Soil water is a vital component of the soil environment and plays a crucial role in the growth and survival of plants. Soil water refers to the water that is held in the soil, including water in soil pores and the water that is tightly bound to soil particles. It is an important resource for plants, as they rely on it for growth and survival.
Water is the most common substance on the earth; it is necessary for all life. The supply of fresh water on a sustained basis is equal to
the annual precipitation, which averages 66 centimetres for the world’s land surface. The soil, located at the atmosphere-lithosphere interface, plays an important role in determining the
amount of precipitation that runs off the land and the amount that enters the soil for storage and future use.
Approximately 70 per cent of the precipitation in the United States is evaporated from plants and soils and returned to the atmosphere as vapour, with the soil playing a key role in water retention and storage. The remaining 30 per cent of the precipitation represents the longtime annual supply of fresh water for use in homes, industry, and irrigated agriculture.
Today’s article contains important concepts and principles that are essential for gaining an understanding of the soil’s role in the hydrologic cycle and the intelligent management of water resources starting with the types of water.
Soil Water Energy Continuum
The soil water energy continuum refers to the concept that soil water is held in the soil profile with various levels of energy or tension, ranging from low to high. The energy of the soil water is determined by the balance between the forces of gravity, matric potential, and osmotic potential.
At the lower end of the soil water energy continuum is gravitational water, which is held in the soil by the force of gravity. As the soil water energy increases, so does the water’s ability to move upward in the soil profile and become available for plant uptake. At the higher end of the soil water energy continuum is capillary water, which is held in the soil pores by the force of surface tension and is readily available to plants.
The soil water energy continuum helps to understand how water moves through the soil profile and how it is available for plant uptake.
As water cascades over a dam, the potential energy (the ability of the water to do work) decrease. If water that has gone over a dam is returned to the reservoir, work will be required to lift the water back up into the reservoir, and the energy content of the water will be restored. Therefore, in a cascading waterfall, the water at the top has the greatest energy and the water at the bottom has the lowest energy.
As water cascades down the falls, the continuous decrease in energy results in an energy continuum from the top to the bottom of the falls. As an analogy, when wet soil dries, there is a continuous decrease in the energy content of the remaining water. When dry soil gets wetter, there is a continuous increase in the energy of soil water.
The continuous nature of the changes in the amount of soil water, and corresponding changes in energy, produce the soil water energy continuum.
Types Of Soil Water & Levels
Soil water can be said to be classified into three types and levels which are;
Adhesion Water
Cohesion Water
Gravitational Water
Hygroscopic Water
Capillary Water
Adhesion Water
Adhesion water refers to the soil water that is held in the soil by the force of adhesion between the soil particles and the water. Adhesion water is closely related to hygroscopic water, which is water that is tightly bound to soil particles and not easily available to plants. Both adhesion water and hygroscopic water contribute to soil structure and stability and help prevent soil erosion.
Adhesion water is important because it helps to maintain soil structure and stability, and also provides some resistance to soil erosion. The amount of adhesion water in the soil depends on several factors, including soil type, particle size, and soil moisture content. In general, soils with larger particles and higher moisture content will have more adhesion water than soils with smaller particles and lower moisture content.
Adhesion water is so strongly adsorbed that it moves little, if at all, and some scientists believe that the innermost layer of water molecules exists in a crystalline state similar to the structure of ice. Adhesion water is always present in field soils and on dust particles in the air, but the water can be removed by drying the soil in an oven.
Adhesion water has the lowest energy level, is the most immobile water in the soil, and is generally unavailable for use by plant roots and microorganisms.
Cohesion Water / Hygroscopic Water
Cohesion water refers to the soil water that is held in the soil by the cohesive forces of the water molecules themselves. These forces are the result of the attractive interactions between water molecules and are what give water its characteristic surface tension. In soil, cohesion water is held in the soil pores by the cohesive forces of the water molecules, which form a thin film around soil particles.
The cohesive forces of water in soil decrease inversely and logarithmically with distance from the soil particle surface. This means that the cohesive forces are strong near the soil particle surface, but become weaker as the distance from the surface increases. As a result, the cohesive forces are most effective in holding water in the smallest soil pores, where the distance from the surface is greatest.
Cohesion water is closely related to hygroscopic water, as both types of water are held in the soil by cohesive forces. However, cohesion water is held in the soil pores by the cohesive forces of the water molecules themselves, while hygroscopic water is held in the soil pores by the cohesive forces between the water and the soil particles
Hygroscopic
Hygroscopic water refers to the soil water that is held in the soil pores by the cohesive forces between the water molecules and the soil particles. This type of soil water is called hygroscopic because it is strongly influenced by the water-holding capacity of the soil, which is largely determined by the hygroscopic properties of the soil particles.
Hygroscopic water is held in the soil pores by the cohesive forces between the water and soil particles, which are influenced by the chemical composition and structure of the soil particles. The cohesive forces between the water and soil particles are stronger in soils with high levels of organic matter, clay, or minerals with high surface area, as these soil components have a greater capacity to hold water through their hygroscopic properties.
Hygroscopic water is an important type of soil water, as it helps to maintain soil structure and stability, and contributes to the overall water-holding capacity of the soil. Understanding the role of hygroscopic water in soil water dynamics is essential for effective water resources management and sustainable agriculture.
Gravitational Water
In soils with claypans, a very slowly permeable subsoil layer permits little water, in excess of the field capacity, to move downward and out of the soil. After a long period of rainfall, water accumulates above the claypan in these soils.
As the rainy period continues, the soil above the claypan saturates from the bottom upward. The entire A horizon or root zone may become water saturated. Soil water that exists in aeration pores, and that is normally removed by drainage because of the force of gravity, is gravitational water.
When soils are saturated with water, the soil volume composition is about 50 per cent solids and 50 per cent water. Gravitational water in the soil is detrimental when it creates oxygen deficiency. Gravitational water is not considered available to plants because it normally drains out of soils within a day or two after the soil becomes very wet.
Gravitational water refers to the soil water that is found in the lower soil profile and is held there due to gravity. It moves very slowly downward and is not readily available to plants. Gravitational water is located at the lower end of the soil water energy continuum.
Capillary water what does it mean for Gravitational water, Adhesion water and Cohesion water
Capillary water is a type of soil water that is held in the soil pores by a combination of forces, including the cohesive forces of the water molecules, the adhesive forces between the water and soil particles, and the forces of gravity. Capillary water is the most readily available type of soil water for plant uptake, as it is held in the soil pores with a balance of forces that allows it to move readily up through the soil profile and into the root zone of plants.
Gravitational water, adhesion water, and cohesion water are also types of soil water, but they are held in the soil by different forces. Gravitational water is held in the soil by the force of gravity and moves slowly downward, making it not readily available to plants. Adhesion water is held in the soil by the adhesive forces between the soil particles and the water and contributes to soil structure and stability. Cohesion water is held in the soil by the cohesive forces of the water molecules themselves.
In summary, capillary water, gravitational water, adhesion water, and cohesion water are all types of soil water, each held in the soil by different forces. Capillary water is the most readily available type of soil water for plant uptake, while gravitational water, adhesion water, and cohesion water play different roles in soil water dynamics.
Conclusion
Soil water is a crucial component of soil and plays a vital role in the growth and survival of plants. There are three types of soil water: gravitational water, held by gravity in the soil; capillary water, held in soil pores by surface tension and readily available to plants; and adhesion/cohesion/hygroscopic water, held in the soil by the force of adhesion or cohesive forces of water molecules.
Adhesion water is closely related to hygroscopic water, which is tightly bound to soil particles and not easily available to plants. It contributes to soil structure and stability and helps prevent soil erosion. Cohesion water is held in soil pores by cohesive forces of water molecules. The soil water energy continuum refers to the concept that soil water is held in the soil profile with different levels of energy or tension, ranging from low to high.
