Soil Water Plant Relationships


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Different forms of Soil Moisture and factors responsible for soil water retention are discussed

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Soil Water Plant Relationships

  1. 1. Soil Water Plant Relationships By M. DHAKSHINAMOORTHY Professor of soil Science
  2. 2. Constituents of soil AIR 25% MINERAL MATTER 45% WATER 25% O M 5%
  3. 3. Soil Plant nutrients Water/air Anchorage Soul Of Infinite Life
  4. 4. NUTRIENTS FOR PLANT GROWTHCH Non limiting/naturally abundantON CaP Major Mg SecondaryK SFe SiMn NaZn ClCo VaCu SeMoB
  5. 5. Soil Water Plant Inter- relatedSoil – 3 phase complex solid, liquid & gas in 50:25:25Solid – made up of Minerals, Organic Matter & Chemical CompoundsLiquid – Water dissolved Minerals & sol. Organic MatterGas – O2 ,CO2 ,N2
  6. 6. Composition of Soil &Atmospheric Air in Percentage O2 Co2 N2 Soil Air 20.05 29.20 0.25Atmospheric 20.97 0.03 78.03 Air
  7. 7. Why study Soil waterAnchorage for plantsMedium for Water & Air CirculationReservoir for Water & NutrientsSpace for beneficiary Micro OrganismsInter relationship between soil pores and its water holding capacityPlant water absorption rate
  8. 8. Soil Properties TextureDefinition: Relative proportions of various sizes of individual soil particlesUSDA classification Very Coarse Sand: 2.0– 1.0 mm Coarse Sand: 1.0– 0.5 mm Medium Sand: 0.5 – 0.25 mm Fine Sand: 0.25 – 0.1 mm Very Fine Sand: 0.1 – 0.05 mm Silt: 0.05 – 0.002 mm Clay: <0.002 mm
  9. 9. Soil Texture Continued – International Classification Coarse Sand: 2.0– 0.2 mm Fine Sand: 0.2 – 0.02 mm Silt: 0.02 – 0.002 mm Clay: <0.002 mm Textural triangle: USDA Textural Classes Coarse vs. Fine, Light vs. Heavy Affects water movement and storage
  10. 10. Importance of TextureStones & Gravel<10%  checks evap., Impr. drainage, seepage .>10%  soil too open, rapid drainage, less water & nutrient intentionSand<40%  soil friable , drainage water & air circulation optimum>40%  rapid evap., percolation & water holding capacityGood Loamy Sand30-40% silt>40% silt  poor drainageClay40-50%  good for dry crops>50%  unsuitable for irrigated crops
  11. 11. USDA Textural Triangle
  12. 12. Broad Textural ClassificationOpen or light textural soils: these are mainly coarse or sandy with low content of silt and clay.• Medium textured soils: these contain sand, silt and clay in sizeable proportions, like loamy soil.• Heavy textured soils: these contain high proportion of clay.
  13. 13. Textural Classification (US Bureau of Soils)Tex. Group SAND % SILT % CLAY % Sand 80-100 0-20 0-20 Sandy loam 50-80 0-50 0-20 Loam 30-50 30-50 0-20 Silt loam 0-50 50-100 0-20 SCL 50-80 0-30 20-30 Silt C L 0-30 50-80 20-30 Clay loam 20-50 20-50 20-30 Sandy clay 50-70 0-20 30-50 Silty clay 0-20 50-70 30-50 Clay 0-50 0-50 30-100
  14. 14. Significant of Soil textureCharacters Sand Loam Silt ClayFeel Gritty Gritty Silky CloddyInternal drainage Excessive Good Fair Fair to PoorPlant Av. water Low Medium High HighDraw bar pull Light Light Medium HeavyTillage Easy Easy Medium DifficultRun off potential Low Low-Med. Med - High HighWater Detachability High Medium Medium LowWater Transportability Low Medium High HighWind erodability High Medium Low Low
  15. 15. Soil StructureAffects root penetration and water intakeand movement
  16. 16. Arrangement of soil particles in-situOrientation of sand, silt, and clayPrismatic, columnar, granular and laminar (platy)Single, massive, aggregateAffect mechanical propertiesAffected by mans action
  17. 17. Soil - Types of StructureSingle Grained } Rapid Granular, CrumbBlocky } ModeratePrismatic, Cloddy
  18. 18. Soil Structure in relation to water movement
  19. 19. Role of Structure in Irrigation ManagementVital role in Soil Air & Water systemIn surface soil str., associated with soil tilth, permeability of Water Air & penetration of rootsSoil porosity bulk density etc…Promotes all plant growth factors
  20. 20. Bulk Density (ρb) Ms ρb = Vb ρb = soil bulk density, g/cm3 Ms = mass of dry soil, g Vb = volume of soil sample, cm3Typical values: 1.1 - 1.6 g/cm3 MsParticle Density (ρp) ρp = Vs ρP = soil particle density, g/cm3 Ms = mass of dry soil, g Vs = volume of solids, cm3Typical values: 2.6 - 2.7 g/cm3
  21. 21. Porosity (φ) volume of pores φ= volume of soil  ρb  φ = 1 − 100%  ρp Typical values: 30 - 60%
  22. 22. Soil Classification Alluvial soils Formed by successive deposition of silt transported by rivers during floods, in the flood plains and along the coastal belts.Alluvial soils textures vary from clayey loam to sandy loam. The water holding capacity of these soils is fairly good and is good for irrigation.
  23. 23. Black soils Weathering of rocks such as basalts, traps, granites and gneisses.Found in Maharashtra, MP, AP, Gujarat and TNHeavy textured with the clay content varying from 40 to 60 %High water holding capacity but poor in drainage.Red soilsFormed by the weathering of igneous and metamorphic rockcomprising gneisses and schist’s.Found in Tamil Nadu, Karnataka, Goa, Daman & Diu,south-eastern Maharashtra, Eastern Andhra Pradesh, Orissa andJharkhand.The red soils have low water holding capacity and hencewell drained.
  24. 24. Laterites and Lateritic soils Laterite is a formation peculiar to India and some other tropical countries,with an intermittently moist climate.Found in Karnataka, Kerala, Madhya Pradesh, Eastern Ghats of Orissa,Maharashtra, West Bengal, Tamilnadu and Assam. These soils have low clay content and hence possess good drainageDesert soilsFound in Western Rajasthan, Haryana, and Punjab,Poor soil development.Light textured sandy soils and react well to the application of irrigation water.
  25. 25. Problem soilsCannot be used for the cultivation of cropswithout adopting proper reclamation measures.Highly eroded soils, ravine lands, soils onsteeply sloping lands etc. constitute one set ofproblem soils.Acid, saline and alkaline soils constituteanother set of problem soil.
  26. 26. Soil WaterMicro Pores Macro PoresWater retained byAdhesion [ Solid surface (soil mass) to Liquid surface (soil water) ]Cohesion - between Liquid MoleculesSurface Tension - total force acting in solid liquid air- force pulling tangentially along the surface of the liquid
  27. 27. Water in SoilsSoil water content Mw θm = Ms Mass water content (θm) θm = mass water content (fraction) Mw = mass of water evaporated, g (≥24 hours @ 105oC) Ms = mass of dry soil, g
  28. 28. Volumetric water content (θv) Vw θv = Vs θV = volumetric water content (fraction) Vw = volume of water Vs = volume of soil sample At saturation, θV = As θm As = apparent soil specific gravity = ρb/ρw (ρw = density of water = 1 g/cm3) As = ρb numerically when units of g/cm3 are used
  29. 29. (cm3) Equivalent Depth(g) (g) (cm3)
  30. 30. Coarse Sand Silty Clay Loam Dry Soil Gravitational Water Water Holding Capacity Available Water Unavailable Water
  31. 31. Soil Water PotentialDescription Measure of the energy status of the soil water Important because it reflects how hard plants must work to extract water Units of measure are normally bars or atmospheres Soil water potentials are negative pressures (tension or suction) Water flows from a higher (less negative) potential to a lower (more negative) potential
  32. 32. Soil Water PotentialComponents ψt = ψ g + ψ m + ψ o ψt = total soil water potential ψg = gravitational potential (force of gravity pulling on the water) ψm = matric potential (force placed on the water by the soil matrix – soil water “tension”) ψo = osmotic potential (due to the difference in salt concentration across a semi-permeable membrane, such as a plant root) Matric potential, ψm, normally has the greatest effect on release of water from soil to plants
  33. 33. Soil Water Release Curve Curve of matric potential (tension) vs. water content Less water → more tension At a given tension, finer-textured soils retain more water (larger number of small pores)
  34. 34. Matric Potential and Soil TextureThe tension or suction created by small capillary tubes(small soil pores) is greater that that created by largetubes (large soil pores). At any given matric potentialcoarse soils hold less water than fine-textured soils. Height of capillary rise inversely related to tube diameter
  35. 35. Soil Moisture Tension1 Atmosphere = 1036 cm Water Column (or) 76.39 cm of Hg1 Bar = 1023 cm Water Column
  36. 36. Soil Moisture Tension RelationshipHeight Water Atmosphere pF (Schofield)Colm. in cm. (Bars) 1 1/1000 0 10 1/100 1 100 1/10 2 346 1/3 2.54 (F.C) 1000 1 3 10000 10 4 15849 15 4.2 (P.W.P) 31623 31 4.5 (H.COEFF)
  37. 37. Classification of Soil WaterGravitational water– Excess water in soil pores– drains out due to gravitational force– Not available for plant growthCapillary water– Water left out in capillary pores after excess water has drained– Held by surface tension – cohesive force 1/3-15 atmp.– Available to plantsHygroscopic water– Water absorbed by a oven dry soil when exposed to a moist air– Held at high tension - tightly held by adhesion force – water ofadhesion 10000-31 atmp., water not available – permanent wiltingpoint
  38. 38. Soil water constantsSoil water proportions which dictate whether the wateris available or not for plant growth.Saturation capacity: Water content of the soil when all the poresof the soil are filled with water. (Maximum water holding capacity)Soil moisture tension almost equal to zero.Field capacity: Water retained by an initially saturated soilagainst the force of gravity.At field capacity, the macro-pores of the soil are drained off, but water isretained in the micropores.Soil Moisture tension at field capacity varies from 1/10 (for clayey soils)to 1/3 (for sandy soils) atmospheres.
  39. 39. •Field Capacity (FC or θ fc) –Soil water content where gravity drainage becomes negligible –Soil is not saturated but still a very wet condition –Traditionally defined as the water content corresponding to a soil water potential of 2.54 (PF)•Permanent Wilting Point (WP or θ wp) –Soil water content beyond which plants cannot recover from water stress (dead) –Still some water in the soil but not enough to be of use to plants –Traditionally defined as the water content corresponding to -15 bars of SWP (pF 4.2)
  40. 40. Permanent wilting pointAs the Plants extract water, the moisture contentdiminishes and the negative (gauge)pressure increases. At one point, the plant cannot extractany further water and thus wilts.Temporary wilting point:this denotes the soil water content at which the plantwilts at day time, but recovers during night or whenwater is added to the soil.Ultimate wilting point:The plant wilts and fails to regain life even afteraddition of water to soil.
  41. 41. Available WaterDefinition Water held in the soil between field capacity and permanent wilting point “Available” for plant useAvailable Water Capacity (AWC) AWC = θfc - θwp Units: depth of available water per unit depth of soil, “unitless” (in/in, or mm/mm) Measured using field or laboratory methods
  42. 42. Field capacity - θ
  43. 43. Permanent wilting point - θpwp
  44. 44. Fraction available water depleted (fd)  θfc − θv  fd =    θfc − θwp  (θfc - θv) = soil water deficit (SWD) θv = current soil volumetric water contentFraction available water remaining (fr)  θv −θwp  fr =    θfc −θwp  (θv - θwp) = soil water balance (SWB)
  45. 45. Total Available Water (TAW)TAW = (AWC) (Rd) TAW = total available water capacity within the plant root zone, (inches) AWC = available water capacity of the soil, (inches of H2O/inch of soil) Rd = depth of the plant root zone, (inches) If different soil layers have different AWC’s, need to sum up the layer-by-layer TAW’sTAW = (AWC1) (L1) + (AWC2) (L2) + . . . (AWCN) (LN) - L = thickness of soil layer, (inches) - 1, 2, N: subscripts represent each successive soil layer
  46. 46. Range of available waterholding capacity of soil % moisture based on Depth of dry wt. of soil available waterSoil texture FC PWP cm per meter depth of soil Sand 6-12(9) 2-6 (4) 6-10(8) Sandy loam 10-18(14) 4-8 (6) 9-15(12) Loam 18-28(22) 8-12 (10) 14-20(17) Clay loam 23-31(27) 11-15 (13) 17-22(19) Silty clay 27-35(31) 13-17 (15) 18-23(21) Clay 31-39(35) 15-19 (17) 20-25(23)
  47. 47. Factors that change AWCOrganic matter content (increase 10 %)Structure (+/- 10 %) Good: granular, blocky, prismatic Bad: platy, massive, single grainCompaction (decrease 20 %)Restrictive layers (increase above 10 %)Depth (5 % per 30 cm depth)
  48. 48. Gravity vs. Capillarity Horizontal movementVertical movement due to capillaritydue largely to gravity
  49. 49. Water InfiltrationDef’n.: the entry of water into the soilInfluencing FactorsSoil textureInitial soil water contentSurface sealing (structure, etc.)Soil crackingTillage practicesMethod of application (e.g., Basin vs. Furrow)Water temperature
  50. 50. Cumulative Infiltration Depth vs. Time For Different Soil Textures
  51. 51. Infiltration Rate vs. TimeFor Different Soil Textures
  52. 52. Water Infiltration Rates and Soil Texture
  53. 53. Infiltration rate for different soil texturesSoil Texture Basic infiltration rate (cm/hr) Sand 2.5-25 Sandy loam 1.3-7.6 Loam 0.8-2.0 Clay loam Sandy clay 0.25-1.5 Clay 0.03-0.5 0.01-0.1
  54. 54. Soil Infiltration Rate vs. ConstantIrrigation Application Rate
  55. 55. Soil Infiltration Rate vs. VariableIrrigation Application Rate
  56. 56. Rooting Characteristic of Plants Shallow Mod. deep Deep Very deep [60cm] [90cm] [120cm] [180cm] Rice Wheat Maize Sugarcane Potato Tobacco Cotton Citrus Onion Castor Sorghum Grape vine Cabbage Groundnut Tomato SunflowerCauliflower Chilli Pearl millet Tree crops
  57. 57. Water requirements of cropsSl. Crop Duration (days) Water Req. (mm)No.1 Rice 135 12002 Groundnut 105 5003 Sorghum 100 5004 Maize 110 5005 Sugarcane 365 20006 Ragi 100 4007 Cotton 165 600
  58. 58. Points to rememberCropped field acts as soil – water reservoirResidual soil moisture and shallow water table contributes to crop water needWater added in excess lost as – deep percolation - lead to nutrient loss, water logging and salinitySoils classified based on textureWater retention capacity differ with soils
  59. 59. FC-upper limit of soil water storageSoil water content between FC and PWP- is total ASW for plant growthCrops differ in ability to withstand diff. depletion of ASWThe growth stage and root characteristics mainly contribute to withstand S-W depletion
  60. 60. ET losses influenced by duration of crops, rate of growth , Pl. popln. , Pl. ht and moisture extrn pattern by rootsRate of loss of water from cropped field depends on climatic factorSolar radiation , temp., humidity and wind important climatic factors influencing ET rateTotal ET value of crops varies based on weather conditions
  61. 61. Soil Water MeasurementGravimetric Measures mass water content (θm) Take field samples → weigh → oven dry → weigh Advantages: accurate; Multiple locations Disadvantages: labor; Time delayFeel and appearance Take field samples and feel them by hand Advantages: low cost; Multiple locations Disadvantages: experience required; Not highly accurate
  62. 62. Soil Water MeasurementNeutron scattering (attenuation)  Measures volumetric water content (θv)  Attenuation of high-energy neutrons by hydrogen nucleus  Advantages:  samples a relatively large soil sphere  repeatedly sample same site and several depths  accurate  Disadvantages:  high cost instrument  radioactive licensing and safety  not reliable for shallow measurements near the soil surface
  63. 63. Soil Water Measurement Neutron Attenuation
  64. 64. Soil Water MeasurementTensiometers Measure soil water potential (tension) Practical operating range is about 0 to 0.75 bar of tension (this can be a limitation on medium- and fine-textured soils)Electrical resistance blocks Measure soil water potential (tension) Tend to work better at higher tensions (lower water contents)Thermal dissipation blocks Measure soil water potential (tension) Require individual calibration
  65. 65. Tensiometer for Measuring Soil Water Potential Water Reservoir Variable Tube Length (12 in- 48 in) Based on Root Zone Depth Porous Ceramic Tip Vacuum Gauge (0-100 centibar)
  66. 66. Electrical Resistance Blocks & Meters