One system of Classification of soils is based on the Fertility and the Productivity of the soils, in other words the Agricultural Value, and takes into account the following factors:
- The parent rock of the soil.
- The degree of weathering of that rock.
- The amount of leaching that has occurred in the soil. Leaching is the washing down of minerals and nutrients from the topsoil to the subsoil and below by rainwater percolating down through the soil.
- The soil texture and the Type of clay present in the soil. The two main types of clay are Kaolinite and Montmorillonite and they both have different properties.
- The amount of un‐weathered material in the soil. This material will contain minerals which have not yet been released but will be released into the soil as the soil becomes more weathered.
- The base saturation of the clay particles in the soil. This refers to the amounts of Calcium, Magnesium, Sodium, Potash, etc. Held on the clay colloids.
- Base exchange capacity of the clay particles. This is the ability of the clay particles to exchange one ion eg: Mg for Ca++.
- The topography of the ground where the soil is situated. The physical features of the region.
- The vegetation of the area.
In this area there is a great range of differing materials, which are derived from the different rocks and geological formations and this affects the internal soil drainage and so the Leaching of soils. In North America, Europe and Asia soil differences are largely due to differences in climate. The amount of rain falling on an area has a marked effect on the soil in that area.
These soils are classified into four levels:
- Soil Order
- Soil Group
- Soil Family
- Soil Series
This classification of soils is, in fact, rather complicated, but in order to give you some idea how it works, the five soil orders and their characteristics are given below:
THE SOIL ORDERS
|Weakly Developed Soils
|Very poor profile development
|Poorly weathered soils with large reserves of weatherable materials unleached in the soil. Base saturation high, the bases being mainly Calcium and Magnesium. Clay soils.
|More intensively weathered and so more strongly leached. Clay mainly Kaolinite, free of sesquioxides which are oxides of iron and aluminium. These soils range from deep red clays to very sandy soils
|Saline, alkaline soils. Also soils containing hard impermeable layers due to the presence of sodium.
|Soils subject to and affected by wetness, either permanent or seasonal
1 – 3 show a broad progression in the degree of weathering, 1 being the least weathered and 3 being the most weathered.
4 & 5 are soil orders affected by water and poor internal drainage.
2. SOIL PROFILES
Soil Profiles are important because they enable us to study how a soil has been formed and from what parent rock. They are also used widely in the classification of farm soils in individual farm plans.
Soils are either weathered directly from underlying rock, and these are called sedentary soils, or they are formed elsewhere and moved; these are called transported soils. It is possible to tell a sedentary soil from a transported soil by digging a Soil Pit and looking at the Soil Profile.
- A HORIZON
- B HORIZON
- C HORIZON
- D HORIZON
- This is the topsoil, the layer under cultivation in which plants are grown. It contains organic matter and humus and is dark in color.
- This is the subsoil and is usually below the level of plant roots. It contains more clay than the topsoil, very little humus. It is less weathered than the topsoil and contains nutrients and minerals which have been leached from above.
- Consists of partly weathered rock.
- Consists of unweathered parent rock.
- In a sedentary soil, there will be an even progression from the A Horizon to the D Horizon, and it will clearly be seen that material in all the horizons is related. In transported soil, the A and B Horizons will be of a different material to the C and D Horizons, and the topsoil and subsoil will be totally unrelated to the underlying rock.
TYPES OF TRANSPORTED SOILS
- Alluvial ‐ These are soils carried by water and deposited where the speed of flow the river is slowed down. Found in old river beds and estuaries. They vary in particle size and are very fertile.
- Colluvial ‐ These are found on slopes and at the foot of cliffs and are formed from material that has been transported downhill by gravity.
- Aeolian ‐ These soils are carried by winds from dry desert areas and deposited (Loes) in more humid areas. They also occur in coastal areas near sand dunes.
- Glacial Drift ‐ Formed from material transported by glaciers (ice) and deposited when the ice melted. The First Ice Age was 1 million years ago, and the last Ice Age was 50 000 years ago.
Soil profiles can be further subdivided as follows:
|Above the Soil Surface
|Undecomposed Organic Matter
Partly decomposed Organic Matter
|Dark colour, high amount of organic matter mixed with mineral matter.
Transition to B
|Transition to A
Minerals leached from above remain here
Transition to C
|Weathering parent material
The A and B Horizons consist of true soil and are called solum
3. SOIL CULTIVATION
Soils are cultivated in order to improve their physical condition so that seeds can germinate evenly and the young plants can become well established. Reasons for cultivation are:
- To allow for the penetration into the soil and the covering of the seed in the seedbed.
- To allow the crop to become established quickly. This requires a good tilth which is the condition of the soil in the seedbed.
- To allow for the best possible growth of the plants free from weeds and pests.
- To improve the aeration of the soil.
- To improve the absorption of soil nutrients and water by the growing plant.
The factors which affect soil cultivation and also the cost of soil cultivation are:
- The soil texture.
- The soil structure.
- The crop to be sown in the seedbed. Crops which have very small seeds such as cabbages or kale require a much finer seedbed and so a better tilth than crops which have larger seeds such as maize or beans.
- The weather during the time when cultivations are taking place. The basic steps required in cultivating soils for a seedbed are:
- Drainage: if necessary to lower the Water Table.
- Sub soiling: to break up the subsoil and any mineral pans in the soil.
- Ploughing: This is normally carried out to a depth of between 150 and 350 mm, depending on the crop to be planted and the depth of the topsoil. Do not plough sub soil onto the surface. Good ploughing with even furrows and no bumps or hollows is most important, because it saves time and money on subsequent cultivations.
Ploughing is carried out in order to:
- Loosen up the soil.
- Bury any surface trash and residues from the previous crop.
- Expose the greatest surface area of the soil to the weathering actions of wind, water, frost and sun.
- Breaking down of the plough furrow from the surface downwards in order to produce a good tilth. The depth of tilth required will depend on the crop to be planted; a fine tilth 75 mm deep is quite satisfactory for wheat, whereas potatoes will require a tilth of 350 mm to allow for the earthing up of ridges as the crop grows. The plough furrow can be broken down by harrows. This will give a shallow tilth. Discs will produce a deeper tilth and also consolidate the soil below the surface. Cultivators will give a deep tilth. It is normal practice to roll light land after ploughing in order to conserve moisture, whereas heavy land is left in the plough furrow in order to dry out.
- Apply fertilizer to and harrow into seedbed. Plant the seed either by hand or by seed drill.
- Harrow with a set of light harrows in order to cover the seed completely and then roll the land in order to consolidate the soil around the seed.
- The aim in carrying out cultivation is to provide a good tilth for the crop so giving the seeds even germination. Further aims are to kill weeds in the seedbed, to conserve moisture in the seedbed and to prevent capping of the soil surface.