I am excited to welcome back Dr. Tuesday Simmons for the third "Tuesdays with Dr. Tuesday" monthly issue. Dr. Simmons will be sharing her perspective on the importance of the soil microbiome to agriculture and the development of new technologies over the next few month.
You can read her full bio at the end of this post.
Soil is a living, breathing ecosystem. As such, it can be classified on a spectrum from “healthy” to “unhealthy”, and where it falls has a direct impact on crop growth (1). But how do we measure soil health?
In this article, we’ll dig into why measuring soil health isn’t so simple, the different aspects of soil that we can measure, and how these metrics can be interpreted to improve soil management.
The complexities of soil health
Much like human health, it is difficult to boil down soil health into a single metric. Doctors might evaluate a patient’s overall health based on BMI (body mass index), but there is so much more to our wellbeing than our height and weight ratio.
The Soil Health Institute (SHI) launched the North American Project to Evaluate Soil Health Measurements (NAPESHM) to understand different soil health metrics, and they determined which are most necessary for classifying a soil’s health (2).
There are many different aspects of soil that we can measure (and I don’t aim to be comprehensive here). These fall into three different categories: physical characteristics, chemical, and biological.
Chemical and physical soil properties
Soil tests became commercially widespread around the mid-1900s, when many farmers were gaining access to chemical fertilizers (4). These “traditional” soil tests measured soil fertility (nutrient content), evaluating levels of macronutrients (N, P, and K). Labs then began to offer measurements for micronutrients, salinity, and pH.
While all important for soil management, none of these individual metrics can provide a good marker of overall soil health.
In addition to standard chemistry tests, many soil labs now offer evaluations of soil carbon (C) and soil structure. Carbon tests include Total Carbon, Total Organic/ Inorganic Carbon (TOC/ TIC), and Active Carbon (aka POXC - Permanganate Oxidizable Carbon).
Organic C levels in the soil are important because this is a food source for the below-ground ecosystem, which includes beneficial insects and microorganisms (see my first post in this series for an intro to the soil microbiome). Organic C was found by the SHI to be one of three metrics that can be used together for the simplest picture of overall soil health (2).
Physical properties that have an impact on crop productivity and can be measured in a lab include: water holding capacity, compaction, bulk density, and aggregate stability. Soil aggregates are soil particles that are stuck together, and their stability is defined by how well they remain stuck together when disturbed by forces such as tilling, water, or wind.
This metric is another found by SHI to be significant to overall soil health (2), and it is related to soil organic matter and biological activity.
The living soil: biological properties
In the past couple of decades, measurements to understand soil biology have become commercially available. These can be sorted into two types of tests: those that identify the types of microbes present and those that measure their impact on the soil.