1. PORE SPACE IN THE SOIL

We have already talked about pore spaces in soils in previous lectures, and by now you should know that the term simply refers to the spaces between the soil particles. These spaces are filled either with air or water depending on the amount of water in the soil. When the soil is saturated, all the pore spaces are filled with water. At field capacity about half the spaces are filled with air and half with water, and this is ideal for the growth of plants. At the permanent wilting point the pore spaces are filled with air with only a thin film of water being held on the surface of the particles. This water is unavailable to the plants so they wilt and die.

If the soil particles were round and all the same size, they could be arranged in either of the two ways shown below in the diagram. With the arrangement in Figure 1, half, or 50% of the soil would be pore space and half would he solid particles. In Figure 2, a quarter or 25% of the soil would be pore space, and three quarters would be solid particles.

Figures 1 and 2: Pore space between coarse and fine soil particles

However, the soil particles are not round, are not equal in size, and are usually bunched together in aggregates or crumbs. In practice, the pore space in soils rarely exceeds 50% and rarely falls below 25%. Also it varies, as soils dry out and the particles and crumbs contract. In a good soil with good  soil structure, 50% of the soil will be pore space at Field Capacity, with half the space filled with air and half filled with water.

One very important point to remember is that Clay Soils, because they have the smallest particles have the highest pore space, and Sandy soils, because they have large particles have the lowest pore space. This is a difficult idea to grasp, and the following simple experiment is designed to show that it is true.

PORES SPACE EXPERIMENT: CARRY OUT WITH AT LEAST TWO DIFFERENT SOILS.

  • Weigh out 50 grams of fine, dry soil without any stones or gravel.
    • Carefully place a portion of the soil in a dry 100 cc measuring cylinder; tap the cylinder in order to settle the soil. Continue to add soil and tap, until all the soil is in the cylinder.
    • Read off the Apparent Volume of the soil in cc. Call this Volume A (Va).
    • Carefully tip out the soil onto a piece of paper.
    • Fill the cylinder with water up to the 50 cc mark.
    • Replace the soil slowly, adding the soil to the water in the cylinder, taking care that no soil is spilt and no water is splashed. Do this slowly, so that air is not trapped in the soil.
    • Read off the volume of water plus soil in cc. Call this Volume B. (Vb). The True Volume of the soil particles ‐ volume B minus 50.

Figure 1: Pore Space Experiment

The apparent volume of the soil is the volume of the soil particles plus the volume of the air in the spaces between the soil particles.

In the second measurement the spaces between the particles are occupied by water, therefore the

true volume of the soil particles is the total volume of soil + water minus the volume of the water.

Table 1: Calculations for percentage of pore space in a soil

    The apparent density is given by  Weight/ Apparent Volume  =    50 /Va
    The true density is given by  Weight True Volume =  50 /Vb – 50
    The volume of air in the soil is given by  Apparent Volume – True Volume
= Va – (Vb – 50)
    The percentage pore space isVolume of air in soil x 100 =
Apparent Volume of soil		  Va – (Vb – 50) x 100 /Va

2.     SOIL AIR

The air which occupies the pore space in the soil has a different composition from that which makes up the atmosphere, as the following table shows:

Table 2: Composition of soil air and atmosphere

 Soil AirAtmosphere
Oxygen20.6 %20.99 %
Nitrogen79.2 %78.95 %
CO20.2 %0.03 %

Soil air is nearly always saturated with water vapor because of the moisture in the soil. The variations in the composition of soil air are much greater than the variation in the composition of atmospheric air. As can be seen from Table 2, there is seven times as much Carbon Dioxide (CO2) in the soil air as in the atmospheric air. This high Carbon Dioxide content is due to the high micro‐organism activity in the soil and is a product of their respiration. Sterile soil stops producing Carbon Dioxide because all the micro‐organisms have been killed off.

There is an exchange of Carbon Dioxide and Oxygen between the atmosphere and the air in the upper surfaces of the soil. This exchange takes place by diffusion into the pore spaces and is, in effect, a type of “soil respiration”.

3.     SOIL TEMPERATURE

For any biological process there is a maximum temperature above which it ceases, a minimum temperature below which it ceases and an optimum temperature at which it flourishes.

For common farm seeds 5°C is the minimum temperature for germination, 38°C is the maximum temperature for germination and 30°C is the optimum temperature for germination.

The effect of temperature on plants influences the permeability of cell walls in the plant, the activity of the plant enzymes, and the rate of transpiration.

TEMPERATURES ABOVE THE OPTIMUM

  • This shortens the period of growth as plants grow too quickly.
    • Causes tall, lanky growth which, in turn, makes the plant liable to disease.
    • Makes the plant produce seeds too quickly, and in the case of cereals, this result in lower yields.

TEMPERATURES BELOW THE OPTIMUM

  • This causes plants to grow slowly, often with a purple colour on their leaves.
    • The plant will fail to ripen and so no seeds will be produced.

The following factors affect the temperature of soils:

  • Colour: Black absorbs more heat than white so that a dark coloured soil (e.g. a peat soil) will warm up more quickly and retain more heat than a light coloured soil.
    • Aspect: Soils which lie facing the sun receive more heat per unit area than those facing away from the sun. In South Africa soils on slopes which face to the North receive more heat from the sun than those which lie facing to the south.
    • Water Content: Water requires five times as much heat as the same weight of dry soil to raise its heat by the same number of degrees. It follows that a dry soil will heat up more quickly than a very wet soil. Although this is not so important in the Tropics, it is a very important factor in temperate climates.

Heat is lost from the soil by radiation and also by conduction up into the air and down to the lower levels of the soil. Temperature variations are much greater in the topsoil than at lower depths.