What is Soil Health?
Soil health (or soil quality) has been defined as the capacity of a soil to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation over a human time scale. In more specific terms, a healthy soil must have: good tilth and drainage, sufficient depth for crop growth, sufficient exchangeable nutrient supply (not excessive or prone to leaching), small population of weeds, insect pests or plant pathogens, large population of beneficial organisms, no toxins, and resilience to adverse conditions. A number of individual soil tests may be used to assess soil health (including those obtained with Routine Soil Analysis); however, a comprehensive evaluation should include a suite of complementary tests to measure soil chemical, physical, and biological properties.
Characteristics of Healthy Soils
(summarized from Cornell's Berry Soil and Nutrient Management – A Guide for Educators and Growers)
What are the characteristics of a healthy soil? Sufficient soil depth for plant root development is important; a soil depth of 8 inches or greater is preferred in the case of berry crops. A healthy soil should have good tilth, water storage and drainage. It should have sufficient but not excessive nutrients and be free of chemicals harmful to plants such as heavy metals, herbicide residues or other contaminants.
Healthy soils should have low populations of plant disease and parasitic organisms such as fungi, bacteria, nematodes, springtails, and so on. Conversely, a healthy soil should contain high populations of beneficial organisms like mycorrhizae and earthworms.
Finally, healthy soils should exhibit resistance to being degraded and along with that – resiliency or the ability to recover quickly from adverse events such as flooding, drought, hurricanes, etc.
Understanding the Three Soil Health Processes
Think of soil health then in terms of the three major realms that impact it: the physical, the chemical, and the biological. These three realms intercept and interact (Figure 2). If any process is compromised, the others are also affected. A healthy soil is balanced in this respect and therefore provides for better growing conditions, crop resiliency and reduced inputs.
Over past decades, chemical aspects of soil were, in general, perhaps overemphasized. While good testing procedures and crop recommendations resulted from this focus, not nearly as much attention was paid to the physical and biological aspects of soil. Research is ongoing in the physical and biological realms today, providing a more complete understanding of soil health and as a result, more comprehensive short-term and long-term management strategies for soil health improvement.
The Chemical Processes
The chemical processes in soil provide essential nutrients for plants. Soil pH is a critical component of the chemical process as it affects nutrient availability. Any changes in soil pH must be addressed, before the planting is established; failure to adjust soil pH to optimal levels for the crop will seriously impact plant establishment as well as future crop production. Soil pH adjustment is more difficult after a perennial crop is established and may reduce the success of the planting.
Chemical processes also includes both macronutrients (nutrients needed in larger quantities, such as N, P and K), secondary nutrients like Ca, Mg and S, and micronutrients required in smaller quantities (such as B and Zn); specific recommendations have been developed for correcting deficiencies of these nutrients essential for berry crop production.
The Physical Processes (summarized)
The physical processes of soil may be limited by inherent or dynamic qualities; some of these may be remediated; others may not.
- Internal drainage – poor internal drainage reduces root growth and function and may support disease development.
- Water availability is a function of soil texture, soil organic matter content, and rooting depth.
- Soil aggregate (crumbs) stability – is a function of adequate soil organic matter which generates humates and other substances that hold soil particles together and contribute to good soil tilth.
- Soil structure – soils with a range of pore sizes are able to provide good drainage, aeration, and rooting, while also retaining moisture.
- Compaction – compaction layers, either near the surface or deeper, can inhibit root penetration and also water drainage contributing to excess runoff or erosion.
The Biological Processes
Understanding soil biology is very much at forefront of our science today. Soil represents a complex environment with highly variable conditions. Most biological activity occurs near the surface of the soil where most of the organic matter is located. There are 3 general types of organic matter found in soil: Living, dead, and very dead. All 3 play important roles in helping produce high yields of healthy crops. Adding organic matter to soil results in many benefits.
- Living Organic Matter - includes plant roots, bacteria, fungi, nematodes, and many other types of organisms. They use resources in soil in various ways, decomposing organic matter, cycling nutrients to make them available for plants, influencing other biota (such as by supressing pathogens), and responding to their chemical and physical environment in very complex ways.
- Dead Organic Matter - is composed of recently dead soil organisms and crop residues that provide food (energy and nutrients) for soil organisms to live and function. Dead organic matter is also called “active” or “particulate” organic matter. This is the other essential partner in mineralizing nutrients for plants, aggregating soils, and forming humus.
- Very Dead Organic Matter - is not a biologically active fraction; rather it consists of well-decomposed organic materials, also called humus. Humus supports the chemical activities of soil; it contains very high amounts of negative charges that hold nutrients and cations in the soil. Humus also has high water-holding capacity, and stores carbon.