Parent Material

Parent Material

Soil Composition
Introduction:
Soil means different things to different people. Earth Scientists look at soil to see how it is formed. Engineers look at soil as a material to build on. Agriculturalists look at soil as a material in which to grow crops. Others think of soil as dirt to play in or get dirty from. Soil can be studied in its natural state or after it is moved to be analyzed. Soil characteristics are diverse and can be used to best determine the uses of different soils.
Objectives:
To develop an understanding of soil forming factors and the different components of soil.
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 lime scale), 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 shape 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.
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 a 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 if 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. Another group are the chalk or limestone soils where the overriding factor is that they are almost exclusively composed of chalk or limestone.
Characteristics of Different Soil Types
Sand, Loamy sand, Sandy loam
These are well drained and aerated and workable for most of the year. They are very light to handle and quick to warm up in spring. Unless they have a very high organic matter content they are prone to drying out too quickly, and additional watering will be needed. This extra watering will also help to wash out the plant foods and lime from the soil, so they are likely to be acid (except for some coastal soils). They are often referred to as “hungry” soils and need lots of extra feeding. With careful management however, they can be amongst the most productive soil types.
Medium loam, Sandy clay loam, Silt Loam
These are the “average” soil types. They achieve a good balance between the ability to be very productive and the minimum of attention. The medium loam group is probably the best in this respect.
Clay, Sandy clay, Clay loam, Silty clay loam, Silty clay, Silt
Although these soils are difficult to work and manage, they usually have good supplies of plant foods and lime. The main drawbacks are the high water holding capacity (which means they are late to get going in spring) and the effort required to work them. You will need to catch just the right weather conditions to avoid hard work and damage to the soil structure. The use of heavy machinery (and especially rotavators) should be avoided at all costs, particularly when the soil is wet.
Peat moss or Fen Soils.
Provided they are not too acid and have effective sub drainage, these are probably the best natural soils available. They are rich in plant foods, are easily workable and early. It is possible to convert your existing soil into peat type soil by adding large amounts of organic matter. Some of the keenest exhibition growers do just this. It can be time consuming and costly at first, but once you get there life becomes much easier. You must avoid making your soil too acid though, and careful choice of organic matter is needed.
Chalk soils and Limestone Soils
These are the soils that contain a high proportion of chalk or lime. So much in fact, that it overrides their normal particle size classification. They are often very shallow soils, and severely limit the types of plants that can be grown successfully in them. If you have a soil of this type and are not happy with the range of plants it will allow you to grow, probably the best thing you can do is move to a new area and check the soil out first. If you can’t move, the most sensible course of action is to limit yourself to the plants that will grow in chalky soils. Trying to change the soil is usually an uphill struggle and quite expensive. For the incurably intrepid, details are given later in the “Golden Rules for Difficult Soils” section.
Five important factors influences the specific soil that develops.
  • Parent Material
Minerals and organic materials present during it's formation. Materials from volcanos, sediment transported by wind, water or glaciers or minerals left behind by drying lakes are good examples of parent materials.
  • Climate
Parent material is broken down into smaller pieces by a process called weathering. Cycles of freezing and thawing, wetting and drying, and the frequency of these occurrences coupled with average temperature and moisture levels of region play an important role in soil formation. These smaller pieces are known as (sand, silt and clay), clay being the smallest size.
  • Living Organisms
Both plants and animals help to create a soil. As they die, organic matter incorporates with the weathered parent material and becomes part of the soil. Living animals such as moles, earthworms, bacteria, fungi and nematodes are all busy moving through or digesting food found in the soil. All of these actions mix and enrich the soil.
  • Topography
Topography is the hilliness, flatness, or amount of slope of the land. Soils vary with topography primarily because of the influence of moisture and erosion. In many areas, moist, poorly drained soils are located in low areas, and depressions of the land. In contract, soils in sloping areas can be drier and well drained. These soils tend to be moderately and well developed. Erosion can remove all or part of the topsoil and subsoil, leaving weakly developed soil.
  • Time
It may take hundreds of years to form one inch of soil from parent material. Only the top few inches are productive in the sense of being able to sustain plant growth. This is why soil conservation is so important.
These horizons collectively are known as a soil profile. The thickness varies with location, and under disturbed conditions: heavy agriculture, building sites or severe erosion for example, not all horizons will be present.
Soil ProfileThe uppermost is called the organic horizon or O horizon. It consists of detritus, leaf litter and other organic material lying on the surface of the soil. This layer is dark because of the decomposition that is occurring. This layer is not present in cultivated fields.
Below is the A horizon or topsoil. Usually it is darker than lower layers, loose and crumbly with varying amounts of organic matter. In cultivated fields the plowed layer is topsoil. This is generally the most productive layer of the soil. This is the layer that soil conservation efforts are focused.
As water moves down through the topsoil, many soluble minerals and nutrients dissolve. The dissolved materials leach downward into lower horizons.
The next layer is the B horizon or subsoil. Subsoil are usually lighter in color, dense and low in organic matter. Most of the materials leached from the A horizon stops in this zone.
Still deeper is the C horizon. It is a transition area between soil and parent material. Partially disintegrated parent material and mineral particles may be found in this horizon.
At some point the C horizon will give up to the final horizon, bedrock.