Soil health can be defined as “the continued capacity of soil to function as a vital living system”. But what does that really mean? A healthy soil is one where there are no restrictions to the soil functioning at maximum capacity. We can test a soil sample for a variety of chemical, physical, and biological parameters, and determine how “healthy” the soil is by comparing it to other soils with similar inherent characteristics. Just as we wouldn’t compare the athleticism of a champion gymnast to the athleticism of a star basketball player, we shouldn’t compare the health of a shallow sandy soil from eastern Oregon to a deep silt loam from the Willamette Valley. The OSU Soil Health Lab is dedicated to providing you with the information about your soil that you need to make positive management decisions for the long-term health and productivity of your soil.
Soil organic matter is the carbon-based material in soil that is derived from dead and decomposing plant, fungal, and microbial matter. Organic matter in soil is important because it can help stabilize aggregates, hold water, and cycle nutrients.
A portion of the total organic matter in the soil is readily available to be consumed by microbes. This “active” carbon is measured in the lab by determining how much C is oxidized by a potassium permanganate solution. Active C tends to respond to management changes sooner than many other indicators.
When soil microbes are growing and reproducing, they respire carbon dioxide, just like humans do. We can measure the amount of CO2 respired and use that as an indicator of the overall amount of microbial activity in the soil.
One of the most important functions of a healthy soil is to cycle nutrients. A mineralizable nitrogen test determines how much N is moved from the organic form, where it is not usable by plants, to a mineral (inorganic) form, which plants can use.
A healthy soil is able to provide water to plants. The “plant available water” is the amount of water that a soil can hold after the initial drainage after a rainfall event, and before it is so dry that plant roots can’t access the water that’s left. A healthy soil will have more water available to plants than an equivalent unhealthy soil.
The mineral particles of soil are bound together as soil aggregates. A good aggregate structure allows for good water infiltration and reduced runoff, healthy root growth, and appropriate gas exchange. We measure aggregate stability using a Cornell rainfall simulator to determine what fraction of aggregates will fall apart during a simulated rain event.
A healthy soil provides nutrients to plants. Since nutrient status can be quickly changed by the addition of mineral fertilizers, we don’t weight it too heavily in our soil health assessment.
Soil pH describes how acidic a soil is. Most plants have a preferred pH range. Outside of that preferred range, nutrients tend to be less available. The pH of a soil also affects what sorts of microbes are able to survive and thrive. Soil pH can be altered through the application of various amendments.
Texture is not affected by management changes, but it does affect how we interpret all of the other indicators. A sandy soil and a clay soil shouldn’t necessarily be judged in the same way. Texture itself isn’t scored, but measuring it makes sure that we are making “fair” comparisons when we score the other indicators.