Soil Water Plant Relationships By M. DHAKSHINAMOORTHY Professor of soil Science
Constituents of soil AIR 25% MINERAL MATTER 45% WATER 25% O M 5%
Soil Plant nutrients Water/air Anchorage Soul Of Infinite Life
NUTRIENTS FOR PLANT GROWTHCH Non limiting/naturally abundantON CaP Major Mg SecondaryK SFe SiMn NaZn ClCo VaCu SeMoB
Soil Water Plant Inter- relatedSoil – 3 phase complex solid, liquid & gas in 50:25:25Solid – made up of Minerals, Organic Matter & Chemical CompoundsLiquid – Water dissolved Minerals & sol. Organic MatterGas – O2 ,CO2 ,N2
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
Why study Soil waterAnchorage for plantsMedium for Water & Air CirculationReservoir for Water & NutrientsSpace for beneficiary Micro OrganismsInter relationship between soil pores and its water holding capacityPlant water absorption rate
Soil Properties TextureDefinition: Relative proportions of various sizes of individual soil particlesUSDA 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
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
Importance of TextureStones & Gravel<10% checks evap., Impr. drainage, seepage .>10% soil too open, rapid drainage, less water & nutrient intentionSand<40% soil friable , drainage water & air circulation optimum>40% rapid evap., percolation & water holding capacityGood Loamy Sand30-40% silt>40% silt poor drainageClay40-50% good for dry crops>50% unsuitable for irrigated crops
Broad Textural ClassificationOpen 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.
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
Soil StructureAffects root penetration and water intakeand movement
Arrangement of soil particles in-situOrientation of sand, silt, and clayPrismatic, columnar, granular and laminar (platy)Single, massive, aggregateAffect mechanical propertiesAffected by mans action
Role of Structure in Irrigation ManagementVital role in Soil Air & Water systemIn surface soil str., associated with soil tilth, permeability of Water Air & penetration of rootsSoil porosity bulk density etc…Promotes all plant growth factors
Bulk Density (ρb) Ms ρb = Vb ρb = soil bulk density, g/cm3 Ms = mass of dry soil, g Vb = volume of soil sample, cm3Typical values: 1.1 - 1.6 g/cm3 MsParticle Density (ρp) ρp = Vs ρP = soil particle density, g/cm3 Ms = mass of dry soil, g Vs = volume of solids, cm3Typical values: 2.6 - 2.7 g/cm3
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.
Black soils Weathering of rocks such as basalts, traps, granites and gneisses.Found in Maharashtra, MP, AP, Gujarat and TNHeavy textured with the clay content varying from 40 to 60 %High water holding capacity but poor in drainage.Red soilsFormed 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.
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.
Problem soilsCannot 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.
Soil WaterMicro Pores Macro PoresWater retained byAdhesion [ Solid surface (soil mass) to Liquid surface (soil water) ]Cohesion - between Liquid MoleculesSurface Tension - total force acting in solid liquid air- force pulling tangentially along the surface of the liquid
Water in SoilsSoil 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
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
Coarse Sand Silty Clay Loam Dry Soil Gravitational Water Water Holding Capacity Available Water Unavailable Water
Soil Water PotentialDescription 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
Soil Water PotentialComponents 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
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)
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
Soil Moisture Tension1 Atmosphere = 1036 cm Water Column (or) 76.39 cm of Hg1 Bar = 1023 cm Water Column
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
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.
•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)
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.
Available WaterDefinition Water held in the soil between field capacity and permanent wilting point “Available” for plant useAvailable 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
Fraction available water depleted (fd) θfc − θv fd = θfc − θwp (θfc - θv) = soil water deficit (SWD) θv = current soil volumetric water contentFraction available water remaining (fr) θv −θwp fr = θfc −θwp (θv - θwp) = soil water balance (SWB)
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
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)
Factors that change AWCOrganic matter content (increase 10 %)Structure (+/- 10 %) Good: granular, blocky, prismatic Bad: platy, massive, single grainCompaction (decrease 20 %)Restrictive layers (increase above 10 %)Depth (5 % per 30 cm depth)
Gravity vs. Capillarity Horizontal movementVertical movement due to capillaritydue largely to gravity
Water InfiltrationDef’n.: the entry of water into the soilInfluencing FactorsSoil textureInitial soil water contentSurface sealing (structure, etc.)Soil crackingTillage practicesMethod of application (e.g., Basin vs. Furrow)Water temperature
Cumulative Infiltration Depth vs. Time For Different Soil Textures
Infiltration Rate vs. TimeFor Different Soil Textures
Points to rememberCropped field acts as soil – water reservoirResidual soil moisture and shallow water table contributes to crop water needWater added in excess lost as – deep percolation - lead to nutrient loss, water logging and salinitySoils classified based on textureWater retention capacity differ with soils
FC-upper limit of soil water storageSoil water content between FC and PWP- is total ASW for plant growthCrops differ in ability to withstand diff. depletion of ASWThe growth stage and root characteristics mainly contribute to withstand S-W depletion
ET losses influenced by duration of crops, rate of growth , Pl. popln. , Pl. ht and moisture extrn pattern by rootsRate of loss of water from cropped field depends on climatic factorSolar radiation , temp., humidity and wind important climatic factors influencing ET rateTotal ET value of crops varies based on weather conditions
Soil Water MeasurementGravimetric Measures mass water content (θm) Take field samples → weigh → oven dry → weigh Advantages: accurate; Multiple locations Disadvantages: labor; Time delayFeel and appearance Take field samples and feel them by hand Advantages: low cost; Multiple locations Disadvantages: experience required; Not highly accurate
Soil Water MeasurementNeutron 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
Soil Water MeasurementTensiometers 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
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)