Hygroscopic soil moisture refers to the water held tightly to soil particles and surfaces due to molecular attraction forces. It is the smallest fraction of soil moisture and is held in the form of a thin film around individual soil particles. This water is not readily available for plant uptake and is often considered unavailable for plant use.
Hygroscopic moisture exists in the soil as a result of the water vapor present in the soil air. When the soil is not saturated, water molecules in the soil air can adhere to the soil particles’ surfaces. The amount of hygroscopic moisture is influenced by factors such as soil texture, temperature, humidity, and the presence of soluble salts.
Soils with finer texture, such as clay or silt, have a larger surface area per unit volume compared to coarser-textured soils like sand. As a result, they can hold more hygroscopic moisture. In contrast, sandy soils have larger pore spaces and lower surface area, leading to relatively lower hygroscopic moisture content.
Hygroscopic moisture is not readily available to plant roots because it is held too tightly by the soil particles. It requires very high energy to extract this water, making it inaccessible for plant uptake. Therefore, plants cannot rely on hygroscopic moisture as a water source for their growth and development.
While hygroscopic moisture is not directly useful for plants, it plays a role in soil-water dynamics. It helps maintain soil structure by providing cohesion between soil particles and contributing to soil stability. It also influences soil water potential, which affects the movement and availability of other forms of soil moisture, such as capillary water and plant-available water.
Understanding the dynamics of hygroscopic moisture is important in managing soil water content and irrigation practices. It is necessary to focus on the more readily available forms of soil moisture, such as capillary water, for plant growth and irrigation scheduling.