Fundamentals of Soil Science (1+1)

Introduction

Introduction Water present in soil pores is called soil water. It is an important component of the soil which influences soil organisms and plant growth. It serves as a solvent and carrier of nutrients for plant growth. It regulates soil temperature and helps in chemical and biological activities of soil. It is essential for soil forming processes and weathering. Forms of soil water: 1. Gravitational water (free water): This form of water is loosely held in soil (in macro pores) and move downwards freely under the influence of gravity. Water in excess of the field capacity is termed gravitational water. The drainage or deep percolation loss of water results from downward movement of this gravity water. It has a suction of less than 1/3 atmosphere. The plants can not absorb it as it drains out of root zone in short period of time. 2. Capillary water: Capillary water is held in the capillary pores (micro pores) with a suction ranging from 1/3 and 31 atmospheres. Capillary water is retained on the soil particles by surface forces. It is held so strongly that gravity cannot remove it from the soil particles. The availability of capillary water to plant roots depends on pore diameter which controls the pressure of water. The narrower the capillary pore, lesser is the availability. 3. Hygroscopic water: This form of soil water is held with a high suction ranging from 31 to 10000 atmospheres. It is held tightly on the surface of soil colloidal particles. Generally, it includes first two molecular layers of water on soil particles. Plants can not absorb this form of water. Soil moisture constants These are of practical importance for irrigation and drainage management. These are also used to compare water retention capacity of different soils. 1. Field capacity: If a soil is saturated, gravity water starts moving downwards. When all the gravitational water is drained away, and then the wet soil is almost uniformly moist. The amount of water held by the soil at this stage is known as the field capacity of that soil. It is the capacity of the soil to retain water against the downward pull of the force of gravity. At this stage only micro-pores or capillary pores are filled with water and plants absorb water for their use. At field capacity water is held with a suction of 1/3 atmosphere. 2. Wilting coefficient: As the soil water content decreases, a point is reached when the water is so firmly held by the soil particles that plant roots are unable to extract water at a rate sufficient to meet the transpiration needs. The plant begins to wilt. At this stage even if the plant is kept in a saturated atmosphere it does not regain its turgidity and wilts unless water is applied to the soil. The stage at which this occurs is termed the wilting point and the percentage amount of water held by the soil at this stage is known as the wilting coefficient. Water at wilting coefficient is held with a force of 15 atmospheres (pF=4.2). 3. Hygroscopic coefficient: Þ The hygroscopic coefficient is the maximum amount of hygroscopic water absorbed by 100 g of dry soil under standard conditions of humidity (50% relative humidity) and temperature (15°C). This tension is equal to a force of 31 atmospheres (pF=4.5). Water at this tension is not available to plant but may be available to certain bacteria. Available water capacity: The available water is the difference in the amount of water at field capacity (- 0.3 bar) and the amount of water at the permanent wilting point (- 15 bars). Maximum water holding capacity: It is the amount of moisture in a soil when all of its pore spaces both micro and macro are completely filled with water. Energy concept of soil water The retention and movement of water in soils, its uptake and translocation in plants and its loss to the atmosphere are all energy related phenomenon. The more strongly water is held in the soil, the greater is the heat (energy) required. In other words, if water is to be removed from a moist soil, work has to be done against adsorptive forces. Conversely, when water is adsorbed by the soil, a negative amount of work is done. The movement is from a zone where the free energy of water is high (standing water table) to one where the free energy is low (a dry soil). This is called energy concept of soil water. The difference between the energy states of soil water and pure free water is known as soil water potential. Forces influencing free energy of water: 1. Gravitational force: This acts on soil water, the attraction is towards the earth's center, which tends to pull the water down ward. This force is always positive. 2. Matric force: It is the attraction of the soil solids for water (adsorption) which markedly reduces the free energy (movement) of the adsorbed water molecules. 3. Osmotic force: It is the attraction of ions and other solutes for water which reduces the free energy of soil solution. Matric and Osmotic potentials are negative and reduce the free energy level of the soil water. These negative potentials are referred as suction or tension. Total soil water potential (ψt) is the sum of gravitational potential (ψg), matric potential (ψm) and the Osmotic potential or solute potential (ψo). o ψt = ψg + ψm + ψo Soil water potential is expressed in terms of atmospheres or bars. Atmosphere is the average air pressure at sea level. Units: Soil water potential is expressed in different units i.e. pF, height in cm of unit water column whose weight is just equals to the potential under consideration, bar, standard atmospheric pressure at sea level which is equal to 14.7 lb/inch2, 760 mm of Hg, or 1020 cm of water. Now a days, megapascal (MPa) which is numerically equal to 10 bars is also used. 10cm height of water column=1 pF= - 0.01 bar = 0.01 atm = -0.001MPa