Soil health is one of those words we hear frequently and seem to understand, yet have no idea what it actually means. Most would assume that a good, healthy soil would have:

1. Proper Soil pH
2. Soil Aggregation/Good Drainage
3. Proper Air Space
4. Good Water Holding Capacity
5. Ample Nutrients
6. Organic Matter
7. Diverse Biological Activity

This would be a great beginning. Soils meeting this criteria would seem to be able to grow any crop desired and be suitable for any use. However, this definition ignores climatic conditions, parent material of the soil, and plant requirements. Too, while many of these characteristics are measurable, they are meaningless until one selects a specific use or plant production system. So, simply listing desirable characteristic of a desirable soils is not enough to provide us a definition of soil health.

The reality is that we cannot define whether a soil is “good” or “bad” (i.e. unhealthy) simply based on the drainage characteristics of a soil. Some plants such as cacti, peanuts, and tobacco perform outstanding in rapidly draining, sandy soils. In contrast, plants such as corn and soybean prefer soils that hold more water than the sandier soils yet also do not hold excessive water. Some plants such as rice, beets, or water lilies can survive in soils that retains excessive water to the point of flooding. While some of these examples may indeed be a bit to the extreme range, they make the point that soil itself is neither good or bad simply based on its drainage capacity. So, until a purpose for the soil has been defined, we have no parameter to quantify whether a soil is “good” or not. We only qualify soils as such when we attempt to use the soil for a specific plant growth, use, or production system.

Some plant can survive in soils with a low soil pH while others may require a near neutral pH of 7.0. In fact, some plants have a very narrow soil pH suitable for good growth while others can survive in a range of soil pH. So again, soil pH value alone does not allow us the ability to define what is a heathy or unhealthy soil. Having thusly stated, the pH is important to monitor. As just mentioned, some plants have a desired soil pH range. When this is known, we should aim to keep soils in the plant’s desired soil pH range. Another critical reason to monitor soil pH is that plant nutrient availability is strongly impacted by soil pH. Improper soil pH can lead to some nutrients transforming to a chemical form unavailable to plants. Other nutrients may increase or even become toxic to plants at an improper soil pH. Lastly, a low soil pH is an indicator of high aluminum within a soil. Aluminum is toxic to many plants. So, knowing the soil pH is critical, but alone, tells little about the soil until management and plant growth are also known.

Qualifying soil as good or healthy because it has a dark color and observable soil particle aggregation is a flawed means of defining soil. Such characteristics are indeed desirable and is often associated with soils that contain high organic matter to give it a dark color. The desirable aggregation is likely a result of silt or clay particles forming aggregates. This type of soil will be well suited for holding water for plant growth as well as good drainage (air space). However, using these properties alone will ignore very healthy soils that are sandy in nature. Sandy soils are often very light to pale yellow in color and most have low organic matter since they do not form from mineral rich and high organic content parent material. As such, the sandier soils inherent properties will not change (See comments regarding organic matter and microbial populations). Too, sandy soils frequently lack any real aggregate structure. Even without the dark color or strong aggregate formation, with ample water, sandy soil are well suited for plant growth, even preferred by some plants. Thus, color or soil aggregation alone is inefficient for determining soil health.

Air space in soils allows water to permeate into the soils during rain instead of pooling on top or running off. Too, the air space allows oxygen into the soil for microbial communities. Without air space in soil, plant growth and biological activity will struggle to survive. This starves plant roots of oxygen, prevents water from draining or infiltrating, and physically blocks roots from expanding through the soil profile.

Fortunately, we have a measure for soil air space. Compacted soils will have a higher bulk density than non-compacted soils due to the lack of air space. Plant limiting bulk density values will vary according to the soil texture. Within Eastern North Carolina where most soils are sandy or loamy sands, this value is 1.8 g/cm3. Generally, soils with a bulk density of 1.5 g/cm3 warrant caution of having poor air space/porosity. However, as noted, this depends upon the soil texture (Some clays have a bulk density of 1.5 g/cm3 so yet are still viable soils for plant growth). When soils have a lower than desired bulk density, this can usually be corrected with increased addition of organic matter, long-term production of cover crops, or deep mechanical tillage.

Soils obtain much of their original nutrient content from the parent material from which they formed. Fertilizer additions and organic matter additions can change this nutrient level. With time, some nutrients are lost or used by plants and microbes or undergoes a chemical reactions. It is a complex process. Fortunately, the nutrient holding ability of a soil can be measured since most plant nutrients have a positive in charge. We call this measurement the Cation Exchange Capacity (CEC). As a comparison, soils with high or clay silt may have a CEC between 15-30 or much higher. Sandy soils may have a CEC less than 5. This does not imply that sandy soils cannot be used to grow plants. It simply means that the sandier soils will not hold as much added plant nutrients (fertilizer) as silt or clayey soils. As such, they all must be managed differently, but will still produce crops. The inherit CEC value affords one the ability to know whether a soil will “hold” the nutrients until a plant needs it or whether one must apply fertilizer in multiple applications because the soil will not “hold” the nutrients. It is an excellent measure of how to manage fertility of the soil. However, using this value alone will not be a good item to use to measure soil health.

Just as the CEC is important to understand, the organic fraction of the soil is also very important. Increasing organic matter of a soil can raise the CEC, increase water holding capacity, and increase microbial activity. As such, the greater the organic matter, the higher potential for more microbes, and subsequently greater nutrient cycling. While this is straightforward, many don’t realize that the soil organic matter measured from soil analysis is actually the microscopic organic content of soil. Yes, recognizable plant residue and other organic matter are indeed considered part of the soil organic fraction, but this is more precisely referred to as soil residue. In other words, this is a limited “part” of the organic matter within a soil. True “organic matter” as reported on soil test reports refers to humus. Humus is relatively stable organic matter particles that has been decomposed by microbes. This portion of the soil impacts the microbial and chemical properties of soil much more so than the recognizable, surface residue. Knowing the humic value will aid in determining the proper soil pH, selection/rate of herbicides, and potential impacts upon the CEC. However, lower values of organic matter do not necessarily relate to poor soil health. In fact, it is not uncommon to find low organic matter in soils with diverse biological composition found within a temperate climate. (See more below)

Some promote soil health measure based upon the population of microbes or the diversity of microbes within a sample. This seems a reasonable assumption but has many flaws and is a quite complex evaluation. First, microbial respiration fluctuates greatly. Thus, the number of samples, temperature of the soil during testing, the type of measurement, the method used to evaluate, soil management (tillage) and even the microbial community composition will provide varying results. As such, a “high” number of microbes does not always relate to high nutrient cycling as assumed. Even should a standardized method be available for comparison, interpretation should be made with caution since many studies show that there is no universal respiration response among differing biomes. This simply demonstrates that the climatic factors where the soil is located is critical to understand as part of the microbial respiration. Equally as important, it is necessary to understand the specific respiration rates of each specific microbe. Regrettably, we do not always know this specific relationship.

To better understand this concept, consider the synergy of soil properties discussed thus far. Climate and type of soil particle are perhaps the dominant factors that will determine the types and number of microbes. As example, sandier soils with lower organic matter within a warm climate will afford microbial respiration almost year-round. In contrast, silt soil within a region where soil temperatures fall below freezing will result in lower or no microbial respiration during winter months. As such, the sandier soils within a warmer climate will continue to decompose organic matter and thus have low organic matter composition. In contrast, the soil that freezes will cause biological activity to cease during winter months. As such, the existing organic matter will remain until spring thaw and over time the organic matter will accumulate. So, knowing the diversity or population alone is not always a good measure of soil health. One must also consider climatic factors.

A last consideration of soil microbe measurement should consider the “philosophy” vs ”reality” of microbial population within a natural environment such as a tropical rainforest compared to monoculture of an agricultural crop. The reality is that natural areas often do indeed have a high number of unique taxa, many with a specialize function to break down difficult to decompose plant or animal parts. Too, these areas generally have a higher fungal population than monoculture production. As such, this tropical biodiversity affords continuous breakdown and nutrient cycling. In contrast, cultivated fields generally have a lower microbial population, and often less diverse, that rapidly increases as climatic conditions favor growth. Field usually have lower fungal populations to break down woody materials.

So this begs the question as to which microbial population is better? Is the diversity needed? In other words, does the lack of fungal growth to break down woody material really impact field crop production? The answer is “ No". Mechanical means provide wood material degradation and remaining surface soil residue provides greater rainfall penetration into the soil.

Regarding diversity, it merits consideration to ask would a greater microbial diversity would decrease field crop diseases? Well, in an ideal world, yes, but reality says otherwise. Natural systems tend to seek entropy. In studies where organisms are introduced to increase biological diversity, these introduced organism tend to decline, and within a short time, the original microbial population dominates without the existence of the new organism. In other words, whatever function these “new organisms” brought to the functional microorganism group was not needed, provided no value to the community, or the organisms introduced was simply not suited to live in this specific environment. Again, nature seeks entropy and this remains true even when man manipulates the environment.

Another flaw in measuring microbes is that simply measuring microbial respiration does not indicate whether the microbes are beneficial or harmful to plant production. Many fungi and bacterial are indeed the reason for major disease losses in crops! The reality is that of all the microbes observed under microscope, we only understand the functionality of about 1% of them. With such a limited knowledge, it seems that using microbial population to justify a good soil health is like trying to explain geopolitical impacts of world trade by applying principles learned from studying the activities of two youth selling lemonade at the street corner. The harsh truth is that we still have much to learn about our microbial soil community. Thus, measurements of microbial respiration alone or using microbial diversity is not a suitable means to quantify soil as health or unhealthy.

The bottom line shows that the synergy of all these factors, including use and management, defines whether a soil is good or bad. The intended use and historical performance should also be a factor in determining soil health. Any other attempt to qualify as “healthy” or “unhealthy” should consider these, and other characteristics in total, not independently. While there are many tests available to measure soil health, these test were created and evaluated for a specific type of soil within a specific region. For regions where these test were developed, they may indeed offer good information. However, using these test universally for all soils within the world to quantify soil health is not wise. A great example of why these test will not function universally is found within the sandy soils of Eastern NC. Based upon the standard of many of these test, these soils will most likely result in reports of “poor soil health” when in fact, our soils are healthy. For a more detailed view of these comments as well as measurement of soil health, read the articles, Soil Health: What Does it Mean in North Carolina?