Use of Water by Plants | Agriculture Optional Notes for UPSC PDF Download

Use of Water by Plants

A plant's root system must exert a negative tension or pressure to draw water from the soil, and this tension must be equivalent to the tension that retains water within the soil. For instance, if the water in the soil is at 0.3 bars (approximately field capacity), the plant needs to generate at least 0.3 bars of negative tension to extract water from the ground.

However, at the wilting point, the maximum negative tension that a plant can generate is balanced by the soil's water tension. At this juncture, the plant becomes incapable of further water uptake from the soil and will experience sustained stress. Several factors come into play when determining the timing, location, and quantity of water a plant will utilize. These factors encompass the plant's daily water requirements influenced by climatic conditions and growth stage, the depth of the plant's root system, and the quality of the soil and water.

Plant Water Need

A plant's water requirements vary at different stages of its growth. Young plants typically need less water compared to when they are in the reproductive phase. As the plant nears maturity, its water demands decrease. These patterns have been documented in daily water requirement curves for various crop types. Figure 8 illustrates a common crop water curve. Perennial crops like alfalfa exhibit similar water use curves to those in Figure 8, albeit with a saw-tooth pattern where water use sharply declines after each cutting and gradually rises until the next harvest.

Plant Root Depth

The depth of a plant's root system determines the extent to which it can access soil water. Young plants typically have shallow roots, and soil water located deeper than their rooting depth isn't beneficial to them. In general, plants draw about 40 percent of their water requirements from the uppermost quarter of their root zone, followed by 30 percent from the next quarter, 20 percent from the third quarter, and only 10 percent from the deepest quarter. As a result, around 70 percent of a plant's water is obtained from the top half of its overall root penetration.

Table 2 provides information on the root penetration depth and the corresponding 70 percent water extraction for various common field crops. Although deeper sections of the root zone can contribute a higher proportion of a crop's water needs if the upper portion becomes depleted, relying on access to deeper water sources can lead to reduced optimal plant growth.

Soil and Water Quality

Soil and water quality are additional factors that influence the availability of soil water to plants. To support healthy plant growth, the soil should allow sufficient space for both water and air movement, as well as for root development. Certain soil management practices can alter soil structure. For instance, excessive tillage can disrupt soil aggregates, and excessive traffic can lead to soil compaction. Both of these practices reduce the amount of pore space in the soil, diminishing the availability of water and air while limiting room for root growth.

Water quality also plays a crucial role in plant development. Irrigation water with a high soluble salt content is less accessible to the plant. Therefore, a higher soil water content must be maintained to ensure water availability for the plant. As the salt content of the water increases, the ability to transport water from the soil to the roots diminishes. Additional water is often required to flush salts below the root zone to prevent salt buildup in the soil. Poor-quality water can have adverse effects on soil structure. Most crops in Kansas exhibit only moderate salt tolerance.