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The following points highlight the five characteristics of soil and water relations of plants. Some of the Characteristics are: 1. Determination of Water Content of Different Types of Soils 2. Determination of Field Capacity (Or Water Holding Capacity) of the Soil 3. Determination of Porosity, Water Holding Capacity and Adorptivity of Soil and Others.
Characteristic # 1
Determination of Water Content of Different Types of Soils:
Experiment:
50gm sample of each of clayey, loamy and sandy soils is taken. Soils are dried in an oven at a temperature of 105°C for approximately 48 hours and the final weights are recorded.
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Results:
The loss in weight determines the amount of water which was present in the soil.
The percentage of water content is calculated from the following formula:
Loss in weight is determined by subtracting the final weight of the soil after drying from the initial weight of the soil before drying. The dry weight of the soil is determined by subtracting the weight of the container from the total weight of container plus soil after drying.
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Discussion:
Size of the particles present in a soil determines-the water holding capacity of the soil. The larger the size of the particles and their proportion, the more is its air holding capacity and the less is the water holding capacity.
The sandy soil particles being of larger dimensions have poor water content. The reverse is true in case of clayey soil, and loamy soil falls in-between.
Characteristic # 2
Determination of Field Capacity (Or Water Holding Capacity) of the Soil:
Experiment:
Some sandy and clayey soils are completely dried in a hot air oven. Two long cylindrical tubes of 3 cm diameter are taken and filled with weighed amount (100gm) of the soil samples after plugging one end with cotton.
Both the tubes are clamped in a vertical position and from the open end of each water are slowly poured and allowed to percolate through the soil by gravitational pull. When the soil is saturated, pouring of water is stopped and the tubes are allowed to stand for some time till all the gravitational water has drained off.
The wet soil is then taken out from the tubes and the amount of water retained by each is found out by weighing and drying method.
Results:
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The amount of water retained by 100gm weight of original dry soil gives the field capacity of the soil which can be calculated from the following formula:
Field capacity or percentage of water holding capacity:
Discussion:
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Field capacity is a measure of the amount of water present in a given soil when it is saturated. The field capacity or water holding rapacity generally depends on the nature of the soil particles, porosity, temperature and the presence of hydrophilic colloidal materials in the soil.
Sandy soil has the least, clayey the maximum and loamy has the intermediate relative values of field capacity.
N.B. In this experiment some errors may creep in due to adherence of soil particles to the container as well as to the cotton plugs. This may be avoided by subtracting the weight of the cylinder plus cotton plug (previously determined) from the final weight.
Characteristic # 3
Determination of Porosity, Water Holding Capacity and Adorptivity of Soil:
Experiment:
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100gm each of dry sand, dry silt (from river bed) and dry clay is taken. Three funnels having folded filter papers are separately filled with three types of soils. 100 ml of distilled water coloured with safranine are slowly poured on each type of soil and allowed to drain out through the stem of the funnel. The drainage is collected in beakers and their volumes are measured (Figure 6).
Observation:
It is seen that the collected volume of water is more in case of sandy, less in silty and least in case of clayey soils. The filtered water shows decrease in the intensity of colour up to different degrees maximum in sandy and minimum in clayey soil. The clay soil adsorbs maximum safranine colour and sand minimum.
Inference:
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The experiment shows that clay has maximum water holding capacity and adsorptivity but least porosity. Sand has highest porosity but least water holding capacity and adsorptivity. Silt shows intermediate values between sand and clay.
Characteristic # 4
Determination of Capillary Flow in Soil:
Experiment:
Three cylindrical glass tubes of the dimension 40 cm long and 2 cm diameter are taken and open end of each tube is plugged with cotton. Air dried soil samples (clay, silt and sand) are taken in each tube separately and uniformly packed up to certain height by moderate tapping. Tubes are allowed to stand vertically in beaker containing water so that the plugged end dips in water.
Observation:
Water gradually rises up through the soil in the tubes. It diffuses upward faster through sand followed by silt and clay.
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Inference:
The capillary spaces between the colloidal particles of clay being narrower and find in dimension exert greater resistance to flow of water and maximum adhesive force. In case of sand the capillary spaces are wider offering lesser resistance to flow, and minimum adhesive force.
Characteristic # 5
Determination of Permanent Wilting Percentage (PWP) or Wilting Coefficient of a Soil:
Experiment:
Physical conditions of the environment should be uniform during this experiment.
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Experimental soil sample is taken in a water-proof pot and a suitable plant is grown in it till it attains maturity. The soil of the pot is watered properly.
The soil surface is covered with a polythene sheet to check evaporation from its surface. The only loss of water from the soil now occurs due to absorption and transpiration by the plant. Further watering of the pot is strictly avoided.
As time elapses, older leave, start wilting and reach a point of permanent wilting and the plant does not recover even after being kept overnight in a chamber having 100% relative humidity.
Results:
The sample of soil from the pot is taken and its water content is determined by oven-drying method.
The permanent wilting percentage or wilting coefficient is calculated as follows:
Discussion:
Wilting coefficient or permanent wilting percentage (PWP) indicates the amount of water in the soil which is actually unavailable to the plant when permanent wilting has set in. Different soils have different wilting coefficients and therefore affect growth of the plant to various extents.
The same soil can, however, have different PWP values for different species of plants. This usually .lies between 1 to 15% and depends upon the nature and texture of the soil. This includes hygroscopic water and bound water of the soil.
Sandy soil has lowest and clayey soil has highest PWP values. Wilting coefficient indicates that amount of water content in a soil below which plants cannot grow and develop normally.
N.B. Wilting coefficient may also be determined from moisture equivalent of the soil which is the amount of water held in a layer of soil 1 cm thick when saturated and subjected to a centrifugal force of 1000g.
Wilting coefficient is determined by the following formula:
