Soil and Water Interactions | Agriculture Optional Notes for UPSC PDF Download

Interactions between Soil and Water

Comprehending the relationships between soil and water, encompassing aspects like soil water content, the soil's capacity to retain water, and soil water tension, holds significant importance. This understanding proves invaluable when making choices related to planting and irrigation.

Soil Water Content

To measure the amount of water present in the soil at any given time, various soil water content parameters are commonly used. These include saturation, field capacity, wilting point, and oven-dried.

Saturation occurs right after heavy rainfall or irrigation, where all the soil's pores are filled with water. At or near saturation, some water drains or percolates due to gravity, referred to as gravitational water. This excess water can be utilized by plants or lost to evaporation over time.

Field capacity, however, is somewhat less precisely defined but is often considered to be at about one-third of atmospheric tension. (Tension will be explained in a later section.) Wilting point denotes the soil water content at which the plant root's ability to absorb water equals the soil's water potential. If the soil water reaches the wilting point, it can lead to crop death. The reference point for determining soil water content is soil that has been completely dried in an oven, removing all soil water.

The quantity of water at any given soil water content varies according to soil type. Specific water-holding capacities are available from various sources, with NRCS County Soil Surveys being a readily accessible option. Figure 3 illustrates the typical amounts of water held at defined soil water content levels for different soil types, such as sand, loam, and silty clay loam soils. The variations in water-holding capacity among soil types will be explained in subsequent sections.

Soil water content can be expressed in inches of available water or as a percentage, with typical values of both expressions presented in Table 1.

How Soil Holds Water?

Soil retains water through two distinct mechanisms: as a thin film coating individual soil particles and as water held within the soil's pores. The water adhering as a thin film on individual soil particles is referred to as "adsorption." Adsorption involves intricate chemical and physical interactions, but in simple terms, it signifies that a delicate film of water clings to the outer layers of soil particle molecules.

On the other hand, water stored within the pores of the soil is termed "capillary storage." An example of this phenomenon can be observed when one end of a glass capillary tube is placed in a container of water. Water within the tube will ascend to a certain height, depending on the diameter of the capillary tube (as illustrated in Figure 4). This capillary action can function in any direction and plays a crucial role in the storage of water within soil pores, as depicted in Figure 5.

Soil Water Tension

The ease with which water can be drawn from the soil is dependent on the soil water tension, also known as soil water potential. Water held in soil by capillary storage or through adsorption is maintained in the soil at a specific tension. As the soil dries out, these tensions increase. It's easier for a plant to extract water when it's held at lower tensions. The tensions associated with the equilibrium points in soil water, which were discussed earlier, are examples of tensions that influence a plant's water uptake.

At saturation, the soil water tension is approximately 0.001 bars. One bar of tension equates to 1 atmosphere of pressure (14.7 psi). Hence, at saturation, it's relatively easy for a plant to access water from the soil. However, saturation is short-lived, so plants only extract a small portion of the water above field capacity. Field capacity, defined at around one-third atmosphere pressure or about 0.3 bars, still allows plants to efficiently access water from the soil.

The wilting point is reached when the potential of the plant's roots balances with the soil water potential, making it impossible for plants to absorb more water. This typically occurs at approximately 15 bars, and plants may perish at this soil water tension level. For reference, the soil water tension in an oven-dried soil sample is approximately 10,000 bars.

A soil water retention curve, also known as a soil water characteristic curve, illustrates the tension relationships (as depicted in Figure 6). These curves vary slightly for different soil types due to differences in soil texture and structure. Water between field capacity and the wilting point is considered available to the plant. However, optimal plant growth and yield are achieved when the soil water content remains within the upper half of the range of plant-available soil water.

Plants regulate the tension, or potential, for moving soil water from the soil into their roots and distributing it through the plant by adjusting the water potential, or tension, within their plant cells. Water potential comprises various components, with solute potential being of particular importance. Solute potential results from the presence of dissolved solutes like sugars and amino acids in the plant cells. The underlying principle is that water always moves from areas of higher water potential to lower water potential. For water to flow from the soil to the roots, stems, leaves, and eventually into the air, the water potential must continuously decrease. This is illustrated in Figure 7, indicating the movement from the soil with high water potential (less negative) to the air with lower water potential (more negative). Tension is often denoted by the symbol "y." Air has consistently low water potential, so water moves from the plant through the air. However, plants have limitations on the extent to which they can adjust these potentials.