Soil Water Lecture Notes PDF

AGRI 51 PRINCIPLES OF SOIL SCIENCE  Soil Physical Properties SOIL WATER 2 SOIL PHYSICAL PROPERTIES Soil Water Nature, Roles and Importance of Soil Water Water content is one of the most variable characteristics of soil. The soil acts as a reservoir for water, making it available...

AGRI 51 PRINCIPLES OF SOIL SCIENCE  Soil Physical Properties SOIL WATER 2 SOIL PHYSICAL PROPERTIES Soil Water Nature, Roles and Importance of Soil Water Water content is one of the most variable characteristics of soil. The soil acts as a reservoir for water, making it available for plants as it is needed. Soil water is part of the global hydrological cycle. How well the soil stores water is of great importance to crop production and the vitality of the land. 3 SOIL PHYSICAL PROPERTIES Soil Water Nature, Roles and Importance of Soil Water Water affects soil formation, structure, stability and erosion Water causes soil particles to: Swell and shrink Adhere to each other Form structural aggregate Water alters the soil profile by dissolving and re-depositing minerals, often at lower levels It leaves the soil less fertile in case of extreme rainfall and drainage 4 SOIL PHYSICAL PROPERTIES Soil Water Nature, Roles and Importance of Soil Water Water is essential to plants for four reasons: a. It constitutes 85%-95% of the plant's protoplasm b. It is essential for photosynthesis c. It is the solvent in which nutrients are carried to, into and throughout the plant d. It provides the turgidity by which the plant keeps itself in proper position 5 SOIL PHYSICAL PROPERTIES Soil Water Ways of Soil Moisture Expression Gravimetric moisture content Volumetric moisture content 6 SOIL PHYSICAL PROPERTIES Soil Water Ways of Soil Moisture Expression Gravimetric moisture content – moisture content by mass 𝑴𝑴𝒘𝒘 (𝑴𝑴𝒕𝒕 − 𝑴𝑴𝒔𝒔 ) % 𝜽𝜽𝒎𝒎 = × 𝟏𝟏𝟏𝟏𝟏𝟏 = × 𝟏𝟏𝟏𝟏𝟏𝟏 𝑴𝑴𝒔𝒔 𝑴𝑴𝒔𝒔 Where: Mw = mass of water Mt = fresh weight of moist soil Ms = oven dried weight of the soil 7 SOIL PHYSICAL PROPERTIES Soil Water Ways of Soil Moisture Expression Volumetric moisture content – moisture content by volume 𝝆𝝆𝒃𝒃 % 𝜽𝜽𝒗𝒗 = %𝜽𝜽𝒎𝒎 × 𝝆𝝆𝒘𝒘 Where: 𝜽𝜽𝒎𝒎 = gravimetric moisture content 𝝆𝝆𝒃𝒃 = bulk density of the soil 𝝆𝝆𝒘𝒘 = density of water 8 SOIL PHYSICAL PROPERTIES Soil Water Ways of Soil Moisture Expression Gravimetric moisture content Volumetric moisture content 9 SOIL PHYSICAL PROPERTIES Soil Water Five categories of water “surrounding” a clay or soil particle: 1. Adsorbed water (hygroscopic water) 2. Water at permanent wilting point 3.Capillary water (water at field capacity) 4.Gravitational water 5.Chemically-combined water 10 SOIL PHYSICAL PROPERTIES Soil Water Five categories of water “surrounding” a clay or soil particle: 1. Adsorbed water (hygroscopic water) – Held on the surface of the particle by powerful forces of electrical attraction and virtually in a solid state of very small thickness. Hygroscopic MC = Wt. of soil at hygroscopic coefficient - Oven dry mass *Hygroscopic Coefficient is the condition when the last micropore is drained of water and only films of water exist surrounding the soil particles. 11 SOIL PHYSICAL PROPERTIES Soil Water 12 SOIL PHYSICAL PROPERTIES Soil Water Five categories of water “surrounding” a clay or soil particle: 2. Water at permanent wilting point –Water that can be removed by oven drying but not by air drying – Not so tightly held 3. Capillary water (water at field capacity) –held by surface tension, generally removable by air drying 13 SOIL PHYSICAL PROPERTIES Soil Water 14 SOIL PHYSICAL PROPERTIES Soil Water Five categories of water “surrounding” a clay or soil particle: 4. Gravitational water – Removable by drainage, can move in the voids between soil grains – A flooded field will drain the gravitational water under the influence of gravity until water's adhesive and cohesive forces resist further drainage and it finally reaches field capacity – Not of much use to the plants 15 SOIL PHYSICAL PROPERTIES Soil Water 16 SOIL PHYSICAL PROPERTIES Soil Water Five categories of water “surrounding” a clay or soil particle: 5. Chemically-combined water –In the form of water of hydration within the crystal structure. Not generally removable by oven drying – Cannot be removed by oven drying at 110oC. – Maybe considered to be part of the solid soil grain 17 SOIL PHYSICAL PROPERTIES Soil Water 18 SOIL PHYSICAL PROPERTIES Soil Water SOIL MOISTURE TENSION (SMT) A measure of the energy state of water Tenacity with which soil holds water It becomes larger when the amount of water stored is smaller (inverse relationship) As soil moisture tension increases, the amount of energy exerted by a plant to remove the water from the soil must also increase Soil moisture tension is negative pressure and commonly expressed in units of bars or kPa 19 SOIL PHYSICAL PROPERTIES Soil Water SOIL MOISTURE COEFFICIENTS a) Saturation pore spaces are fully filled with water a b c SMT = 0 bars = 0 kPa b) Field capacity amount of water 2-3 days after the rain SMT= 1/3 bars = 33kPa c) Permanent wilting point turgid plants wilt SMT = 15 bars = 1500 kPa 20 SOIL PHYSICAL PROPERTIES Soil Water 21 SOIL PHYSICAL PROPERTIES Soil Water Qualitative Descriptions of Soil Wetness 1. Maximum water holding capacity 2. Field capacity (FC) 3. Permanent wilting point (PWP) 4. Available water 22 SOIL PHYSICAL PROPERTIES Soil Water Qualitative Descriptions of Soil Wetness 1. Maximum water holding capacity – When all soil pores are filled with water, the soil is saturated and it is at maximum water holding capacity – Matric potential is close to zero and the volumetric water content is essentially the same as total porosity 23 SOIL PHYSICAL PROPERTIES Soil Water 24 SOIL PHYSICAL PROPERTIES Soil Water Qualitative Descriptions of Soil Wetness 2. Field capacity (FC) –Moisture content of the soil after gravity has removed all the water it can. Usually occurs 1-3 days after a rain –Smaller pores remain full of water, many of the intermediate pores are partially filled and most of the large pores are nearly empty –Plants must exert a suction force of 1/3 bar to obtain water at field capacity 25 SOIL PHYSICAL PROPERTIES Soil Water 26 SOIL PHYSICAL PROPERTIES Soil Water Qualitative Descriptions of Soil Wetness 3. Permanent wilting point (PWP) – Soil moisture percentage at which plants cannot obtain enough moisture to continue growing – Large and medium pores are completely empty. Water is held very tightly by the mineral and particles of the soil – Plants exert a suction force of 15 bars in trying to obtain water at this moisture condition 27 SOIL PHYSICAL PROPERTIES Soil Water 28 SOIL PHYSICAL PROPERTIES Soil Water Qualitative Descriptions of Soil Wetness 4. Available water – Amount of liquid water stored in the soil and subsequently released for use by the plants – Varies directly with the total amount of medium-sized pore spaces (diameter in the range of 0.2 – 10 micrometers (0.0002 – 0.01 mm) – Pore spaces wider than 10 micrometers drain water readily and pore spaces narrower than 0.2 micrometer release the retained water too slowly – AW = Field Capacity – Permanent Wilting Point 29 SOIL PHYSICAL PROPERTIES Soil Water 30 SOIL PHYSICAL PROPERTIES Soil Water CAPILLARITY Ability of liquid to flow against gravity Due to adhesion and cohesion forces 31 SOIL PHYSICAL PROPERTIES Soil Water Related terms but different: Capillarity: refers to the ability of a liquid to flow in narrow spaces or tubes against the force of gravity. Capillary Rise: specifically describes the upward movement of a liquid in a narrow tube or a porous material due to capillary action. Capillary Action: movement of a liquid within the spaces of a porous material or narrow tube due to the combination of adhesive and cohesive forces. Capillary Forces: Forces that allows the upward movement (adhesion and cohesion forces) 32 SOIL PHYSICAL PROPERTIES Soil Water CAPILLARITY Macropores are wide enough that water in the pores cannot be held against the force of gravity Micropores are small enough to hold water due to cohesion/ adhesion forces 33 SOIL PHYSICAL PROPERTIES Soil Water Water movement by capillarity 34 SOIL PHYSICAL PROPERTIES Soil Water Capillary action involves two types of attractions: Adhesion - some water molecules are held rigidly at the surfaces of soil solids Cohesion - water molecules hold other water molecules – Capillary forces are greater in small pores than in large pores 35 SOIL PHYSICAL PROPERTIES Soil Water Evidences of the forces of cohesion and adhesion as a drop of water is held between the fingers 36 SOIL PHYSICAL PROPERTIES Soil Water Enables plant roots to make use of water from the wetter portions of the soil profile particularly from a water table The smaller the pores, the higher the water will rise above the water table The higher the rise, the tighter the Water water will be held to soil particles to overcome the force of gravity Fine Sandy Silt Clay sand loam loam 37 SOIL PHYSICAL PROPERTIES Soil Water Water Flow in Soils Water moves through soil due to the forces of gravity, osmosis and capillarity At 0 – 1/3 bar suction, water moves through soil due to gravity; this is called saturated flow. At higher suction, water movement is called unsaturated flow 38 SOIL PHYSICAL PROPERTIES Soil Water Three Main Types of Forces which Contribute to the Energy State of Soil Water Gravitational Potential Osmotic Potential Matric Potential 39 SOIL PHYSICAL PROPERTIES Soil Water Three Main Types of Forces which Contribute to the Energy State of Soil Water 1. Gravitational Potential – Water has a positive energy and can flow out of the soil through the large pores – A point when cohesive forces are not large enough to hold onto the water – Significant only when soils are saturated 40 SOIL PHYSICAL PROPERTIES Soil Water Three Main Types of Forces which Contribute to the Energy State of Soil Water 2. Osmotic Potential – Osmosis - the movement of water from regions of higher potential (activity) to regions of lower potential (activity). – Potential is due to the attraction that salts have for water through the phenomenon of osmosis – Attributable to the presence of solutes in the soil solution. The greater the concentration of solutes, the lower the osmotic potential – This energy is negative relative to free water 41 SOIL PHYSICAL PROPERTIES Soil Water Three Main Types of Forces which Contribute to the Energy State of Soil Water 2. Osmotic Potential Water flows from the solution with the lower solute concentration into the solution with higher solute concentration 42 SOIL PHYSICAL PROPERTIES Soil Water Three Main Types of Forces which Contribute to the Energy State of Soil Water 3. Matric Potential – The potential energy of water attracted to soil solids – Operational in unsaturated soil above the water table – In an unsaturated soil, matric potential results from the capillarity and adhesion forces –Plants must overcome the energy of matric potential to extract water from the soil 43 SOIL PHYSICAL PROPERTIES Soil Water Relationship between the potential energy of pure water at a standard reference state (pressure) and that of soil water. 44 SOIL PHYSICAL PROPERTIES Soil Water Three Main Types of Forces which Contribute to the Energy State of Soil Water Gravitational Potential Osmotic Potential Matric Potential 45 SOIL PHYSICAL PROPERTIES Soil Water A plant root attempting to remove water from a moist soil would have to overcome all three forces simultaneously. 46 SOIL PHYSICAL PROPERTIES Soil Water Whether concerning matric potential, osmotic potential, or gravitational potential, water always moves to where its energy state will be lower. In this case the energy lost by the water is used to turn the waterwheel. 47 SOIL PHYSICAL PROPERTIES Soil Water Factors controlling water infiltration in soils 1. Soil texture 2. Soil structure Fine-textured soils with granular structure are most favorable to infiltration of water. 3. Amount of organic matter Organic matter is best, and if on the surface, helps prevent the destruction of soil structure and the creation of crusts 48 SOIL PHYSICAL PROPERTIES Soil Water Factors controlling water infiltration in soils 4. Depth of soil to impervious layers such as hardpans or bedrock 5. Amount of water already in the soil 6. Soil temperature – Warm soils take in water faster while frozen soils may not be able to absorb 49 SOIL PHYSICAL PROPERTIES Soil Water Soil Water Content Measurement 1.Hand-feel method The faster method but is prone to error since it requires experience and can be subjective An acceptable method if the observer has experience in the hand-feel method and is comfortable with the method Will allow for more locations to be sampled 50 SOIL PHYSICAL PROPERTIES Soil Water Soil Water Content Measurement 2. Gravimetric method A direct measurement of soil water content A sample of moist soil is weighed and then dried in an oven at a temperature of 105oC for about 24 hours, and finally weighed again The weight loss = soil water 51 SOIL PHYSICAL PROPERTIES Soil Water Soil Water Content Measurement 3. Electrical Resistance Blocks method The electrical resistance block uses small blocks of porous gypsum, nylon, or fiberglass, suitably embedded with electrodes When placed in moist soil, it absorbs water in proportion to the soil moisture content 52 SOIL PHYSICAL PROPERTIES Soil Water Soil Water Content Measurement 3. Electrical Resistance Blocks method Inexpensive and can be used to measure approximate changes in soil moisture during one/more cropping seasons However, the accuracy and range of soil moisture contents measured by these devices are limited 53 SOIL PHYSICAL PROPERTIES Soil Water Soil Water Content Measurement 4. Neutron Scattering method A neutron scattering probe contains a source of fast neutrons and a detector for slow neutrons When fast neutrons collide with H atoms, the neutrons slow down and scatter The number of slow neutrons counted by a detector corresponds to the soil water 54 content SOIL PHYSICAL PROPERTIES Soil Water Soil Water Content Measurement 5. Tensiometer method A tensiometer is a water-filled tube closed at the bottom with a porous ceramic cup and at the top with an airtight seal Once placed in soil, water in the tensiometer moves through the porous cup into the soil until water potential in the tensiometer is the same as the matric water potential in the soil 55 SOIL PHYSICAL PROPERTIES Soil Water Soil Water Content Measurement 5. Tensiometer method A tensiometer is a water-filled tube closed at the bottom with a porous ceramic cup and at the top with an airtight seal Once placed in soil, water in the tensiometer moves through the porous cup into the soil until water potential in the tensiometer is the same as the matric water potential in the soil 56  Thanks! ANY QUESTIONS? Message me. 57