscopic soil organisms are highly beneficial in terms of nutrient cycling, soil tilth, and soil health. Because of their important roles in these crucial soil properties and their direct interactions with plants, beneficial soil microorganisms are also absolutely critical to soil fertility and plant nutrition. Unfortunately, the rapid acidification of soils in the inland Pacific Northwest is having detrimental impacts on the populations and effectiveness of beneficial soil microorganisms.
Every teaspoonful of soil typically contains hundreds of millions of microorganisms, including bacteria, fungi, protozoa, and nematodes, the majority of which are absolutely essential to healthy, productive soils (Chaparro et al. 2012). Microbes in soil are important to healthy soil processes and good soil quality. Many aspects of most nutrient cycles are controlled by soil microbes. For example, without microbes, organic matter decomposition simply wouldn’t occur, legumes would not be able to fix nitrogen, and ammonia would not be converted to plant-available nitrate. Without important beneficial soil fungi, most plants would be much more limited in their ability to acquire nutrients and water from the soil, resist drought, and produce economically viable yields. Additionally, soil microbes also play a key role in the breakdown and degradation of a huge number of widely used herbicides (Forlani et al. 1999). (For more on herbicides and soil acidity, please see How Soil pH Affects Activity and Persistence of Herbicides in this series.)
As shown in Figure 2 of Soil Acidification: Implications for Management in this series, soil acidity influences many chemical and biological characteristics of soil, including availability of nutrients and toxicity of metals (McBride 1994), which can also affect microbial communities in many ways (Sylvia et al. 2005). The silt loam soils of the Palouse region, and most soils around the world, are composed primarily of aluminum-silicate minerals. These minerals are solid or crystalline at neutral pH (a pH of 7), but exhibit pH-dependent aluminum (Al) solubility. This means the amount of Al available to plants and microbes in a soil increases dramatically as soil pH drops below roughly 5.5 (Figures 1 and 2). Because Al can cause plant toxicity, the effects of soil acidity on crop yields are, in large part, due to Al toxicity in acid soils (Foy 1984).
Acidity, Microbes, and Nutrient Cycling
Beneficial soil microbes and plants prefer a near-neutral pH range of 6 to 7, so increased soil acidity is often accompanied by shifts in the types of microbes in soils and their activities. This means significant changes in the rate of decomposition which can lead to immobilization of basic nutrients and decreased nutrient availability to plants (Figure 1).