Hygroscopic Water: The Part of Field Capacity Water with Loose Chemical Bonds

Introduction:
When discussing the properties of soil and its ability to retain water, field capacity is an important parameter to consider. Field capacity refers to the maximum amount of water that soil can hold against gravity after excess water has drained away. It is the point at which the soil is saturated and all the gravitational water has been drained.

However, field capacity water is not a homogeneous entity. It consists of different types of water that are held in the soil by different forces. One of these types is hygroscopic water, which is water held tightly to soil particles through chemical bonds.

Hygroscopic Water:
Hygroscopic water is the water that is held by the soil particles due to the presence of hygroscopic substances within the soil. These substances have a strong affinity for water molecules and can hold water even against strong forces of evaporation. Hygroscopic water is tightly bound to soil particles and is not available to plants for uptake.

Loose Chemical Bonds:
The term "loose chemical bonds" refers to the relatively weak forces of attraction between the hygroscopic substances and the water molecules. These bonds are not as strong as covalent or ionic bonds but are strong enough to hold the water molecules in place.

Other Types of Field Capacity Water:
In addition to hygroscopic water, field capacity water also consists of other types of water held by different forces:

1. Gravity Water: This is the water that drains freely under the influence of gravity after the soil has been saturated. It is the excess water that does not contribute to the soil's ability to retain water.

2. Capillary Water: Capillary water is held in the soil against the force of gravity due to capillary action. It fills the small spaces between soil particles and is available for plant uptake.

3. Residue Water: Residue water is the water that remains in the soil after the gravitational and capillary forces have been satisfied. It is held in the larger pores and is not easily available to plants.

Conclusion:
Hygroscopic water is the part of field capacity water that is held by loose chemical bonds formed between hygroscopic substances in the soil and water molecules. It is tightly bound to soil particles and is not available for plant uptake. Understanding the different types of field capacity water is crucial for determining the water-holding capacity of soils and their suitability for various applications, such as agriculture and construction.