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topic 5 The physical properties of soil dr, mactal.ppt

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Physical properties of the soil

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The physical properties of soil
• Soil color • Soil Texture • Soil structure • Soil density • Soil moisture • Soil temperature
Significance of physical properties of soils The soil physical properties profoundly influence how soil functions in an ecosystem and how they can be best managed It influence plant growth by controlling penetration of plant roots, drainage, aeration, retention of moisture and availability of plant nutrients The success and failure of agricultural and engineering projects is often hinged on the physical properties of the soil
1. Soil Color • - it is the property of the soil that depend on the wavelength of light it reflects or emits. • Colors are derived from the color of Fe oxides and organic matter that coat the surfaces of soil particles high OM – Dark color High Fe oxides- Red to yellow Measurable variables in color 1. Hue- most dominant spectral color 2. Value – degree of darkness or lightness (zero is black) 3. Chroma – intensity or brightness (zero is neutral gray)
topic 5 The physical properties of soil dr, mactal.ppt
Hue - The dominant color • 10 R is red; 2.5YR has some yellow, 7.5YR are tans and browns, 2.5 Y is yellow, G is green). • As soils age they oxidize and change from yellow to brown to red (e.g., 2.5Y to 10YR to 7.5YR to 5 YR to 10R). Value - The relative darkness or lightness • from 1 (dark) to 8 (light). The value is often a function of the amount of humic organic material in the soil. Darker soils have more organic material. Very black horizons may be buried charcoal or accumulations of MnO2
. Whiter horizons may be the result of leaching as in an E horizon, or the accumulation of carbonate or gypsum. Chroma - The strength or intensity the color from 0 (least with none of the hue) to 8 (most vivid). This is indicative of the amount pigmenting material present, but it is strongly influenced by the texture of the soil. • Example: a soil with color 10YR5/6 is 10YR hue, 5 value, and 6 chroma,
Interpreting soil color variations A- horizon is usually darker than B horizon due to OM Dry soils generally have lighter color than moist soil Bright colors throughout the profile indicates well drainage Gray, bluish green denotes anaerobic condition Red soils are typical of tropical and subtropical areas while dark gray and brown are typical of temperate regions
2. Soil Texture • It is defined as the relative proportion of sand, silt and clay in the mineral matter fraction of the soil • Most permanent physical property Soil textural class 1. Sandy soils/coarse textured soils/light soils 2. Loamy soils/medium textured 3. Clayey soils/fine textured/heavy soils
Name Diameter (mm) ISSS Diameter (mm) USDA Number of particles/gram Surface area (mm²/g) Coarse gravel Fine gravel Coarse sand Medium sand Fine Sand Very fine sand Silt clay 15 2-15 0.2-2 - 0.02-0.2 - 0.002-0.02 ≤0.002 - 2-1 1-0.5 0.5-0.25 0.25-0.1 0.1-0.05 0.05-0.002 ≤0.002 - 90 722 5777 46,213 722,074 5,776,674 50,260,853,860 - 11.3 22.4 45.4 90.7 226.9 453.7 11,342.5 Table 1. Some characteristics of soil separates
Table 2. Generalized influence of soil separates on some properties and behavior of soils Property/ Behavior Rating Associated with Soil Separates Sand Silt Clay Water holding capacity Aeration Drainage rate Soil OM Decomposition of OM Warming up in spring Compactibility Susceptibility to wind erosion Susceptibility to water erosion Shrink-swell potential Sealing of ponds/dams Suitability to tillage after rain Pollutant leaching potential Ability to store nutrients Resistance to pH change Low Good High Low Rapid Rapid Low moderate Low Very low Poor Good High Poor low Medium to high Medium Slow to medium Medium to high Medium Moderate Medium high High Low Poor Medium Medium Medium to high medium High Poor Very slow High to medium Slow Slow High Low Low if aggregated Moderate to V high Good Poor Low High High
Importance of soil texture • The effect of soil texture can be beneficial or detrimental to plants, usually high clay in the subsoil is desirable 1. Relative resistance to root penetration – high silt and clay may retard root growth 2. water holding capacity sandy – low WHC clay – high WHC 3. Soil Fertility – clayey is more fertile than sandy 4. Soil aeration – heavy soil (aggregated – high, Non-aggregated – low) - sandy- high
• Methods of determining soil texture 1. Feel and Roll 2. Sieve 3. Hydrometer 4. pipet
topic 5 The physical properties of soil dr, mactal.ppt
• Sand – feels gritty and rough • Silt – feels floury or powdery • Clay – feels sticky and plastic
Sieve method
3. Hydrometer method
topic 5 The physical properties of soil dr, mactal.ppt
3. Hydrometer method- based on Stoke’s law V = kd² where: k – constant that is relative to the acceleration due to gravity and density and viscosity of water d – Diameter of particle
Stoke’s law V= h/t; and V = d²g(Ds-Df)/18η where: g = gravitational force = 9.81 Newtons/kg η = viscosity of water at 20°C= 1/1000 N-sec/m² Ds = Density of solid particles, 2.65 x 10³ kg/m³ Df = Density of fluid (water) = 1 x 10³ kg/m³
• Therefore: d² x 9.81 Newtons/kg (2.65 x 10³ kg/m³- 1 x 10³ kg/m³) V = 18 X 1/1000 N-sec/m² 9.81 N/kg (1.65x 10³ kg/m³) x d² V = 0.018Ns/m² V = 16.19 x 10³ N/m³ x d² 0.018Ns/m2
(9 x 10⁵) . d² V= sm V = K d² Calculate the settling time (sec) of silt particles at a depth of 10 cm. The diameter of silt is 2 x 10⁻⁶ m Note: V = K d² ; h/t = K d² Therefore: t = h/ K d²
t = 0.1m/ (9 x 10⁵/sm) (2 x 10⁻⁶ m)² t = 27,777 sec or 7.72 hrs
Using the textural triangle, find the soil texture if % clay = 30%, % silt = 25% and % sand= 45%
topic 5 The physical properties of soil dr, mactal.ppt
• seatwork Calculate the settling time of smallest sand particle (0.05 mm) at a depth of 10cm.
3. SOIL STRUCTURE - refers to the arrangement of soil particles and their aggregates into certain well defined patterns Types of soil structure 1. Spheroidal/Granular – soil separates combine to form small, rounded and loose or porous aggregates. Most surface soils have this kind of soil structure. - This enhances aeration and drainage. Soils rich in organic matter and calcium assumes this form of structure
• 2. Platy – The soil separates assumes the form of sheets one on top of the other lying horizontally . Soils of this structure have poor drainage and root penetration. • 3. Blocky – Soil separates combine together to form cube-like blocks. This kind of structure is generally found in the subsoil and has something to do with good soil drainage, aeration and root penetration a) angular blocky – with distinct and sharp angles b) sub-angular blocky – with somewhat rounded edges
• 4. Prism like – soil separates assume a post-like appearance standing upright a) columnar – with rounded tops b) prismatic - with flat tops • 5. Structureless – soil separates do not assume any definite form a) single grained – typical to most sandy soils b) massive – typical to most lowland rice soils
topic 5 The physical properties of soil dr, mactal.ppt
Factors affecting aggregation • 1. climate – alternate wetting and drying due to rainfall causes the soil to expand and contract allowing the particles to group or orient themselves into aggregates • 2. vegetation – aside from the effect of OM on aggregation, the physical effect of plant roots assists in the process of aggregate formation • 3. microbial activity- microorganisms excrete substances as by-product of metabolism and these are very useful as cementing materials. Fungi and other filamentous organisms produce mucilaginous substances for aggregate formation
Implication of desirable and undesirable structure • 1. Desirable structural condition – it is the one with high proportion of medium sized particles, a low bulk density and an appreciable large amount of large pores a) it implies that it is highly permeable to water b)have satisfactory water infiltration and retaining capacities c) readily penetrated by plant roots d) resist compaction of farm implements
• 2. Undesirable structure – with high bulk density, a few large pores and low content of water -stable aggregates a) it implies that permeability of air and water is slow b) there is resistance to root extension c) anaerobic conditions prevail
Density and Pore Spaces Particle Density (g/cc)– weight per unit volume of soil particles not including the pore spaces.  the particle density of most mineral soils varies from 2.6 to 2.75 g/cc  the particle density of OM varies from 1.2 to 1.7 g/cc PD = weight of soil/volume of solids
• Particle density increases as the amount of heavy minerals such as magnetite, limonite, hematite increases while PD decreases as the amount of OM increases. • It is usually determined using the Pycnometer method
• Bulk Density (Apparent density)– ovendry weight of a unit volume of soil including the pore spaces BD = ODW/total volume of soil  The bulk density of uncultivated soil varies from 1.0 to 1.6 g/cc.  BD increases with compaction  It helps in estimating the weight of soils per unit area
Porosity – refers to the percentage of soil volume which is occupied by pore spaces. % pore space = 100 – (BD/PD x 100)  Porosity varies with texture, shape of individual particles, soil structure, amount of OM an degree of compaction
• 1. Calculate the weight of 1 hectare furrow slice to a depth of 15 cm if the soil BD is 1.4 g/cc. What is the percentage porosity if PD is 2.65g/cc. 2. Fresh soil sample weighs 300 g and has a volume of 150 cc. If the ovendry weight is 200g, what is BD?
Soil Moisture • Most variable soil component hydrology – study of the water cycle.
The occurrence of water • Integral part of living cell • It occurs in great depth in earths crust • 99% of all the earths water occurs in the troposphere. It reaches an average height of 15 km • The average depth to which the water occurs in the ground is about 3 km and in some locations about 8 km.
topic 5 The physical properties of soil dr, mactal.ppt
Forces holding water in the soil 1. Adhesion – attraction between soil and water 2. Cohesion – attraction between water molecules
• Expression of forces holding water in the soil 1. Height of a unit column of water in cm (h) 2. Bar or atmosphere (atm) 1 bar = 1 atm 1000 cm = 1 bar = 1 atm 3. pF – log of free energy difference measured on a gravity scale pF = log h
Moisture constants Bars/atm Depth (cm) pF Ovendryness 10,000 10,000,000 7 Hygroscopic coefficient - 31,623 4.5 Wilting point 14.8 or 15 15,340 4.18 Field capacity 1/3 341 2.53 saturation 0.001 1 0
Soil moisture measurement 1. Gravimetric method/ovendrying method – ovendrying a moist soil sample at 105°C for 8-24 hrs. %water = weight of water/ ODW of soil x 100 2. Tensiometer method – suction tensiometer 3. Gypsum block
Soil Moisture Constants 1. Saturation – a soil whose pores are completely filled with water. - the water in the soil is at zero tension 2. Field Capacity (1/3 atm)– Percentage of water after the soil has been saturated and allowed to drain for 2-3 days. - macropores is without water but micropores are filled - water being used by plants
 it is approximately 1/3bar or a pF of 2.53  soil is best tilled at a moisture of pF 2.8 to 4.4 because the soil is friable and maintains its small aggregates  organic soil can be tilled even wetter than pF 2.8 because they have stable aggregates
3. Wilting point –the moisture condition at which the ease of release of water to the plant roots is just barely too small to counterbalance the transpiration losses. - the wilting point is also called as wilting coefficient or permanent wilting percentage. - it is about 15 bars or a pF of 4.18
4. Hygroscopic Coefficient – the moisture tension in which the soil water is in equilibrium with an atmosphere of 98 percent water vapor saturation ( 98% RH) - this corresponds to pF of 4.5 or 30.5 atm - note that 100 % Relative humidity has zero tension, hence soil is saturated with water.
5. Oven dry – soil is oven dry when it has reached equilibrium with the vapor pressure of an oven at 105oC - the oven dry condition corresponds to a relative humidity of zero or a pF near 7
Biological Classification of Soil water 1. Superfluous water – free or drainage water 2. Available water – moisture held between FC and PWP 3. Unavailable water – moisture held at the PWP
• Physical classification of soil moisture 1. Free (drainage water) –loosely held, < 0.1 atm 2. Capillary water – held between FC and hygroscopic coefficient - functions as soil solution and not all are available to plants 3.Hygroscopic water (vapor water) – held at hygroscopic coefficient, 31 to 10,000 atm
Classification of soil moisture Kinds of Water pF Water of constitution and interlayer water Hygroscopic water Capillary water Gravitational water Groundwater Above pF 7 7-4.5 4.5-2.5 2.5 – 0 Tension free
Water in relation to plant growth – water makes up 80% of the weight of green plants 1. Absorption of water – moisture enters plant roots thru osmosis osmosis – movement of water through a semi-permeable membrane cased by unequal concentration on the two sides.
2. Movement of nutrients in soil moisture – nutrients dissolved in water usually move with it 3. Water requirement of crops – water lost through evaporation and transpiration (evapotranspiration) Consumptive use – inches of water lost by evapotranspiration in the production of a crop
Crop Kg of water transpired per kg dry plant tissue Corn Beans sorghum 349 736 277
4. Water-use efficiency – any practice that promotes plant growth and photosynthesis to increase dry matter production increases water use efficiency
1. After a large soaking rain, a soil was sampled as it dried. The following weights were obtained: a) immediately after the rain – 300 g b) 2 days after the rain - 270g c)5 days after the rain - 250g d) when plant growing wilts - 230g e) when the soil is airdry - 220g f) when the soil is ovendry - 200g Calculate the the moisture content (%) at saturation, field capacity, 5 days, PWP and airdryness. Also find the available moisture at FC, 5 days, and gravitational water.
2. A sample of soil at field capacity weighs 130 g. After airdrying it weighs 105 g and after ovendrying it weighs 100 g. What are the percentages of capillary and hygroscopic water.
3. A sample of soil weighs 200 g and contains 15% water by weight. Its volume is 150 cc and its Particle density is 2.65 g/cc. Calculate the percentage of the pore space (by volume) that is filled with water after 30 g of water was added to the original sample.
4.A soil has the following characteristics: Hygroscopic coefficient - 8% Field capacity - 34% PWP - 14% Bulk Density - 1.3 g/cc Considering the first 20 cm of the soil in the field, calculate the following in centimeters of water a)Total available water b) amount of water needed to bring the moisture content from hygroscopic coefficient to 60%of TAW
5. The following information about a soil were obtained in the laboratory BD – 1.19 g/cc 1/3 atm percentage - 40% 15 atm percentage- 25% If you were asked to irrigate the first 30 cm depth of the soil a) calculate the amount of water (in cm) that you must apply if the field moisture content is 30% B) If the field is grown to corn and is now wilting permanently, how much water will you apply to the soilto bring the MC to FC?

More Related Content

topic 5 The physical properties of soil dr, mactal.ppt

  • 2. • Soil color • Soil Texture • Soil structure • Soil density • Soil moisture • Soil temperature
  • 3. Significance of physical properties of soils The soil physical properties profoundly influence how soil functions in an ecosystem and how they can be best managed It influence plant growth by controlling penetration of plant roots, drainage, aeration, retention of moisture and availability of plant nutrients The success and failure of agricultural and engineering projects is often hinged on the physical properties of the soil
  • 4. 1. Soil Color • - it is the property of the soil that depend on the wavelength of light it reflects or emits. • Colors are derived from the color of Fe oxides and organic matter that coat the surfaces of soil particles high OM – Dark color High Fe oxides- Red to yellow Measurable variables in color 1. Hue- most dominant spectral color 2. Value – degree of darkness or lightness (zero is black) 3. Chroma – intensity or brightness (zero is neutral gray)
  • 6. Hue - The dominant color • 10 R is red; 2.5YR has some yellow, 7.5YR are tans and browns, 2.5 Y is yellow, G is green). • As soils age they oxidize and change from yellow to brown to red (e.g., 2.5Y to 10YR to 7.5YR to 5 YR to 10R). Value - The relative darkness or lightness • from 1 (dark) to 8 (light). The value is often a function of the amount of humic organic material in the soil. Darker soils have more organic material. Very black horizons may be buried charcoal or accumulations of MnO2
  • 7. . Whiter horizons may be the result of leaching as in an E horizon, or the accumulation of carbonate or gypsum. Chroma - The strength or intensity the color from 0 (least with none of the hue) to 8 (most vivid). This is indicative of the amount pigmenting material present, but it is strongly influenced by the texture of the soil. • Example: a soil with color 10YR5/6 is 10YR hue, 5 value, and 6 chroma,
  • 8. Interpreting soil color variations A- horizon is usually darker than B horizon due to OM Dry soils generally have lighter color than moist soil Bright colors throughout the profile indicates well drainage Gray, bluish green denotes anaerobic condition Red soils are typical of tropical and subtropical areas while dark gray and brown are typical of temperate regions
  • 9. 2. Soil Texture • It is defined as the relative proportion of sand, silt and clay in the mineral matter fraction of the soil • Most permanent physical property Soil textural class 1. Sandy soils/coarse textured soils/light soils 2. Loamy soils/medium textured 3. Clayey soils/fine textured/heavy soils
  • 10. Name Diameter (mm) ISSS Diameter (mm) USDA Number of particles/gram Surface area (mm²/g) Coarse gravel Fine gravel Coarse sand Medium sand Fine Sand Very fine sand Silt clay 15 2-15 0.2-2 - 0.02-0.2 - 0.002-0.02 ≤0.002 - 2-1 1-0.5 0.5-0.25 0.25-0.1 0.1-0.05 0.05-0.002 ≤0.002 - 90 722 5777 46,213 722,074 5,776,674 50,260,853,860 - 11.3 22.4 45.4 90.7 226.9 453.7 11,342.5 Table 1. Some characteristics of soil separates
  • 11. Table 2. Generalized influence of soil separates on some properties and behavior of soils Property/ Behavior Rating Associated with Soil Separates Sand Silt Clay Water holding capacity Aeration Drainage rate Soil OM Decomposition of OM Warming up in spring Compactibility Susceptibility to wind erosion Susceptibility to water erosion Shrink-swell potential Sealing of ponds/dams Suitability to tillage after rain Pollutant leaching potential Ability to store nutrients Resistance to pH change Low Good High Low Rapid Rapid Low moderate Low Very low Poor Good High Poor low Medium to high Medium Slow to medium Medium to high Medium Moderate Medium high High Low Poor Medium Medium Medium to high medium High Poor Very slow High to medium Slow Slow High Low Low if aggregated Moderate to V high Good Poor Low High High
  • 12. Importance of soil texture • The effect of soil texture can be beneficial or detrimental to plants, usually high clay in the subsoil is desirable 1. Relative resistance to root penetration – high silt and clay may retard root growth 2. water holding capacity sandy – low WHC clay – high WHC 3. Soil Fertility – clayey is more fertile than sandy 4. Soil aeration – heavy soil (aggregated – high, Non-aggregated – low) - sandy- high
  • 13. • Methods of determining soil texture 1. Feel and Roll 2. Sieve 3. Hydrometer 4. pipet
  • 15. • Sand – feels gritty and rough • Silt – feels floury or powdery • Clay – feels sticky and plastic
  • 19. 3. Hydrometer method- based on Stoke’s law V = kd² where: k – constant that is relative to the acceleration due to gravity and density and viscosity of water d – Diameter of particle
  • 20. Stoke’s law V= h/t; and V = d²g(Ds-Df)/18η where: g = gravitational force = 9.81 Newtons/kg η = viscosity of water at 20°C= 1/1000 N-sec/m² Ds = Density of solid particles, 2.65 x 10³ kg/m³ Df = Density of fluid (water) = 1 x 10³ kg/m³
  • 21. • Therefore: d² x 9.81 Newtons/kg (2.65 x 10³ kg/m³- 1 x 10³ kg/m³) V = 18 X 1/1000 N-sec/m² 9.81 N/kg (1.65x 10³ kg/m³) x d² V = 0.018Ns/m² V = 16.19 x 10³ N/m³ x d² 0.018Ns/m2
  • 22. (9 x 10⁵) . d² V= sm V = K d² Calculate the settling time (sec) of silt particles at a depth of 10 cm. The diameter of silt is 2 x 10⁻⁶ m Note: V = K d² ; h/t = K d² Therefore: t = h/ K d²
  • 23. t = 0.1m/ (9 x 10⁵/sm) (2 x 10⁻⁶ m)² t = 27,777 sec or 7.72 hrs
  • 24. Using the textural triangle, find the soil texture if % clay = 30%, % silt = 25% and % sand= 45%
  • 26. • seatwork Calculate the settling time of smallest sand particle (0.05 mm) at a depth of 10cm.
  • 27. 3. SOIL STRUCTURE - refers to the arrangement of soil particles and their aggregates into certain well defined patterns Types of soil structure 1. Spheroidal/Granular – soil separates combine to form small, rounded and loose or porous aggregates. Most surface soils have this kind of soil structure. - This enhances aeration and drainage. Soils rich in organic matter and calcium assumes this form of structure
  • 28. • 2. Platy – The soil separates assumes the form of sheets one on top of the other lying horizontally . Soils of this structure have poor drainage and root penetration. • 3. Blocky – Soil separates combine together to form cube-like blocks. This kind of structure is generally found in the subsoil and has something to do with good soil drainage, aeration and root penetration a) angular blocky – with distinct and sharp angles b) sub-angular blocky – with somewhat rounded edges
  • 29. • 4. Prism like – soil separates assume a post-like appearance standing upright a) columnar – with rounded tops b) prismatic - with flat tops • 5. Structureless – soil separates do not assume any definite form a) single grained – typical to most sandy soils b) massive – typical to most lowland rice soils
  • 31. Factors affecting aggregation • 1. climate – alternate wetting and drying due to rainfall causes the soil to expand and contract allowing the particles to group or orient themselves into aggregates • 2. vegetation – aside from the effect of OM on aggregation, the physical effect of plant roots assists in the process of aggregate formation • 3. microbial activity- microorganisms excrete substances as by-product of metabolism and these are very useful as cementing materials. Fungi and other filamentous organisms produce mucilaginous substances for aggregate formation
  • 32. Implication of desirable and undesirable structure • 1. Desirable structural condition – it is the one with high proportion of medium sized particles, a low bulk density and an appreciable large amount of large pores a) it implies that it is highly permeable to water b)have satisfactory water infiltration and retaining capacities c) readily penetrated by plant roots d) resist compaction of farm implements
  • 33. • 2. Undesirable structure – with high bulk density, a few large pores and low content of water -stable aggregates a) it implies that permeability of air and water is slow b) there is resistance to root extension c) anaerobic conditions prevail
  • 34. Density and Pore Spaces Particle Density (g/cc)– weight per unit volume of soil particles not including the pore spaces.  the particle density of most mineral soils varies from 2.6 to 2.75 g/cc  the particle density of OM varies from 1.2 to 1.7 g/cc PD = weight of soil/volume of solids
  • 35. • Particle density increases as the amount of heavy minerals such as magnetite, limonite, hematite increases while PD decreases as the amount of OM increases. • It is usually determined using the Pycnometer method
  • 36. • Bulk Density (Apparent density)– ovendry weight of a unit volume of soil including the pore spaces BD = ODW/total volume of soil  The bulk density of uncultivated soil varies from 1.0 to 1.6 g/cc.  BD increases with compaction  It helps in estimating the weight of soils per unit area
  • 37. Porosity – refers to the percentage of soil volume which is occupied by pore spaces. % pore space = 100 – (BD/PD x 100)  Porosity varies with texture, shape of individual particles, soil structure, amount of OM an degree of compaction
  • 38. • 1. Calculate the weight of 1 hectare furrow slice to a depth of 15 cm if the soil BD is 1.4 g/cc. What is the percentage porosity if PD is 2.65g/cc. 2. Fresh soil sample weighs 300 g and has a volume of 150 cc. If the ovendry weight is 200g, what is BD?
  • 39. Soil Moisture • Most variable soil component hydrology – study of the water cycle.
  • 40. The occurrence of water • Integral part of living cell • It occurs in great depth in earths crust • 99% of all the earths water occurs in the troposphere. It reaches an average height of 15 km • The average depth to which the water occurs in the ground is about 3 km and in some locations about 8 km.
  • 42. Forces holding water in the soil 1. Adhesion – attraction between soil and water 2. Cohesion – attraction between water molecules
  • 43. • Expression of forces holding water in the soil 1. Height of a unit column of water in cm (h) 2. Bar or atmosphere (atm) 1 bar = 1 atm 1000 cm = 1 bar = 1 atm 3. pF – log of free energy difference measured on a gravity scale pF = log h
  • 44. Moisture constants Bars/atm Depth (cm) pF Ovendryness 10,000 10,000,000 7 Hygroscopic coefficient - 31,623 4.5 Wilting point 14.8 or 15 15,340 4.18 Field capacity 1/3 341 2.53 saturation 0.001 1 0
  • 45. Soil moisture measurement 1. Gravimetric method/ovendrying method – ovendrying a moist soil sample at 105°C for 8-24 hrs. %water = weight of water/ ODW of soil x 100 2. Tensiometer method – suction tensiometer 3. Gypsum block
  • 46. Soil Moisture Constants 1. Saturation – a soil whose pores are completely filled with water. - the water in the soil is at zero tension 2. Field Capacity (1/3 atm)– Percentage of water after the soil has been saturated and allowed to drain for 2-3 days. - macropores is without water but micropores are filled - water being used by plants
  • 47.  it is approximately 1/3bar or a pF of 2.53  soil is best tilled at a moisture of pF 2.8 to 4.4 because the soil is friable and maintains its small aggregates  organic soil can be tilled even wetter than pF 2.8 because they have stable aggregates
  • 48. 3. Wilting point –the moisture condition at which the ease of release of water to the plant roots is just barely too small to counterbalance the transpiration losses. - the wilting point is also called as wilting coefficient or permanent wilting percentage. - it is about 15 bars or a pF of 4.18
  • 49. 4. Hygroscopic Coefficient – the moisture tension in which the soil water is in equilibrium with an atmosphere of 98 percent water vapor saturation ( 98% RH) - this corresponds to pF of 4.5 or 30.5 atm - note that 100 % Relative humidity has zero tension, hence soil is saturated with water.
  • 50. 5. Oven dry – soil is oven dry when it has reached equilibrium with the vapor pressure of an oven at 105oC - the oven dry condition corresponds to a relative humidity of zero or a pF near 7
  • 51. Biological Classification of Soil water 1. Superfluous water – free or drainage water 2. Available water – moisture held between FC and PWP 3. Unavailable water – moisture held at the PWP
  • 52. • Physical classification of soil moisture 1. Free (drainage water) –loosely held, < 0.1 atm 2. Capillary water – held between FC and hygroscopic coefficient - functions as soil solution and not all are available to plants 3.Hygroscopic water (vapor water) – held at hygroscopic coefficient, 31 to 10,000 atm
  • 53. Classification of soil moisture Kinds of Water pF Water of constitution and interlayer water Hygroscopic water Capillary water Gravitational water Groundwater Above pF 7 7-4.5 4.5-2.5 2.5 – 0 Tension free
  • 54. Water in relation to plant growth – water makes up 80% of the weight of green plants 1. Absorption of water – moisture enters plant roots thru osmosis osmosis – movement of water through a semi-permeable membrane cased by unequal concentration on the two sides.
  • 55. 2. Movement of nutrients in soil moisture – nutrients dissolved in water usually move with it 3. Water requirement of crops – water lost through evaporation and transpiration (evapotranspiration) Consumptive use – inches of water lost by evapotranspiration in the production of a crop
  • 56. Crop Kg of water transpired per kg dry plant tissue Corn Beans sorghum 349 736 277
  • 57. 4. Water-use efficiency – any practice that promotes plant growth and photosynthesis to increase dry matter production increases water use efficiency
  • 58. 1. After a large soaking rain, a soil was sampled as it dried. The following weights were obtained: a) immediately after the rain – 300 g b) 2 days after the rain - 270g c)5 days after the rain - 250g d) when plant growing wilts - 230g e) when the soil is airdry - 220g f) when the soil is ovendry - 200g Calculate the the moisture content (%) at saturation, field capacity, 5 days, PWP and airdryness. Also find the available moisture at FC, 5 days, and gravitational water.
  • 59. 2. A sample of soil at field capacity weighs 130 g. After airdrying it weighs 105 g and after ovendrying it weighs 100 g. What are the percentages of capillary and hygroscopic water.
  • 60. 3. A sample of soil weighs 200 g and contains 15% water by weight. Its volume is 150 cc and its Particle density is 2.65 g/cc. Calculate the percentage of the pore space (by volume) that is filled with water after 30 g of water was added to the original sample.
  • 61. 4.A soil has the following characteristics: Hygroscopic coefficient - 8% Field capacity - 34% PWP - 14% Bulk Density - 1.3 g/cc Considering the first 20 cm of the soil in the field, calculate the following in centimeters of water a)Total available water b) amount of water needed to bring the moisture content from hygroscopic coefficient to 60%of TAW
  • 62. 5. The following information about a soil were obtained in the laboratory BD – 1.19 g/cc 1/3 atm percentage - 40% 15 atm percentage- 25% If you were asked to irrigate the first 30 cm depth of the soil a) calculate the amount of water (in cm) that you must apply if the field moisture content is 30% B) If the field is grown to corn and is now wilting permanently, how much water will you apply to the soilto bring the MC to FC?