- PROPERTIES OF SAND AND SILT
We know from the three previous lectures that soils are made up of sand, silt and clay particles produced from rocks by a process called weathering. We are now going to look at the properties of these soil fractions.
- Sand: These are the largest particles in the soil, between 2 mm and 0,02 mm in size. Sand has no chemical properties at all and acts as a skeleton for the soil. It provides anchorage for the plant roots, and allows air and water to travel slowly down through the soil.
- Silt: These are the particles between 0,02 and 0,002 mm in size; they are like sand particles and have no chemical properties.
2. PROPERTIES OF CLAY
These are the very smallest particles, smaller than 0,002 mm in diameter. They are called Colloids, and are produced by a chemical process (see Lecture 3). Because of this, they have Chemical Properties.
- When they become wet, they absorb water and swell.
- When they dry out, they shrink. This causes the cracks which appear in clay soils when they dry out.
- They can be moulded when they are wet and will retain their shape on drying out.
- They can hold water very tightly indeed. The smallest clay particles are smaller than the root hairs of plants so that it is impossible for root hairs to penetrate between the smallest clay particles and extract water. Furthermore, the water on the surface of these very small particles has a very high surface tension.
- Because the particles are small, their total surface area is great. This means that Water can be drawn up towards the surface of the soil by a process known as capillary attraction.
- The most important property of the very small clay particles, or clay colloids, is that each particle carries a negative electrical charge. This causes the flocculation of clay particles, which controls the Acidity and Alkalinity of the soil, and is the method by which certain nutrients are held in the soil.
Figure 1: Soil Particle
(COLLOIDS: one of the forms in which minerals exist ‐ the opposite of crystalloid)
FLOCCULATION OF CLAY PARTICLES
Agricultural Lime is really a chemical called Calcium Carbonate with a chemical symbol CaCO3. When lime is added to water, it breaks up into Calcium (Ca) and Carbonate (CO3). The calcium part carries a positive electrical charge which can be shown like this ‐ Ca++. When lime is added to a clay soil, and remember that all soils that have dried out completely still contain some water, the Calcium (Ca++) is attracted to the ‘negatively’ charged soil particles; some of the Calcium sticks to the surface of the soil particles changing the charge of the particle from negative to positive. The clay particles then
stick together, form larger particles slowly growing in size. In this way a clay soil that has been limed tends to behave more like a sandy soil because the particles are larger; it is better drained, does not retain so much water, dries out more quickly and is easier to work. The process of clay particles sticking together is known as flocculation, and when the soil is in this condition, it is said to be flocculated.
Figure 2: The process of Deflocculation and Flocculation
Particles are repelling Particles joining together Particles settled out Each other
3. SOIL ACIDITY
As we have seen, the clay particles, or colloids as they are called, carry a negative electrical charge, and can attract substances such as Calcium that carry a positive charge. These positively charged substances are called ions. When substances like Lime and Common Salt are dissolved in water they become ionised as follows:
Na, Ca, CO3, etc., are simply symbols used to save writing out the full name of each compound.
The same sort of thing happens with Acids. All acids contain Hydrogen (H), and when they are in solution, e.g. when they are liquids, the Hydrogen is in the form of H+, or Hydrogen Ions. Examples of acids are:
Carbonic Acid is a very important acid in soil science and plant nutrition, because most rainwater is, in fact, a very weak solution of this acid. It is formed by Carbon Dioxide in the air being dissolved in rainwater as it falls to the ground.
When the soil water contains a lot of Hydrogen Ions (H+), many of these are attracted onto the clay particles, and the soil is said to be Acid. Acidity and Alkalinity can be measured quite easily in a laboratory and all soils that are sent for analysis are tested for acidity. The results are given on what is called the pH scale, and this is just a measure of the hydrogen ions in the soil. The pH scale goes from 1 to 14, with the mid‐point of 7 being neutral. Anything below 7 is Acid, and anything above 7 is Alkaline. The best reading for an agricultural soil is 6,5 which is slightly Acid.
Figure 3: The pH Scale
The Acidity of the soil can be altered by adding Lime which is highly alkaline and neutralizes the acid in the soil. It is a common agricultural practice to add Lime in the form of Calcium Carbonate to the soil in order to make it more alkaline.
Alkaline: The effect in the soil is to replace the Hydrogen Ions (H+) with Calcium Ions (Ca) both in the soil water and, more important, on the clay particles. The hydrogen Ions are then washed out of the soil.
Figure 4: The following diagram shows what is happening on the clay particles
However, as we have said, rainwater contains very small amounts of Carbonic Acid, and as rain is added to the soil, more and more Carbonic Acid is gradually added to the soil. The Hydrogen Ions from the acid gradually replace the Calcium Ions from the lime that has been added to the soil, and the soil slowly becomes more Acid. This is illustrated in the following diagram:
Figure 5: Illustrates how acids are formed on the soil particle
This replacement of the Calcium Ions (Ca++) by Hydrogen Ions (H+) is a slow process, and Lime need not be applied to the soil every year. In the U.K. the ‘normal Practice is to apply 5 tons of Lime per hectare every 5 years. In Central Africa it is applied more often and in smaller quantities because rain falls here as storms and has a greater washing effect on the soil. However, Calcium Ions are held in the soil by the clay particles, and this is a very important fact to remember.
4. BASE EXCHANGE
We have just seen how Calcium Ions are held in the soil by being attracted to the clay particles in the Soil. However, Calcium is not the only substance which is held in the soil in this way, and some important plant nutrients also behave like this. Other elements are:
Table 1: Other elements present in the soil
|This is one of the major plant nutrients.
|This is a source of Nitrogen for the plant.
|This is always present in the soil, but excess will cause Brackish Soils (see Lecture 9).
|An important soil mineral.
|The rest are all important minerals and Trace Elements in the soil and in plant nutrition. Some are required by plants in very small quantities.
Note: some of these elements carry one positive charge and some carry two ++, but this is not important.
Base Exchange or Cation Exchange is the ability of the clay particles in the soil to exchange one Ion,
- Mg, for another ion e.g. K+. Some ions are more easily exchanged than others, but, in general, the Base Exchange capacity of a soil is a measure of the amount of clay particles in the soil. This is an important concept which you should take note of: the idea that certain elements are held in the soil by being attracted to the negative charge on the clay particles. This is the reason why clay soils have a much better nutrient‐holding capacity than sandy soils.
Finally, knowledge of soil acidity is important because it:
- Influences plant growth
- Affects the activity of soil micro‐organisms
- Affects the farmer’s choice of fertilizer.
Acid Tolerant crops are Oats, Rye, Maize, Potatoes, Sugar Cane, Fruit trees and vines, Tobacco, Cotton, Groundnuts, Peas, Soya beans, Sorghum and Coffee. These crops like a pH of 5,5 to 6.
Acid Sensitive crops are Wheat, Barley, Sugar Beet, Fig Trees, Cabbages, Kale, Lucerne, Clover, Beans and Onions. These crops like a pH of 7.