chapter 3 soil water and irrigation practice1

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  • IrrigationEngineering:LectureNote

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    Chapter 3 Soil Water and Irrigation Practice Introduction

    Soil plant water relationships relate to the properties of soil and plant that affect the movement retention and use of water.

    Soil Serves as a storehouse of water. Irrigation water and rain water become available to plants through the soil. Irrigation water and rain water after due infiltration in to the soil get stored in micro & macro pores of

    the soil. The water stored in the soil pores within the root zone constitutes the soil water. Water in soil medium is involved in many processes and soil characteristics influence those greatly. An understanding of the relation ship between soils and water is essential to make the most efficient use

    of water in crop production. Soil A system

    Soil is a three-phase system consisting of solid, liquid and gases. The minerals and organic matters in soil constitute the solid phase. Water forms the liquid phase The soil air forms the gaseous phase. The mineral matters comprise the largest fraction of soil and exist in the form of particles of different

    sizes and shapes encompassing the void space called soil pore space. Amount and geometry of soil pores, depend on the relative proportion of different sizes and shapes of

    soil particles, their distribution and a management. The pore space remains filled with air and water in varying proportions, which are mainly manipulated by

    the amount of water present in the soil. The soil air is totally expelled from soil when water is present in excess amount as in water logged soil,

    while water in liquid form may be absent in dry sands of deserts. Volumes of the soil components vary widely. A typical silt loam soil contains about 50% soil solids 30%

    water and 20% soil air. Soil serves as a medium of plant growth. Soil components when exists in proper amounts offer a favorable condition for plant growth.


    Reservoir of water Reservoir of Nutrients For anchorage Habitat for organisms

    Water Plants grow on soils that provide them with water & nutrients. They absorb the water from soils mainly through roots and use only 1.0 to 1.5 percent of the volume of water absorbed for building their vegetative structures and performing various physiological and biochemical activities. The rest of water absorbed is lost through transpiration. A close relationship exists between soil water and plant and that should be clearly understood to decide up on the time and depth of irrigation and make the most efficient use of irrigation water. An excess or deficit of soil water hinders the plant growth and reduces the yield.

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    Role of water in plants It is a structural constituent of plant cells. It is source of two essential elements oxygen, hydrogen required for synthesis of Carbohydrate during

    photosynthesis. It serves as a solvent of substances and allow metabolic reactions to occur. It serves as a solvent of plant nutrients and helps in up take of nutrients from soils. It helps to transport manufactured to various parts of the plant in soluble form.

    Soil Physical Properties Influencing Soil Water Relationship The important physical properties of soil affecting the soil-water relationship relate to soil characteristics governing the entry of water in to the soil during irrigation or rain, water movement through the soil, retention of water by the soil and availability of water to crop plants. The two main physical properties of soil influencing soil-water relationship are soil texture and soil structure. Soil Texture Soil texture refers to the relative sizes of soil particles in a given soil. The sizes of particles making up a soil determine its texture. In other words soil texture refers to the relative proportion of the various size groups (soil separates) of mineral particles in a given soil. According to their sizes soil particles are grouped in to gravel, sand, silt and clay sand, silt and clay are called soil separates. The relative sizes of sand, silt and clay as proposed by the united state Department of Agriculture (USDA) and international soil science society is given below.

    Soil separates


    Particle diameter (mm)

    Coarse Sand 1.0 0.5 2.0 0.2 Medium Sand 0.5 0.25 -

    Fine sand 0.25 0.10 0.2 0.02 Very fine sand 0.10 0.05 -

    Silt 0.05 0.002 0.02 0.002 Clay < 0.002 < 0.002

    The percentage contents of soil separates in a soil are determined by Mechanical analysis. Based on the percentage content of sand, silt and clay present, the textural class of soil is determined by using textural triangle given below.

    If a soil sample is analyzed for mechanical fractionalization and the result indicates that is made up of 25% clay, 45% silt and 30% sand. Line may be traced on the textural triangle. Thus the above soil is indeed Loam soil.

    ISSS Very Coarse Sand

    ISSS Very Coarse Sand 2.0 1.0 - Coarse Sand 1.0 0.5 2.0

    0.2 Very Coarse Sand

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    Figure : Textural Triangle

    Physical Characteristics of Textural Classes of soils

    Sandy soil

    Loose and single grained Individual grains can be seen or felt. Give a rough feeling when rubbed between fingers Dry sands remain loose when pressed. Slightly moist soil tends to form a ball when pressed in palm but the same brakes when the pressure

    is released. A moist soil forms a ball with impressions of fingers on it, but the same brakes at the release of the

    pressure. Has a low water holding capacity and availability of water to plants is quite low. Has high infiltration rate Light soil and can be tilled very easily

    A sand group includes all soils comprising sand fraction by 70 percent or more of the material weight. The properties of such soils are characteristically sandy in nature. Specific classes:- Sandy soil and loam sand.

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    Loam soil

    Contains sand, silt and clay fractions almost in equal proportions. When felt between fingers, it gives the feeling of the presence of small grits. When a lump of slightly moist soil is pressed in palm, it forms a ball and does not break when pressure is

    released, but falls a part when dropped on the ground from above. A wet soil forms a ball that does not disintegrate when the pressure is released; it breaks when dropped

    from a height with particles not separated out fully. Has a good water holding capacity and Can be tilled comfortably Provides favorable physical condition for crop growth. Specific classes:- Sandy loam Silty loam Clay loam

    Clay soil It may have clay fraction more than 50 percent. The particles are fine and give a talcum powder feeling when rubbed between fingers. It forms very hard clods on drying. A wet soil can be puddled easily and it impounds water for a long time. Difficult to get good tilth during land preparation. Very elastic & becomes very sticky when wet Water holding capacity is high Low infiltration rate

    Soil Structure

    Soil structure refers to the manner in which soil particles are arranged in groups or aggregates. The structure of soil is dynamic and it changes constantly with soil management practices. Cementing/bounding agents clay, organic matter, microbial glue mineral cementing agents. Soil aggregates may be temporary or stable depending on the amount and nature of cementing agents.

    Three main types of soil structures 1. Single grained Consists of one grain which is structure less 2. Massive grained Consists of very large lumps of soil 3. Compound aggregates structure Forms a small clods

    Depending on the shape, the structures are classified in to platy, columnar, prismatic, blocky, angular

    blocky etc. A soil structure is important in plant growth as it influences The amount and nature of porosity Regulates water, air and heat regimes in the soil Mechanical properties of soil Soil management aims at obtaining soil structure favorable for plant growth & yield besides ensuring

    soil conservation and good infiltration and movement of water in soils. Common methods of soil structure management include addition of organic matter and adoption of

    suitable tillage, soil conservation and cropping practices.

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    Volume and Mass Relationships of Soil Constituents Soil has solids, liquid and air and their relative masses and volumes are often required for proper soil and crop management. A schematic diagram of soil shown below may be useful to define the volume and mass relationship of the three soil phases. The diagram shows the presence of the three phases in relative proportions both in masses and volumes. Where Ma = Mass of air (Negligible) MW = Mass of water Ms = Mass of solids Mt = Total mass = Ma + Mw + Ms Va = Volume of air Vw = Volume of water Vs = Volume of solids Vp = Volume of pores = Va + Vw Vt = Total volume = Vp + Vs = Va + Vw + Vs Dry Bulk Density Dry Bulk density is the weight of oven dry soil per unit volume of soil.

    Dry bulk Density, DBD 3/ cmginVM



    Where, DBD= dry = bulk density, g/cm3 Ms = Mass of oven dry soil, g Vt = Volume of soil, cm3

    Typical values: 1.1 - 1.6 g/cm3 For the determination of bulk an undisturbed soil core is taken from the field by a core sampler (sampling cylinder) an