Soil Classification & Soil Health

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Soil classification deals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use. For soil resources, experience has shown that a natural system approach to classification, i.e. grouping soils by their intrinsic property (soil morphology), behaviour, or genesis, results in classes that can be interpreted for many diverse uses. Differing concepts of pedogenesis, and differences in the significance of morphological features to various land uses can affect the classification approach. Despite these differences, in a well-constructed system, classification criteria group similar concepts so that interpretations do not vary widely. This is in contrast to a technical system approach to soil classification, where soils are grouped according to their fitness for a specific use and their edaphic characteristics. Edaphic is a nature related to soil. Edaphic qualities may characterize the soil itself, including drainage, texture, or chemical properties such as soil pH. Edaphic may also characterize organisms, such as plant communities, where it specifies their relationships with soil.


What is soil?

Photo courtesy of

Photo courtesy of

Soil is a dynamic interface between the lithosphere (rock), atmosphere (air), hydrosphere (water), and biosphere (living things). It is the zone in which rocks and organisms, and the air and water that move in and through and around them, interact. Soil is not just the physical parts that make it up, but also the active interactions between its various physical, biological, and chemical parts. A soil’s characteristics determine how that soil functions as a foundation of the ecosystem it is part of, whether natural or managed by humans. When we discuss soil health, we are primarily concerned with the interactive processes involved with this functioning and how human management in influences these processes.

Physically, soil is made up of a mixture of materials, including various solids, air, and water in varying proportions. The solid components of soil include mineral and organic fractions (both living and non-living). This composition of soil strongly in influences how it functions. (Cornell, 2016)

Soil Profile: Most naturally occurring, undisturbed soils have three distinct layers of variable thicknesses. The layers are the topsoil, subsoil, and parent material. Each layer can have two or more sublayers called horizons. Collectively, the horizons make up the soil profile.

soil horizon.jpg

Soils’ properties vary with the soil depth. The surface soil, or topsoil layer (O and A horizon), usually contains less clay, but more organic matter and air, than the lower soil layers. Topsoil is usually more fertile than the other layers and has the greatest concentration of plant roots.

The subsurface layer (B and C horizon), known as subsoil, usually has a higher clay content and lower organic matter content than the topsoil.

Soil properties often limit the depth to which plant roots can penetrate. For example, roots will not grow through an impenetrable layer. That layer may be bedrock, compacted soil, or a chemical barrier, such as an acidic (very low) pH. A high water table can also restrict root growth due to poor soil aeration. Few big trees grow in shallow soils because big trees are unable to develop a root system strong enough to prevent them from toppling over. Shallow soils also tend to be more drought-prone because they hold less water and thus dry out faster than deeper soils. Water lost to runoff on shallow soils would instead be absorbed by a deeper soil. In addition, deep soils allow the roots to explore a greater volume, which means the roots can retain more water and plant nutrients. The optimal conditions for cannabis root systems are in well-aerated soils with relatively high organic matter.

Soils change in three dimensions. The first dimension is from the top to the bottom of the soil profile. The other two dimensions are north to south and east to west. The practical meaning of this three-dimensional variability is that as you move across a state, a county, or even a field, the soils change. Five factors of soil formation account for this variation:

  • Parent material
  • Biological activity
  • Climate
  • Topography
  • Time

Differences in even one of these factors will result in a different soil type. Soils forming from different parent materials differ. Soils forming from the same parent material in varying climates differ. Soils at the top of a hill differ from soils at the bottom. The top of the hill loses material due to natural erosion; the bottom gains the material from above.  Globally, more than 20,000 different soil series occur. Neighborhood level soil series can be found by typing “Web Soil Survey” into any Internet search engine.


Mineral Solids: The large majority of the solids (in most soils) are the mineral parts, consisting of stone fragments, sand, silt, and clay. These particles are defined by their sizes, although they differ in the way they in influence soil functioning beyond simply their size-related effects. The relative proportions of sand, silt and clay determine a soils texture and textural class (Figure 1).

Figure 1

Figure 1

Texture is one of the fundamental characteristics important for quantifying how a soil is functioning. For example, the amount and type of clay, in particular, can greatly in influence the ability of soils to hold and exchange nutrients, and to store organic matter. Clays have a lot of surface area because they are very small, layered, platy particles. The surfaces of most clays are negatively charged, so that positively charged nutrient ions can electrostatically ‘stick’ to them. This ability of soil particles to hold onto positively charged nutrient ions and exchange them with the soil water, or soil solution, is referred to as the soil’s cation exchange capacity (CEC), and the surfaces to which the ions can ‘stick’ are the exchange complex. (Cornell, 2016)

The relative proportions of sand, silt, and clay determine a soil’s textural class (Figure 1). For example, a soil that is 12% sand, 55% clay, and 33% silt is in the clay textural class. Soil texture is a permanent feature, not easily changed by human activity. Consider a typical mineral soil that is 6 inches deep on 1 acre. That soil weighs about 2 million pounds. To change the sand content just 1% would require adding 20,000 pounds (or 10 tons) of sand. A 1% change in sand content would have minimal effect. A significant effect might require a 10% change, which would mean adding 100 tons of sand.

Figure 2 (NC State University)

Figure 2 (NC State University)

Adding organic matter is a more economically feasible alternative for improving soil. Adding organic matter does not change a soil’s texture—the percentage of sand, silt, and clay in the soil—but adding organic matter will alter soil structure by increasing the pore space and improving drainage. Gardeners can be successful with any soil texture, as long as they know the attributes and limitations of that soil.

Typically, laboratory procedures are used to determine the soil texture. It is possible, however, to use the procedure outlined in Figure 2 to determine the textural class by the “feel” method. It takes practice and calibration, but it can provide a reasonable estimate of the soil texture.

Sandy or Coarsely Textured Soils

Low in organic matter content and native fertility.
Rapidly permeable and do not hold soil moisture.
Nutrient leaching is a concern, so proper fertilization is a must. Apply smaller amounts of nutrients, and apply them more frequently.
Low in cation exchange and buffer capacities.
Well-suited for road foundations and building sites.
Feel gritty.

Loamy or Medium-Textured Soils

Contains more organic matter.
Permit slower movement of water and are better able to retain moisture and nutrients.
Are generally more fertile.
Have higher cation exchange and buffer capacities.
Feel crumbly.

Clayey or Finely Textured Soils

Higher nutrient-holding capacity.
Higher available water-holding capacity.
Finely textured soils exhibit properties that are somewhat difficult to manage or overcome.
Often too sticky when wet and too hard when dry to cultivate.
May have shrink-and-swell characteristics that affect construction uses.
Feel slippery.

Organic Matter: Soil organic matter (SOM or OM) is largely made up of carbon, and is any material that originated from living organisms. OM is of profound importance for soil function. It contributes to the soil’s ability to hold onto nutrient ions, similarly to clay, but for an even greater range of ionic nutrients. It can also contain nutrients in its molecular structure. As soil biota (living things – see the following page on Life in the Soil) decompose the OM, nutrients can be released and become available to plants. Some of the very small particles of well decomposed organic materials become bound to ne soil mineral particles and can become protected from further biological activity inside very small soil aggregates. There it will remain more stable as part of the soil’s structure. This process is known as carbon sequestration, an important process for mitigating climate change. Stabilized soil organic matter contributes to soil function in numerous ways, including those related to soil structure such as its capacity to store water and thus provide drought resilience.

Porespace is a higher percentage in ideal soils. PHOTO: NC STATE

Porespace is a higher percentage in ideal soils. PHOTO: NC STATE

Pores: The spaces between the solid soil particles, as mentioned previously, are called pores. These are filled with air, water, and biota. Water and air are essential for all life in the soil. Water is the medium that facilitates nutrient transport through the soil and enables plant nutrient uptake. It also allows microbes such as nematodes and bacteria to move through the soil. Air is constantly moving into and out of the soil, providing oxygen required for cell functioning in aerobic organisms including plant roots and most of the biota discussed in soil biology.

The balance of air and water depends on weather conditions, and also on the size of the pores. Pore sizes are determined in part by the sizes of the particles between which the spaces are formed: for example, clay soils tend to have smaller pores than sandy soils. But just as important as the sizes of the primary particles in this in influence, is the aggregation, or ‘clustering’ of these particles into soil crumbs or aggregates, bound together by particle surface chemistry, fungal hyphae, and microbial and plant exudates.

Just as the primary particles are of multiple sizes, soil aggregates can be of varying size, with larger aggregates made up in turn of smaller aggregates. This is referred to as soil structure, or popularly as ‘tilth’. A healthy, well aggregated soil has a range of sizes of both stable crumbs and pores (Figure 1).

Pore sizes and their continuity determine how water moves in soil. For example, after a soil becomes wet, gravity will drain larger pores more readily than smaller ones. Due to the same forces responsible for capillary action, smaller pores will store
a fraction of the water that in infiltrates into the soil. Plants can access water from all but the smallest pores, which hold water too tightly to release it to plants. Thus, a well-structured soil with a range of pore sizes allows plant roots and soil dwelling organisms to have access to a good balance of air from the larger pores that drain readily through gravity, and water from the smaller pores that store water.

    Color: Organic matter, the soil minerals present, and the drainage conditions all influence soil color. Color alone is not an indicator of soil quality, but color does provide clues about certain conditions. For example, light or pale colors in grainy topsoil are frequently associated with low organic matter content, high sand content, and excessive leaching. Dark soil colors may result from poor drainage or high organic matter content. Shades of red indicate a clay soil is well-aerated, while shades of gray indicate inadequate drainage. (NC State University)

    How Do Soil Types Affect Gardeners?

    Compaction: Compaction occurs when pressure is applied to soil particles and the air and water are pushed out of the pore spaces. Large, cubic sand particles are not easily compacted. Clay particles, small and platelike, are easily aligned and can compact, especially when wet. Compaction inhibits the movement of water, gases (air), and roots. Compacted soils have less infiltration, greater runoff, a higher risk of erosion, and more restricted root growth than soils without compaction. Water drains slowly, which may increase the likelihood of plant root diseases.

    Erosion: Sand particles are heavy, so they are not easily picked up and moved by water or wind. Clay particles are sticky, so they are not easily moved. Silty loam particles are light and not sticky, so erosive forces easily move them. Eroded soils are usually harder to till and have lower productivity than soils without erosion. The main causes of soil erosion in North Carolina are insufficient vegetative or mulch cover, and improper equipment and methods used to prepare and till the soil (Figure 1–12).

    Soil erosion can be minimized by following a few preventive measures:

    • Choose plants suited to the soil so they establish well.
    • Mulch the surface each year with organic materials 1 inch to 3 inches deep.
    • Adequately fertilize to promote vigorous, but not excessive, plant growth.
    • Create a water diversion, such as a grass waterway, to capture and slow water movement.
    • Align rows to follow the land’s contour so that water flowing downhill is slowed.
    • Use proper tillage methods, such as not tilling when the soil is overly wet and not overtilling.
    • Plant a winter cover crop.
    • Consider installing rain gardens to capture sediment and runoff.

    Surface Area: The most active part of a soil particle is its surface area. A particle’s surface is where nutrient exchange takes place. Sand particles have a small surface area relative to their mass, meaning they do not hold onto nutrients well. Clay particles have a large surface area relative to their mass, so a small amount of clay can add a significant amount of surface area to a soil, increasing the nutrient-holding capacity. (NC State University)


    soil health

    Soil health is the foundation of productive farming practices. Fertile soil provides essential nutrients to plants. Important physical characteristics of soil-like structures and aggregation allow water and air to infiltrate, roots to explore, and biota to thrive. Diverse and active biological communities help soil resist physical degradation and cycle nutrients at rates to meet plant needs. Soil health and soil quality are terms used interchangeably to describe soils that are not only fertile but also possess adequate physical and biological properties to “sustain productivity, maintain environmental quality and promote plant and animal health” (Doron 1994).

    According to the (USDA) Natural Resource Conservation Service, “Soil quality is how well soil does what we want it to do.” In order to grow our crops, we want the soil to hold water and nutrients like a sponge where they are readily available for plant roots to take them up, suppress pests and weeds that may attack our plants, sequester carbon from the atmosphere, and clean the water that flows through it into rivers, lakes, and aquifers.

    Healthy, high-quality soil has:

    • Good soil tilth
    • Sufficient depth
    • Sufficient, but not excessive, nutrient supply
    • Small population of plant pathogens and insect pests
    • Good soil drainage
    • Large population of beneficial organisms
    • Low weed pressure
    • No chemicals or toxins that may harm the crop
    • Resilience to degradation and unfavorable conditions

    soil and soil dynamics, weathering and biosphere


    Soil and Soil Dynamics In this video Paul Andersen explains how soils are formed and classified. Weathering of rock creates particles which are mixed with water, air, and organic material. Soils are classified according to particle size, chemical makeup, and horizon distribution.