Water Movement in Soil Plant System

The total quantity of water required for the essential physiological functions of the plant is usually less than 5 per cent of all the water absorbed. Most of the water entering the plant is lost in transpiration. But failure to replace the water loss by transpiration results in the loss of turgidity, cessation of growth and death of plants due to dehydration.

The following are the main areas of water movement in plant system:

• Water absorption

• Water adsorption

• Water conduction and translocation

• Water loss on transpiration

The path of water movement may be divided into four sequential processes as follows:

• The supply of liquid to root surface–Adsorption

• The entry of water into the root–Absorption

• The passage of water in the conducting tissues–(Xylem) Translocation or conduction.

• Movement of water through and out of leaves–Transpiration or loss of water.

The rate of water movement is directly proportional to potential gradient i.e., higher potential to lower potential and inversely proportional to the resistance to flow.

A. Mechanism of Water Absorption

In plants, water is absorbed through root hairs, which are in contact with soil water. The wall of the root hairs are permeable and consists of pectic and cellulose substances which are strongly hydrophilic (water loving) in nature. There are two types of absorption viz., (a) Active absorption, and (b) Passive absorption.

(a) Active absorption – Here the process of osmosis plays an important role. The soil plant water movement can be effected due to forces of imbibition, diffusion and osmosis.

Significance of Osmosis

• Large quantities of water are absorbed by roots from soil by osmosis.

• Cell to cell movement of water and other substances takes place through this process.

• Opening and closing of stomata depends upon the turgor pressure of guard cells.

• Due to osmosis the turgidity is maintained and give a shape to the plants.

(b) Passive absorption – It is mainly due to transpiration and the root cells do not play active role. Passive absorption takes place when rate of transpiration is very high. Rapid evaporation from the leaves during transpiration creates a tension in water in the xylem of the leaves. These tension is transmitted to the water in xylem of roots through the xylem of stem. Due to this, water rises upward to reach the transpiring surface. As a result, soil water enters into the cortical cells through the root hairs to reach xylem of the roots to maintain the supply of water. The force for this entry of water is created in leaves due to rapid transpiration and hence the root cells remain passive during this process. It is otherwise known as transpiration pull.

B. Factors Affecting Absorption of Water

(i) Available soil water – Capillary water is available to plants. Hygroscopic water and gravitational water are not available to plants. The capillary water is absorbed by the plants, which in turn reduces the soil water potential. Hence, the water from higher potential area tends to move to lower potential area and root will absorb this water. This is the chain of process involved in water uptake.

(ii) Concentration of soil solutions – High concentration affects the process of osmosis.

(iii) Soil air – Sufficient amount of O2 should be there and excess amount of CO2 affects the availability of water by root suffocation.

(iv) Soil temperature – Up to 30oC favours absorption. Very low and very high temperature affects absorption.

(v) Soil texture

Clay – Neither good nor bad

Sand – Not good for absorption

Loamy – Good for absorption

C. Crop Response to Irrigation and Fertilizers

The requirement regarding the number and their timings vary widely for different crops. It has been observed that water requirement of crops vary with the stages of its growth. When the water supply is limited, it is necessary to take into account the critical stages of crop growth with respect to moisture. The critical stages of crop growth is commonly used to define the stage of growth. Certain critical stages at which if there is shortage of moisture, yield is reduced drastically. When there is shortage of water, it is better to take care of the critical stages first to obtain increased water use efficiency.

(i) Water and fertilizer – Water is the key factor in all the three mechanisms (mass flow, diffusion, transpiration pull) of nutrient uptake. Root intercepts more nutrient ions when growing in a moist soil than dry soil. In moist soil, the effective root zone area will be more and extensive which in turn absorbs more water and nutrients.

This is especially important for calcium and magnesium. If the applied fertilizer uptake is more, it enhances the growth and increases the yield under irrigated condition than dry condition which in turn increases the water use efficiency. Hence, it is concluded that there is a close relationship between soil moisture and nutrient uptake by plants.

The application of fertilizer or nutrients without adequate moisture in root zone is not useful to plants. Similarly, mutual benefits are also obtained from fertilizer. For e.g., in drought situation balanced fertilized crops is able to withstand drought, than relatively low fertilized crop. Even well balanced fertilized crop may not show its normal growth and development unless adequate moisture is available. This is not only due to poor uptake, but also due to poor ET and which in turn reducing the use of absorbed nutrients for photosynthesis.

(ii) Fertilizer use efficiency can be increased by :

• Soil test to evaluate nutrient deficiency and use of proper quantity of the needed fertilizer. Applying fertilizer based on soil test values.

• Placement of fertilizers rather than broadcasting.

• Split does of application at suitable time interval rather than bulk application.

• Controlled application of water to avoid leaching of fertilizers to deeper layers. In most cases there is significant correlation between soil moisture regime, fertilizer requirement and the availability of fertilizer for plant use.

(a) Nitrogen – Mineralization of nitrogen increases as the water content of soil increases from PWP to FC and to saturation. When the fertilizer is applied to the surface soil, its uptake is inhibited when the soil dries.

(b) Phosphorus – Increase in soil moisture to an optimum level is generally possible because of reduced aeration and root penetration or the increased activity of sesqioxide fraction on ‘P’ fixation under reduced condition. In dry areas ‘P’ applied close to the seed is more effective than the broadcast application. The availability and uptake of P is less in dry or rain fed condition.

(c) Potassium – Soil moisture content affects the level of exchangeable ‘K’ in the soil. In high soil moisture zone, availability of k is increased. The results of studies on fertilizer-irrigation relationship lead to the following conclusions.

• Water use efficiency is raised by fertilizers by increased DMP (DRY matter production) and yield

• The response of fertilizer is generally of a higher order under irrigated condition than under unirrigated condition. Response to frequent irrigation is generally enhanced by increased levels of fertilizer application, particularly crops grown for its vegetative plant parts.

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