The composition of Soil is a complex topic that’s often hard to apprehend. If you’re unsure about soil quality and composition, then this article will help you learn what to look for in the ground surrounding your property.
- Most gardens require a 6.0 to 7.0 pH to support either flowers or produce. Finding the right combination of soil properties to promote drainage while retaining moisture ensures healthy plants.
- Adding organic material and fertilizer can correct most soils.
- Composting effectively recycles many kinds of yard wastes and provides organic matter to replenish soils.
- Mulch protects good soil by reducing the need for watering. Just 4″ of mulch can reduce soil temperature by as much as 20 degrees.
- Soil testing provides the most effective method for soil evaluation. Contact the county extension service or local garden center for more information.
- Prepare garden soil during autumn to increase benefits through the winter months. Good spring soil preparation improves productivity during the growing season.
Composition – Soils are classified according to the
proportions of different-sized particles they contain.
In the lab, special ultra-fine sieves will be used to separate the particles, but an easy rule of thumb guide can easily be undertaken at home.
The technical term for this subject is “Soil Texture” but I think “Composition” is more expressive, so I will use it.
You will need a small sample of the soil that is typical of the area you want to test, some water (distilled or rainwater is best if your water comes from a source that is hard or produces limescale), and a tall flat bottomed straight-sided glass container at least half an inch or so in diameter.
At a pinch, a large test tube will do, but something that shapes with a flat bottom like a tall thin jam jar is ideal. The amount of soil you need depends on the size of the container. It needs to be about half full of soil.
What is soil Organic Matter?
Soil organic matter is the fraction of the soil composed of anything that once lived. It includes plant and animal remains in various states of decomposition, cells, and tissues of soil organisms, and substances from plant roots and soil microbes. Well-decomposed organic matter forms humus, a dark brown, porous, spongy material that has a pleasant, earthy smell. soils generally are less than 2 percent organic matter. (from National Soil Survey Center et al.)
Put the soil in the container and fill to three-quarters full with water
Close the top with a lid or cork and shake it vigorously for a minute or two, so that all the soil particles are broken down into suspension in the water. Then put it somewhere to settle where it won’t be disturbed at all for a day or so, If you have heavy soil, it might need a few days, and in this case, put it somewhere dark or the algae and bacteria will start to turn it green!
What happens is that the laws of gravity take over
The heaviest (largest) particles sink to the bottom first, and the fine clay particles are the last to settle out of suspension. Organic matter which is not decomposed either floats or sinks to the surface after the clay particles. Occasionally it will settle as a band before the clay.
You will be surprised how easy it is to see the separate bands of particle sizes as they are laid down, and by using a ruler up the side of the tube as a measure and a bit of imagination, you can get a total column height and are often able to measure the individual depths of sand, silt, and clay-sized grains, together with organic matter to give a proportion of each as a percentage of the whole sample.
A Few Soils Cannot Be Assessed in this Way
These are the peat moss or fen soils where the proportion of organic matter is perhaps more than 50% of the total sample. Other groups are the chalk or limestone soils where the overriding factor is that they are almost exclusively composed of chalk or limestone. These are described in the section dealing with the characteristics of different soil types.
Knowing the composition of your soil gives a better understanding of what your soil type is and, if you are an enthusiast, it provides the baseline against which to measure the effect of future improvements. For example: how much organic matter is now left in the soil from that lot you put on two years ago?
Assessing the Composition Mix of your Soil
For those who want to go a stage further, you can plot your results on a three-sided chart which is widely used to classify soil types. Unfortunately, although they are similar, international standards vary, so your country might be slightly different from the one shown which is my own, based on several years of research and practical testing of a wide range of soils.
The diagram shows a three-sided
grid with each side representing the content of a particular
article on a scale from 0% to 100%. The bottom line is the sand
content, starting at 0% at the bottom right-hand corner, and rising
to 100% in the bottom left-hand corner.
Moving clockwise around the triangle, the line from the bottom left corner to the top point of the triangle is the clay content, starting at 0% in the bottom left corner and rising to 100% at the top.
Continuing clockwise, the silt starts at 0% at the top of the triangle and increases to 100% at the bottom right-hand corner. By following the percentage lines for sand silt and clay to wherever they meet inside the triangle you can determine which category your soil falls into.
To locate your exact point on the grid you should first calculate the percentage of sand, silt, and clay in the total sample without including the organic matter in the total. Then take the percentage of sand as your starting point and, using the bottom of the triangle grid to find the corresponding percentage, begin to draw a line upwards and to the left (parallel with the right-hand side of the triangle) at the exact percentage.
Next, take the percentage of clay and, using the left-hand slope of the triangle grid find your percentage point. Starting at that point, draw a line horizontally across (parallel with the bottom of the triangle) toward the right slope of the triangle. It will cross the sand line at some point.
The final confirmation that you have done it right is that the percentage of silt, located by the same process on the right-hand slope of the triangle and following a line drawn downwards toward the bottom left will cross at the same point as the other two.
The intersection of these three lines will fall within one of the colored areas on the grid, and this is your soil type.
If it is in the middle of the colored area it will be typical of that classification, but if it is near to the edge of two areas, then it will behave with a mix of the characteristics of both soil types.
Bacteria in Soils
As well as plants, other life arose in the soil. Some of it is more fascinating than the world we see each day. In one tablespoonful of soil, there are more bacteria than there are people on the entire planet. A quarter of a million of them could sit on the full stop at the end of this sentence. They can live in the air, water, extremes of heat and cold, and can function without sunlight.
Some bacteria can take animal excrement and purify it. Others can take nitrogen from the air in the soil and convert it into nitrates that are needed by higher plants for growth. Being contained within a single cell, they cannot eat solid but feed by secreting enzymes to dissolve their surroundings to a form which they can digest, then re-absorb as lunch.
Another of the most impressive things about bacteria is the range of material they can break down to digest. Carbon compounds like naphthalene present them with no problem, something we as humans find difficult to do unless we have a laboratory.
Fungi in Soils
Essential to the breakdown of woody organic matter, fungi are another mystery in the soil, Some are parasites on live or dead plants, others live in harmony with plant roots, helping to create the ideal conditions for both to flourish.
Algae in Soils
Like plants, but more simple in composition, algae can take up carbon dioxide from the surface air (although a few do it deep in the soil) and convert it to oxygen as part of their food production process. We, of course, are happy with this, because oxygen is replenished by such means and we get to live.
Microscopic Animals of Soils
From simple celled amoeba and protozoa running their lives in the soil moisture, through nematodes that can damage the roots of the plants we want to grow, your soil is teeming with unseen life all of which plays its part in the complex chain of interdependency that is life.
The most obvious are the earthworms. The gardener’s friend, they play a huge part in mixing organic matter from the surface into the lower depths of the soil, and in doing so, they provide the source of food for countless numbers of other organisms who feed on the organic matter. Their burrowing also leaves (by comparison) huge aeration channels and fissures in the soil, along which air can diffuse and water drain. It is estimated that there are somewhere between 200 and 600 worms in every square meter of your garden.
Then there are the beetles that assist in clearing up the decaying organic matter, and who themselves provide food for small animals up the food chain to man.
So, as we have seen, the soil is a hugely complicated microscopic world, teeming with interdependent life chains.
The soil is the unifying factor in world crop production. Understanding the fundamental properties of soils proceeds to a better appreciation of soil fertility and fertilization, as well as soil management and conservation.
Integrating nutrient management, soil management, and conservation leads to considering soil quality or health. Simply put, soil quality is the ability of a particular soil to function as desired. Some functions include crop production, water control, or structural support.
What exactly is soil?
There are many, many definitions. Specific ones depend on the viewpoint of the user. Some stakeholders consider soil to be the living, breathing substance near the earth’s surface that fosters life on all scales. Others consider it to be the aggravating skim on the earth’s surface.
The study of soil is not the study of dirt. Dirt does not have life-sustaining properties. Greenhouse media is not soil either. It does not have unique three-dimensional characteristics that can only evolve as a function of place, time, and the environment.
More information on soil properties and formation are available on the Master Gardener reference manual links below.
Soil Taxonomy System
Someone coming into soils may think they are learning a foreign language. Soil folks do use specific terminology in discussing soils in a holistic sense. Soil classification and nomenclature have been a challenge for centuries. Currently, most of the world uses a system developed in the United States. An Alfisol in South America can be assumed to have similar characteristics to an Alfisol in Carroll County, Mississippi.
In the U.S. Taxonomy, soil properties are used to name them. Most agriculturalists are familiar with series names, however series is only the final step of the system. The hoped-for strength of the system is a complete soil name to convey lots of information in a relatively brief phrase.
The following is adapted from materials obtained from Professor Ed Nater. It contains some technical terms (argillic?) but is candid in discussing the strengths and weaknesses of the system.
- Order: The broadest category in the system. Distinctions between orders are based largely on horizon morphology, with (unfortunately in some cases) soil genesis as an underlying factor. In general, each order is presumed to contain soils whose common properties suggest similar genesis. There are 12 orders in the taxonomy, but the key hasn’t been updated to include the 12th yet. Eight of the 12 have been identified in Mississippi. More information on 11 of the orders is linked below.
- Suborder: The suborders are subdivisions of the order based on factors such as wetness, climate (temperature and moisture), mode of deposition, texture, or diagnostic horizons. The number of suborders varies from four to seven within orders.
- Great Group: Diagnostic horizons are often used to differentiate great groups within a suborder. For example, the presence or absence of an argillic horizon might distinguish one great group from another. (Argillic horizons are layers with observable clay accumulation in the soil profile. There are many soils in Mississippi containing argillic.)
- Subgroup: The subgroups are subdivisions of the great groups. The typical, or central, concept of the great group makes one subgroup (Typic). Often, other subgroups are intergrading between the current great group and the central concepts of other great groups (i.e., metallic subgroups of Alfisols).
- Family: The family category allows the grouping of members of a subgroup by such things as common texture, mineralogy, pH, soil temperature, coarse fragment content, or soil depth. This level of the system is often the most useful one for interpretations because it is the most descriptive.
Series: Soil series represents a collection of soils essentially uniform in most differentiating characteristics and the arrangement of horizons. This is the level most often identified by farmers and the NRCS. Names are usually based on towns near where the soil was first identified. There are about 700 named soils in Mississippi. Common series in the Mississippi Delta are Alligator, Sharkey, Dundee, Dobbs, and Forestdale. Prentiss, Ruston, Grenada, Memphis, Oktibbeha, and Smithdale are found in the hill portions of the state.