organic matter

By Deirdre Griffin Lahue, Washington State University

What is “soil health” anyway? 

Soil health has become a popular topic in agricultural and environmental sectors over the last few years, and soil scientists around the world have rejoiced that others are recognizing the importance of soils to the systems on which our societies rely. However, like any term that is borne into a buzz word, we need to keep coming back to reflect on its meaning, to make sure that we are in fact striving to steward healthy soils. So, what is soil health, anyway?

Some say that soil health is just a new way of referring to soil quality. While many of the concepts and promising management practices are indeed similar, I see soil health as building on soil quality, encompassing a deeper recognition of the importance of the life in the soil, the organisms that drive so many of the functions we associate with high-quality soils. Soils are alive, and new analysis tools are allowing us to look more closely at what organisms are there, what they are doing (breaking down organic matter, making nutrients available to plants, building soil structure), and how soil management affects them. This is a cornerstone of soil health research.

How do we know if our soils are healthy?
Just as vital signs help give a picture of human health, soil health indicators are being developed to help us tell a healthy soil from an unhealthy one. These indicators include measurements of properties that are chemical (e.g. organic matter content, pH), physical (e.g. soil compaction, water holding capacity), and biological (e.g. microbial biomass or microbially available carbon). Less quantitative tests can also be done quickly and cheaply in the field (Figure 1 A and B). These properties relate to key functions we look for from soils, including supporting plant growth, cycling and storing nutrients and carbon, filtering water, and maintaining these functions in the face of disturbances (resilience).

But just as a 25-year-old man might have different target vitals than a 45-year-old woman, soil types differ in what might be “healthy” values for these indicators. Therefore, there is still a lot of work to do to define these ranges for different soils, particularly in the PNW’s huge diversity of soils and cropping systems. We also need to better understand which indicator measurements are most meaningful and useful in different situations (e.g. this paper by Morrow et al., 2016). If you have a limp, will your blood pressure tell me what I need to know about what is wrong or do I need to do more diagnostic tests? And when should measurements be done? Like any living system, soils are dynamic, and some of the soil properties we measure are constantly changing in response to the environment. Still, these measurements are a helpful guide in understanding the relative functioning of different soils.

slake test

 

shirts

Figure 1. Soils with differing history of organic matter inputs are subjected to the slake test (A, above) and the t-shirt decomposition test (B, below). These inexpensive tests provide a quick way to get a sense of the stability and activity of your soil. (Photos by Deirdre Griffin LaHue)

How do we improve the health of our soils?
At the risk of overusing this metaphor—we know that we can usually keep our bodies healthy by following a few general principles: getting regular exercise, eating nutritious foods, and avoiding destructive activities. However, our exercise routines or diet habits vary from person to person. Similarly, not every land manager can adopt the same practices into their management regime. But we can follow general principles that seem to move us in the right direction: 1) feeding the soil with some form of organic matter, be it plant residues, living roots, compost, or biosolids; 2) reducing soil disturbance, particularly when the soil is very wet or very dry and soil structure is more fragile; 3) having living roots and soil cover on the ground as much as possible.

In each system, we must think: What is the main challenge we need to combat? Which ecosystem services are most critical? How do we value the services that are provided by caring for the soil?   
The popularization of soil health has facilitated a renewed focus on soils as a central part of our ecosystems. Several US states have or are working to pass Soil Health Initiatives, including Washington and Oregon (Figure 3). Ideas of a Federal Soil Protection Act have been discussed among prominent soil scientists. The Ma ̅ori of New Zealand have elevated soils to personhood status and are using this value system to promote a soil resource management framework that can be applied more broadly. Many now recognize that soil conservation plays a critical role in combating climate change and food insecurity, and I am hopeful that we can continue to work toward finding economically feasible approaches to improve the health of our soils and value the societal benefits that are gained. 

healthy soils us

Figure 2 - Map showing Healthy Soils legislation across the US (from Soil4climate.org, https://www.soil4climate.org/news/category/healthy-soils-legislation)