It is known that the objective of the irrigation process in general is to supply plants with irrigation water in a quantity that meets the needs of plants for growth and production of the appropriate crop.
Irrigation water is delivered to plants through rivers, canals, or wells, according to the availability of irrigation water to supply the plants with the water they need for plant life, good growth and the production of the appropriate crop.
As for the relationship between plants, soil and irrigation water, all these elements must be studied, as well as the relationship between them to reach the desired goal, which is to obtain the optimal method for delivering water to plants with absolute efficiency.
We will learn about the nature and composition of these elements by studying the nature and composition of soil. And its relationship to irrigation water.
First, the soil:
What is meant by soil is the material that forms the surface layer of the earth’s crust, which ranges in depth from approximately 1-2.5 meters, through which the roots of plants can penetrate and the passage of water and air inside it, and it is able to retain moisture to meet the water needs of plants.
Soil components differ from one place to another, and there may be variation in the nature of the soil in the same place depending on the depth, but all types of soil share together that it consists of four general parts:
1- The solid part:
The solid part of the earth’s crust is the basic soil structure and it consists of particles such as gravel, sand or clay.
2- The organic part:
The organic part of the earth’s crust is the remains of living organisms, plant remains, or any other organic matter that bacteria and fungi decompose.
3- Underground water:
Ground water is the water present in the soil and it occupies all or part of the spaces between the soil particles.
The air in the soil is the air that occupies all or part of the spaces between the soil particles that do not have water.
The proportions of these four parts are not fixed and are constantly changing according to humidity, natural factors, bacterial activity and chemical influences.
And it is considered the best benefit for plants when there is a balance between moisture in the root area and air, which is called the water-air balance. .
The physical properties of soil particles:
First, the texture of the soil.
If we consider that the soil grains are in the form of balls of varying size, then when dividing these grains into groups according to the size or according to the diameter of these grains, each group can be defined by a certain agreed name.
The following is a breakdown of these groups according to size:
1- Slate, which is about 2 mm in diameter.
2- Coarse sand, with a diameter of 2-0.2 mm.
3- Fine sand, with a diameter of 0.02 – 0.20 mm.
4 – silt, which is 0.002 – 0.02 mm in diameter.
5- Clay, which is 0.002 mm in diameter.
Of course, agricultural soil consists of several groups of particles in varying proportions, in addition to some organic materials.
It is possible from mechanical analysis to know the percentages of soil components of sand, clay and silt, which range from 0.002 to 0.2 mm.
On the basis of knowing these ratios of the three components, the type of soil is classified.
For example, if the proportion of sand is 30%, the proportion of clay is 50%, and the proportion of silt is 20%, then this soil is considered clay soil.
Coarse soil in which the proportion of sand increases, its permeability increases, that is, it facilitates the penetration of water and air through it, while its ability to hold water decreases, and vice versa for soft soils in which the content of small particles increases to a high degree that may prevent water intrusion. These lands are called light yellow lands If the proportion of clay in it is less than about 50%..
As for the yellow lands, the proportion of mud in it is from 20-30%, and therefore the proportion of mud leads to an increase in the ability to retain water.
However, if the percentage of clay exceeds a certain limit, the land may not be suitable for agriculture, because it does not allow water intrusion, and the best types of land are those that contain a percentage of clay ranging between 30-40%.
Water division by soil:
Soil water is divided into the following:
1 – Hygroscopic water: It is a thin membrane of water that adheres to the soil particles with great force, as it absorbs the soil particles from the surrounding air, but this membrane is water that the roots cannot absorb this water.
2- Capillary water: this water surrounds the hygroscopic membrane and its thickness is greater than the thickness of this membrane. The roots can absorb this water as there is air between the voids of the soil particles. This capillary water is found where the soil is not saturated with water.
3- Free water: Free water is the amount of water in excess of capillary water that the soil particles cannot absorb, so it is called free water, and it moves downwards. With the presence of this water, the soil becomes saturated with water.