Soil Acidity Impacts Beneficial Soil Microorganisms

Soil Acidity Impacts Beneficial Soil Microorganisms

FS247E
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Dr. Tarah Sullivan, Assistant Professor, WSU Department of Crop and Soil Sciences, Victoria Barth, USDA Farm Service Agency, Prosser, WA, Dr. Rick Lewis, Postdoctoral Research Associate, WSU Department of Crop and Soil Sciences
Beneficial soil microorganisms are also absolutely critical to soil fertility and plant nutrition, but soil acidification in the Pacific Northwest threatens their effectiveness. This publication outlines the response of beneficial soil microbial populations to acidification and how understanding this response may provide valuable in enacting sustainable approaches to soil acidification problems.
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Abstract

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.

Introduction

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).

Figure 1. Importance of microbial communities in decomposition of organic matter and release of nutrients from organic compounds in soils and rooting systems of plants. (Original artwork created by Ricky W. Lewis, PhD)
The structure and function of the soil microbial community can be both directly and indirectly affected by soil pH. The direct interaction of hydrogen ions (H+) ,which are at high concentrations when pH is low, with microbial cells may influence microbial communities in a number of ways, including disruption of cell membranes, altered enzyme production, and limited reproduction. This equates to reduced overall microbial function towards the health and productivity of soils (Birgander et al. 2014).  Also, soil fungi are favored by

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Copyright 2017 Washington State University

Published Febuary, 2017

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