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Identifying Indicators for Soil Health

Identifying Indicators for Soil Health

Breaking down our knowledge of soil enzymes

Post written by Marshall McDaniel, Assistant Professor in the Department of Agronomy at Iowa State University

As mentioned in a recent Washington Post article, there is a zoo beneath our feet in the soil. There are three properties of the soil, which are physical, chemical, and the biological properties. The emphasis on soil biology is, in large part, what separates soil health from the concepts of soil quality and the physical properties of soil (also known as soil tilth).  After all, only something that is living can be healthy (or unhealthy).  Many soil organisms are like us humans in that they require carbon as their main source of food in order to grow and reproduce.  Extracellular enzymes are proteins produced by microorganisms in soil to acquire carbon and nutrients from soil organic matter.

The McDaniel Lab was one of five to receive the Soil Health Literature and Information Review Grants from the Soil Health Institute. We will do a quantitative literature review on two of these enzymes – beta-glucosidase and polyphenol oxidase.  Beta-glucosidase can generally be thought of as being used for easily broken down, or labile, forms of soil carbon, and polyphenol oxidase for recalcitrant carbon.  In other words, think of labile carbon as a buffet of ‘yummy and healthy’ food that is nutritious and easy-to-digest for soil microbes, while recalcitrant carbon can be thought of as the equivalent of broccoli stems to human digestion.  We want to manage soils so that there is a large amount of the ‘yummy and healthy’ soil carbon for microbes to eat, and less of the ‘broccoli stems’.

Where the enzymes come in is that soil microbes will produce more of the beta-glucosidase enzyme if there is more ‘yummy and healthy’ forms of carbon in the soil, because it helps them to metabolize this form of carbon.  Conversely, if all you have left in the soil are ‘broccoli stems’, then as a soil microbe you are going to produce more polyphenol oxidase to metabolize this difficult to break down source of food.  Therefore, the ratio of these two enzymes holds promise as a good biological soil health indicator since it is an index of supply-and-demand for ‘yummy and healthy’ microbe food over ‘broccoli stems’.

What does this have to do with water?

Soil health and water quality go hand in hand. Improved soil health has the potential to increase water infiltration, increase water holding capacity, decrease surface runoff, decrease soil erosion, increase nutrient retention in the soil for plants, and more. By improving understanding of our soil biology, we can both better serve our natural resources and crop production.

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