ProAgri Zambia acknowledges Grain SA for the use of this series which originally appeared in Afrikaans in SA Graan/Grain.
Martiens du Plessis, Soil Scientist, NWK Limited & Prof Cornie van Huyssteen, Lecturer: Soil Science, University of the Free State
In the series ”Soil: The producer’s most important asset” the most important soil properties that influence plant production, were discussed. The effect thereof was explained, as far as possible, in a practical and simplified way.
Soil is a permanent resource and cannot be replaced. It can only be improved to a slight extent, but may also be totally destroyed. The management of the farm must be adapted according to soil properties, so that the soil properties are not degraded and that the farm may remain sustainable in the long term.
This article is the final summary in a series that highlighted this particular resource.
What does soil comprise?
In Part 1, the composition of soil was discussed. The minerals, organics, water and air fractions were explained.
The origin of soil is rocks that have weathered (Part 2). Various rocks lead to the provision of various minerals which, in turn, lead to various soil properties.
Some rocks weather to primary minerals and then the various clay types crystallise out of them (Parts 3 and 4). Some rocks had their origin in clay and when they weather, they lead once again to clay. Sandstone, in contrast, formed out of sand which, when it weathers, once again forms sand.
Physical soil properties
Particle size distribution (texture) is surely the most well-known physical soil property (Part 5). Soils in which the finest particles dominate, are the clays, while sand has a coarse texture.
Concepts such as gross density, porosity, aeration and specific surface, determine root growth and water movement and it is important to take note of them.
Soil structure refers to the aggregation (clinging together) of individual soil particles to form larger structures (aggregates or peds) (Part 5). The structure of soil is usually strongly developed to form clear structural units. In contrast, the structure of sand is poorly formed and it is described as structureless. A stable, fine crumbly structure is beneficial to plant growth.
Soil strength refers to the resistance soils offer to distortion (Part 5.3). This is important, as plant roots have to distort the soil as they grow through it. Soils that are compacted (densified), offer a lot of resistance to distortion and thus inhibit root development. Soil temperature plays an important role in biological activity in particular (for example germination of seed and growth of microbes).
Chemical soil properties
Clay plays a very important role in the chemical properties of soil. The clay structure results in it having a nett negative charge. This is known as the soil’s Cation Exchange Capacity (CEC) (Parts 6 and 7).
It is the CEC that lends the soil its property of retaining or exchanging positively charged plant nutritional substances (such as Ca++, Mg++, and K+). The anion exchange capacity retains negatively charged plant nutritional substance such as H2PO42- and NO3-. The CEC of the various clay types differ significantly from each other. The base saturation refers to the percentage of the exchange complex which is filled with basic cations (such as Ca++, Mg++, Na+ and K+).
Soil acidity and the neutralisation thereof (Part 8) has a major influence on the fertility of the soil. Under acidic and alkaline conditions, the majority of plant nutritional substances become inaccessible to plants. This is the main reason why the majority of plants grow best in a neutral to slightly acidic pH.
Ground water levels are very dynamic and vary between soil types and seasons. One of the most important roles of soils is to store water and then make it available to plants. The composition of the water molecule plays a major role in the process of storage and movement of water in the soil (Part 9).
Concepts such as matrix potential, gravitation water and capillary movement are important for effective ground water management.
These properties lead to certain ground water constants such as: upper limit of plant available water (field water capacity), lower limit of plant available water (permanent wilt point) and plant available water (Part 10). The movement of water in the soil is also important, as excess water must be able to flow out of the profile and water must also be able to move to the plant root.
With the above hydrological concepts in mind, the ground water balance equation may be used to manage the ground water in such a way that the soil may be able to store the maximum amount of water and make it available to the plants (Part 11). The next aspect that is important, is that the plant roots can absorb the ground water (Part 12). Of these, root distribution and soil/water contact are probably the most important.
Respiring plant roots and soil microbes use O2 and deposit CO2 in the soil air system. Oxygen and CO2 must therefore be exchanged between the soil and the atmosphere (Part 13). This exchange takes place at the soil surface and, when the soil surface is sealed, the
exchange cannot take place and the CO2 builds up in the soil.
When dead plant material is added to the soil, parts of it are mineralised into plant nutritional substances via biological activities, while parts thereof are converted into stable humus in the soil (Part 14). When soil is tilled, an excess of air and oxygen in particular enters into the soil, and then the humus is broken down into plant nutritional substances much quicker. The waste product is CO2 which is then released from the soil into the air (which then contributes to earth warming).
There are a wide variety of organisms that live in the soil. Some are beneficial, while others are detrimental (Part 15). The microbes comprise fungi, bacteria, protozoa, actinomycetes, and algae.
Although the organic fraction of the soil is small, it has a significant effect on the soil’s chemical, physical and biological properties (Parts 16 and 17). Humus makes a major contribution to the CEC of the soil. Adequate organic material leads, for example, to a stable soil structure and organic material is also the food for the soil organisms.
When salts build up in the soil, we say the soil has become brackish (Part 18). There are two forms of brackishness, namely salt brack (white brack) and sodium brack (black brack). The reclamation of soil when it has become brackish, entails the washing out of the salts and the sodium is replaced with calcium and washed out. However, this must be done via consultation and expert advice. It is of particular importance in the management of irrigation soils.
Soil degradation can be divided into three groups, viz. physical, chemical and biological degradation (Part 19). The most important physical properties that usually degrade, are: texture and mineralogical composition, structure, formation of surface crusts and gross density (compaction).
The most important chemical properties of soil that degrade are: lowering in fertility, acidification, chemical imbalances, salination and pollution (Part 19).
In contrast, the most important degradable biological properties are: decrease in quantity of organic material and specific humus, ground fauna and flora and an increase in pathogens.
In Parts 20 and 21, wind and water erosion were discussed. During erosion, it is especially the fertile top soil which is lost. The clay and humus is very light and easily goes into suspension and is then washed away.
The most important factors which lead to a specific soil type, are the
- mother material from which the soil develops;
- topography which supports water provision and plant growth;
- climate, which provides water
- organisms, which live in and upon the soil;
- time (Part 22).
The processes (Part 23) that are driven by the soil formation factors, are mineralisation and humification; leaching and illuviation; gleysol and plinthite formation, as well as inversion bioturbation.
The above processes and factors lead to the formation of a specific soil type under a specific set of conditions. It means that soil types in bodies of soil may be identified and mapped (Part 24).
The South African soil classification system was compiled specifically with the South African soils in mind. A specific horizon sequence is classified as a soil type. A soil type therefore communicates the factors and processes that are active in a body of soil.
These factors and processes also have an influence on plant growth that occurs on the soil. The processes and factors give rise to certain soil properties, which may be interpreted. The probable effect thereof on a certain form of soil usage, may then be deduced.
In Parts 24, 25 and 26, the processes and factors which give rise to the formation of the 74 soil types that are currently acknowledged, are broadly discussed. The properties of the soil groups are highlighted, especially with interpretation for agricultural usage in mind.
Soil is the producer’s most important asset, without which food and fibre cannot be produced. A better understanding of the chemical, physical and biological soil properties, is of particular importance in increasing the biological productivity of the soil, without these properties being degraded.
The soil resource is extremely valuable because one cannot make new soil. Thousands of hectares are lost to agriculture annually as a result of urbanisation, as well as physical and chemical degradation. In addition, the human population is growing at a drastic rate. Sustainable utilisation of soil is therefore of the utmost importance for the continued existence of humans on earth.
For further information, please contact the authors on: Martiens du Plessis: 072-285-5414 / firstname.lastname@example.org or Prof Cornie van Huyssteen: 051-401 -9247 / email@example.com