by Ty Fischer, Riparian Health Restoration Intern

You may not often offer it much consideration when you are walking over it or planting in it, but soil is a wonderfully complex and extraordinarily important thing that both us and our native wildlife owe much of our lives to.

Soil is necessary for the growth of plants, which are primary producers. These are organisms that harvest energy from the sun and transform it into chemical energy, thereby forming the basis of all food webs. It is not just a crucial part of our natural ecosystems, though; we as humans are deeply reliant on healthy soil since nearly all our food is produced directly from crops grown in it or from animals which feed on herbaceous plants that grow in it. Soil also plays other essential roles such as cycling water and nutrients, mitigating environmental damages by intercepting, absorbing, and storing inactive pollutants (Bridges, Baren, 1997), and sequestering carbon dioxide from the atmosphere (McBratney et al., 2014). Overall, it is a vitally important component of our natural environment for both us and for our wildlife. Despite all this, soils are often trivialized and simply seen as something that grows everything, instead of being bestowed the accurate (yet complex) definition that they deserve.

So, what exactly are they? Soils are natural bodies that form on the earth’s surface as a result of complex biogeochemical and physical processes, and which are capable of supporting life chiefly by acting as a medium for plant growth (Brevik, Arnold, 2015). They often contain a mix of organic matter (from decomposing vegetation, fungi, and other dead things), and inorganic matter (often minerals of varying particle size such as rocks, sand, clay, and silt) (Gardener’s Supply, 2023).

Soils are also considered to be among the most biologically rich habitats on the planet, with greater biodiversity per unit than that found aboveground. Many of the species they support – from microbes to macroinvertebrates – provide critical ecosystem functions and services (Nielsen et al., 2015). Finally, it is important to know that soil is considered to be a nonrenewable natural resource because its rate of formation is so slow (Stockmann et al., 2014). This is an important thing to understand and realize since soils globally are showing evident degradation (Amundson et al., 2015) due to several threats that undermine their ability to provide their vital goods and services (Dazzi, Lo Papa, 2022).

What does healthy soil look like? There is no easy answer to this question. Soil health indicators can be broadly classified as physical, chemical, or biological. Physical indicators include soil pore size and water infiltration/storage. Chemical indicators include toxin content, amount of mobile nutrients and heavy metals, and soil acidity, while biological indicators include nutrient mineralization, microbial biomass, pathogens, and biodiversity. There are two important considerations here, though.

Firstly, many of these soil characteristics are interconnected; for example, phosphate (an essential nutrient) is a chemical parameter, but phosphate levels are largely regulated by microbial mineralization and the rate of plant uptake, which are biological processes (Lehmann et al., 2020). The second consideration is even though there are some fairly universal truths for what makes soil healthy, the ideal combination of these characteristics will largely depend on the local context. Both of these considerations are reasons why focusing on a more broad soil characteristic, soil organic content (SOC), is so valuable. Since SOC is directly influenced by many of the above indicators, it integrates the physical, chemical, and biological health all in one (Kumar et al., 2022), thereby providing a clear snapshot of overall soil health in nearly any ecosystem. Generally, higher SOC indicates better soil health, as it improves soil structure, and the capacity of it to retain water and nutrients (Kumar et al., 2022). A side benefit is that, given that soil is the largest terrestrial carbon pool globally, assessing SOC simultaneously has implications for the current climate crisis (Kell, 2012).

Another vastly important aspect of soil health is the mineral particle size. This is because it directly controls soil texture, which has massive influences on not only moisture availability for plants, but also oxygen flows through the soil. These are both essential factors for maintaining healthy root systems. Mineral particles are typically either (i) sand, the largest, coarsest particles that feel gritty when rubbed between fingers; (ii) silt, the intermediate size that feels similar to flour when dry; or (iii) clay, extremely fine and sticky and gummy when wet. Each has different soil texture properties. For example, clay has very low to no pore space, while sand has a lot. Different soils can also have different nutrient levels due to drainage differences. For example, clay is often high in nutrients, while sand is not.

Since each mineral particle has different trade-offs, a combination of them takes advantage of the best features of each. Curious about the soil type(s) you have on your property? You can find out for yourself by making a “mudshake” (Gardener’s Supply, 2023)! First, fill a clear container with straight sides (such as a mason jar) two-thirds full of water, and then add enough soil to nearly fill the jar. Second, add a drop of laundry detergent to help promote separation of the soil components. Third, shake the jar vigorously and then set it somewhere. Observe it for the next couple of days as the particles delineate into layers – sand will settle to the bottom, followed by silt, and then clay. Organic matter will be floating on top or below the surface of the water.

The fourth and final step is to measure the width of each of the layers of sand, silt, and clay and translate them into percentages of each component. Once you do that, you can then compare it to the soil textural triangle to discover your soil type. You can use this information to help pick which plants are appropriate for your property on our Native Plant Database, as you can filter by soil type in addition to other local influences like moisture level, light exposure, and hardiness zone (climate).

In conclusion, soil is an essential resource that is responsible for providing a whole host of critical ecosystem functions and services. It is a humble thing that is far more fascinating than one often thinks about. The next time you are on a walk in a forest, a grassy field, or in a garden, be sure to take a moment to consider the complex and incredible medium supporting you from underneath.

 

Resources
Amundson, R., Berhe, A. A., Hopmans, J. W., Olson, C., Sztein, A. E., & Sparks, D. L. (2015). Soil and human security in the 21st Century. Science, 348(6235). https://doi.org/10.1126/science.1261071
Brevik, E. C., & Arnold, R. W. (2015). Is the traditional pedologic definition of soil meaningful in the modern context? Soil Horizons, 56(3), 0. https://doi.org/10.2136/sh15-01-0002
Bridges, E.M., Van Baren, J.H.V. (1997). Soil: An Overlooked, Undervalued, and Vital Part of the Human Environment. Environment Systems and Decisions (vol. 17(1), pp. 15-20, Springer
Dazzi, C., & Lo Papa, G. (2022). A new definition of soil to promote soil awareness, sustainability, security and governance. International Soil and Water Conservation Research, 10(1), 99–108. https://doi.org/10.1016/j.iswcr.2021.07.001
Gardener’s Supply. (2023, April 25). Sand? clay? loam? What type of soil do you have? Gardeners Supply. https://www.gardeners.com/how-to/what-type-of-soil-do-you-have/9120.html
Jhariya, M. K., Meena, R. S., Banerjee, A., Meena, S. N. (2022). Natural Resources Conservation and advances for Sustainability. Elsevier.
Kell, D. B. (2012). Large-scale sequestration of atmospheric carbon via plant roots in natural and agricultural ecosystems: Why and how. Philosophical Transactions of the Royal Society B: Biological Sciences, 367(1595), 1589–1597. https://doi.org/10.1098/rstb.2011.0244
Kumar, S., Meena, R.S., Sheoran, S., Jangir, C.K., Jhariya, M.K., Banerjee, A., Raj, A. (2022). Remote sensing for agriculture and resource management. In Jhariya, M. K., Meena, R.S., Banerjee, A., Meena, S. N (Eds.), Natural Resources Conservation and Advances for Sustainability (pp. 63-64). Elsevier.
Lehmann, J., Bossio, D. A., Kögel-Knabner, I., & Rillig, M. C. (2020). The concept and future prospects of Soil Health. Nature Reviews Earth & Environment, 1(10), 544–553. https://doi.org/10.1038/s43017-020-0080-8
McBratney, A., Field, D. J., & Koch, A. (2014). The dimensions of Soil Security. Geoderma, 213, 203–213. https://doi.org/10.1016/j.geoderma.2013.08.013
Nielsen, U. N., Wall, D. H., & Six, J. (2015). Soil Biodiversity and the environment. Annual Review of Environment and Resources, 40(1), 63–90. https://doi.org/10.1146/annurev-environ-102014-021257
Stockmann, U., Minasny, B., & McBratney, A. B. (2014). How fast does soil grow? Geoderma, 216, 48–61. https://doi.org/10.1016/j.geoderma.2013.10.007
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