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Soil Water Estimations Measurement and calculation of soil water by mass Measurement and calculation of soil water by volume Objective: This lesson is to help you to understand how water is held in the soil in different forms and to identify the forms of soil water. You will also learn how to use mathematical calculations to make practical use of the different forms of soil water.
Soil Water The presence of adequate water in the soil is vital to plant growth, not only because plants need water for their physiological processes but also because water contains nutrients in solution. Plants use a tremendous amount of water. The combined evaporation and transpiration may be as high as 1 cm per day or about 100,000 kg per ha per day. It is the soil that holds this water and supplies it to the plants. The soil acts as a large reservoir of water. Solid particles of varying sizes and shapes make up the skeleton of the soil. Between the soil particles are interconnected pore spaces that vary continuously in size and shape. In a completely dry soil, all of the pore space would be filled with air; and in a completely saturated oil, water (soil solution) would occupy all of the pore space. Hence, the ability of soil to store water is determined by the percentage of the total soil volume that is occupied by soil solids and pore space. These concepts can be expressed quantitatively by soil porosity and soil water content.
Soil water content Water is held in the soil because of the attraction between soil solids and water. This force can be measured by moisture tension. A knowledge of the amount of water held by the soil at the various tensions is required if we are to calculate the amount of water that is available to plants, the water that can be accommodated before percolation starts, the amount of water that needs to be used for irrigation, etc. For reasons of practical use and for the determination and tabulation of soil moisture data, it is necessary to select definite tension levels as reference points. These are expressed as soil moisture constants. Let us understand, briefly, these soil moisture constants. Soil moisture constants Saturation: A soil where pores are completely filled with water. Field capacity: The moisture content of the soil when downward movement of water has virtually ceased. The soil moisture tension will be around 0.33 atmospheres. Wilting point: The moisture content of the soil when the plants loose their ability to recover from water deficits. The soil moisture tension will be around 15 atmospheres. Hygroscopic coefficient: The hygroscopic coefficient is the percentage of water remaining in an air-dry soil. Forms of soil water The water content at field capacity, wilting point, and the hygroscopic coefficient are all based on the OVEN-DRY reference mass. The percentage of water held under each of these conditions can therefore be used to define the following and other forms of soil water.
Each of these forms of water can be calculated from the appropriate soil mass
Measurement and calculation of soil water by mass It is important to measure either the soil moisture or the amount of water present in a soil. Several different approaches are available for making these measurements. Tensiometers, gypsum blocks, and Bouyoucos moisture meters are used in the field to measure the soil moisture tension. More often these moisture meters are used to schedule irrigations to crops. In field research, soil moisture is estimated by measuring the percentage of water present in the soil. The amount of water present in the soil is determined gravimetrically by weighing a soil sample before and after oven drying.
Percent water calculations Amounts of soil water are most commonly expressed as mass percentages. Please remember that the reference mass for calculating percent water is always based on the OVEN-DRY MASS of the soil. Let us solve an example. 15 g Let us calculate the percentage of water at soil moisture constants. A soil having the following mass per unit volume will serve as an example to calculate the soil moisture constants.
81 - 58 71 - 58 Available water = Field capacity - Wilting point = 40 - 22 = 18 % A soil having the following mass per unit volume will serve as an example to calculate the soil moisture constants.
= 35 % Hygroscopic water = water in air dry soil 64 - 55 Capillary water = Field capacity - Hygroscopic water = 47 % - 16 % = 31 %
Measurement and calculation of soil water by volume
Earlier you have seen that: Water by mass = grams of water (Mw)/ Mass of solids (Ms) Now you will learn that: Water by volume = Vol. of water (Vw)/ Vol. of solids (Vs) Vol. of water = grams of water / density of water Vol. of soil = bulk vol. of the soil Hence: Vol. of water = (grams of water/density of water)/ bulk vol. of the soil water by volume = Vol. of water x 100 %
A cube of soil measures 10 x 10 x 10 cm and has a total wet mass of 1740 g of which 270 g is water. Find the percentage of water content in the soil by mass and by volume. 1. Water content by mass = water (g) / mass of dry soil (g) = 270 / ( 1740 - 270 ) = .18 Water content by mass = .18 x 100 = 18 % 2. Water content by vol. = vol. of water / bulk vol. of soil =( 270 / 1 g per cc) / (10x10x10) = .27 cc Water content by vol. = .27 x 100 = 27 %
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