Physical Soil Composition

Physical Composition of Soil

Soil is composed of solids and spaces. Solids include soil minerals and organic matter; the spaces contain air and water. Ideally, the spaces should be composed of 50% air and 50% water. Soil tilth is good texture and drainage. Soil in good tilth holds water without becoming soggy and allows air to circulate to plant roots and soil organisms. Soil in good tilth allows roots to penetrate the soil easily and grow and is easy for gardeners to work.

Texture, structure and aggregation, density, drainage and water-holding capacity are important components of the physical characteristics of soil tilth. Gardeners can improve some of these characteristics, but not all.


Texture refers to the proportions of sand (large particles), silt (medium particles) and clay (very fine particles) in the soil. The particles are loose sedimentary mineral material from fine clay particles to grains of sand and aggregates. Many of the traditional garden plants we are most familiar prefer loam soil, rich in humus. Loam contains from 7% to 27% clay, 28% to 50% silt and less than 52% sand. This would be ideal.

There are twelve main textural classes or groupings: clay, sand, silt, loam, sandy clay, sandy clay loam, sandy loam, loamy sand, clay loam, silty clay, silty clay loam and silty loam. Any extreme textural class is undesirable.

Soil texture does not take organic matter into account, only the composition of clay, silt and sand in the soil. Adding organic matter does not change the texture of the soil. Only by adding huge quantities of sand, silt or clay would gardeners be able to change the soil texture -- and this is often not practical.

Caliche is defined as a crust or a succession of crusts of calcium carbonate that form within or on top of stony soil in arid and semi-arid regions and is often combined with clay. Caliche is lacking in organic matter. Its crusting tendencies often prohibit drainage.

Structure, Aggregation and Density

One of the keys to good soil tilth is soil structure -- the way the soil particles are grouped together. Particles grouped together form aggregates (crumbs or granules), blocky units, flattened units often called plates or in a vertical structure called columns or prisms, or unstructured.

Plants grow best in soil with good aggregate size, or crumb structure. This structure would be loose with adequate pore spaces for air and water circulation (capillary action). along with this, roots more easily penetrate the soil and the soil is better able to hold water and nutrients for the growth of the plants. Think of a good crumb structure as being similar to worm castings. If you've never seen worm castings, the castings are small, somewhat rounded and moist where you can readily see the individual castings or crumbs, but when squeezed in a ball, holds it's shape. Worm castings, with a high organic composition, holds in water and nutrients until the plants is ready for it.

Heavy clay soil and sandy soil can both have good soil structure that crumbles easily. Heavy clay soils that do not crumb but form big clods that are difficult to break has poor soil structure. Just the same, sandy soil that does not hold together has poor structure.

Structure and aggregation is the physical quality that can be improved or destroyed, depending on management techniques. Good structure and aggregate formation is a result of biological activity. Humus plays an important role in aggregate formation and earthworms secrete the sticky gums that hold the particles together.

Adding organic matter to the soil loosens clay soil and helps bind sandy soil together through the actions of the soil organisms. Walking on or tilling wet soil compacts and destroys the soil aggregates and structure.

Drainage and Water Holding Capacity

The drainage quality of soil is equally important. Soil drainage is the rate and extent of water and air movement in the soil either across the surface or downward. Soil that is soggy or water logged means there is not enough air in the soil pores or spaces. Many plants cannot live in this environment. Likewise, soils that drain quickly leach water and nutrients out of the soil.

Drainage problems are caused by high water tables, collections points such as swales or areas flooded by roof runoff, compacted soils or hardpan. Sometimes the solution is installing a raised bed garden, planting in containers or channeling water away from the area. Breaking through the compacted soil or hard pan is necessary for roots to penetrate, or even double digging the area in your home garden. Adding organic matter, the inoculation and stimulation of beneficial microbes and earthworms and planting deep rooted plants that can breach hardpan provide more permanent solutions to the drainage issue.

Most high and medium water-use plants require average drainage, low water-use plants require good to very good drainage. Tight compacted clay and caliche soils have poor drainage. If you have clay soil that forms cracks 1/8 inch wide or larger, most likely you have poor drainage. When you notice these cracks, pour some worm castings or compost down the cracks -- a less labor intensive way to begin to improve the soil.

Amendments v.s. Fertilizer

Soil amendments are added to the soil to improve the soil tilth and provide nutrients for soil organisms. A soil amendment is a material that is added to improve the organic content of the soil, feed the biological soil life, and improve the structure of the soil. Soil amendments are not immediately available to plants, but their addition to the soil, through the workings of soil organisms, makes the nutrients available to plants.

For initial amending, a good quality compost is recommended, usually compost cottonseed hulls, certified organic composted manure, composted leaf litter, and composted garden and grass clippings. Some other locally available organic soil amendments are cottonseed meal, horticultural molasses, corn meal and corn gluten meal, alfalfa meal, alfalfa pellets, and worm castings. Add these other amendments according to the recommendations on the bag, not "inches thick" of these amendments as you would compost.

Inorganic amendments improve soil structure. Newer inorganic amendments on the market also have the ability to retain water and nutrients. When the term inorganic amendments is used, this is not a reference to synthetic chemicals. Organic amendments contain carbon atoms, inorganic amendments do not.

Fertilizer is a chemical or material applied primarily as a nutrient for your plant. This includes high analysis fertilizers (N-P-K analysis, that is nitrogen, phosphorus and potassium) and synthetic chemicals. Fertilizer does not build the soil or the soil life, but because of its salt content aids in destroying soil structure.

Benefits of Organic Matter, Compost, & Humus

How much organic matter does your soil have? In Texas, organic content ranges from .5% to 3%, with the 3% in the most fertile regions of the state, not necessarily the Texas Panhandle. If you’ve not previously amended your soil with organic matter, you will be closer to the 1/2%. (Have your soil tested for organic content, Texas Plant and Soil Lab,, 956-383-0739). I’ve topdressed my turf since 1999 until 2007, often twice a year with a ¾ inch layer of compost and had it tested in January, 2005 (I now use Turfmate from SoilMender). I dug out a slice down to about seven inches, about ½ wide, and 5 inches long and could plainly see how the compost had worked its way down to 5 ½ inches in 5 ½ years from gravity, machine and earthworm aeration. My lawn tested at 3.68% organic content. I was advised to keep topdressing my fescue turf. Unfortunately, I did not have my soil tested prior to topdressing, which would have given me a starting point to note improvement. It's always best to have your soil analyzed for organic content before beginning an improvement program. Be sure to specify you'd like the organic content analyzed, it may be an extra charge.

Amending with organic matter can also corrects some drainage issues. The main advantage to adding organic matter is to improve fertility, such as compost or humus. Fertility is closely related to the biological life of the soil. Humus is a form of organic matter that has undergone some degree of decomposition. Humus gets used up by plants over time and needs to be replenished. Humus improves soil in many ways:

  • Improves water retention (thereby making it more drought resistant).
  • When ample minerals and nutrients are available, plants require less water for their uptake.
  • Aids in better soil structure (by allowing air and water to circulate easier, and holds soil particles together better in a more desirable structure from the sticky gums secreted by microbes through the process of humus formation).
  • Promotes the growth of mycorrhizal fungi, the beneficial fungi that grow in decomposing matter.
  • Increases growth-promoting fungi, which help control diseases, root rot, and damping off fungi.
  • Earlier soil warming.
  • Better nutrient retention and long-term nutrient release.
  • Balances the pH of the soil (lowers alkalinity).
  • Recycles plant waste products, acts as a buffer to chemicals and reduces soil toxicity.

Inorganic Amendments

If you have tight compacted clay and caliche soil, you should add a mix of organic matter with one of the newer inorganic amendments. I recommend adding an inorganic amendment for heavy clay soil, such as Turface® (calcined clay), Tru-Grow® (expanded blue shale), Ecolite™ (zeolite) or Axis® (diatomaceous earth) and Profile™ for sandy soil. Expanded blue shale is also available from Soil Mender and is called natural expanded shale. These are a few of the better inorganic amendments that retain water and nutrients, in addition to creating more spaces for air and water.

Other inorganic amendments with much more limited ability to retain water and nutrients are crushed granite, granite and lava sand, greensand, glass sand, and finally, regular sand. If your clay soil develops cracks during summer droughts, please consider adding the inorganic as well as organic amendments.