Soil water Plant Relations

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Slide 1:

Soil Water Plant Relationships M. DHAKSHINAMOORTHY By Professor of soil Science

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Constituents of soil AIR 25% MINERAL MATTER 45% WATER 25% O M 5%

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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 – O 2 ,CO 2 ,N 2

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Composition of Soil & Atmospheric Air in Percentage O 2 Co 2 N 2 Soil Air 20.05 0.25 29.20 Atmospheric Air 20.97 0.03 78.03

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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 Why study Soil water

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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

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Coarse Sand: 2.0– 0.2 mm Fine Sand: 0.2 – 0.02 mm Silt: 0.02 – 0.002 mm Clay: <0.002 mm Soil Texture Continued – International Classification Textural triangle: USDA Textural Classes Coarse vs. Fine, Light vs. Heavy Affects water movement and storage

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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 Sand 30-40% silt >40% silt  poor drainage Clay 40-50%  good for dry crops >50%  unsuitable for irrigated crops

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USDA Textural Triangle

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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.

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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 Textural Classification (US Bureau of Soils)

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Characters Sand Loam Silt Clay Feel Gritty Gritty Silky Cloddy Internal drainage Excessive Good Fair Fair to Poor Plant Av. water Low Medium High High Draw bar pull Light Light Medium Heavy Tillage Easy Easy Medium Difficult Run off potential Low Low-Med. Med - High High Water Detachability High Medium Medium Low Water Transportability Low Medium High High Wind erodability High Medium Low Low Significant of Soil texture

Soil Structure:

Soil Structure Affects root penetration and water intake and movement

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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

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Soil - Types of Structure Single Grained } Rapid Granular, Crumb Blocky } Moderate Prismatic, Cloddy Platy } Slow Massive

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Soil Structure in relation to water movement

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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

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Bulk Density (  b )  b = soil bulk density, g/cm 3 M s = mass of dry soil, g V b = volume of soil sample, cm 3 Typical values: 1.1 - 1.6 g/cm 3 Particle Density (  p )  P = soil particle density, g/cm 3 M s = mass of dry soil, g V s = volume of solids, cm 3 Typical values: 2.6 - 2.7 g/cm 3

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Porosity (  ) Typical values: 30 - 60%

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Soil Classification Alluvial soils F ormed 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.

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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 rock comprising gneisses and schist’s. Found in Tamil Nadu, Karnataka, Goa, Daman & Diu, south-eastern Maharashtra, Eastern Andhra Pradesh, Orissa and Jharkhand. The red soils have low water holding capacity and hence well drained.

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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 drainage Desert soils Found in Western Rajasthan, Haryana, and Punjab, Poor soil development. Light textured sandy soils and react well to the application of irrigation water. •

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Problem soils Cannot be used for the cultivation of crops without adopting proper reclamation measures. Highly eroded soils, ravine lands, soils on steeply sloping lands etc. constitute one set of problem soils. Acid, saline and alkaline soils constitute another set of problem soil.

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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

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Water in Soils Soil water content Mass water content (  m )  m = mass water content (fraction) M w = mass of water evaporated, g (  24 hours @ 105 o C) M s = mass of dry soil, g

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Volumetric water content (  v )  V = volumetric water content (fraction) V w = volume of water V s = volume of soil sample At saturation,  V = As  m As = apparent soil specific gravity =  b /  w (  w = density of water = 1 g/cm 3 ) As =  b numerically when units of g/cm 3 are used

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(g) (g) (cm 3 ) (cm 3 ) Equivalent Depth

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Coarse Sand Silty Clay Loam Gravitational Water Water Holding Capacity Available Water Unavailable Water Dry Soil

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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

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Components  t = total soil water potential  g = gravitational potential (force of gravity pulling on the water)  m = matric potential (force placed on the water by the soil matrix – soil water “tension”)  o = osmotic potential (due to the difference in salt concentration across a semi-permeable membrane, such as a plant root) Matric potential,  m , normally has the greatest effect on release of water from soil to plants Soil Water Potential

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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)

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Height of capillary rise inversely related to tube diameter Matric Potential and Soil Texture The tension or suction created by small capillary tubes (small soil pores) is greater that that created by large tubes (large soil pores). At any given matric potential coarse soils hold less water than fine-textured soils.

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Soil Moisture Tension 1 Atmosphere = 1036 cm Water Column (or) 76.39 cm of Hg 1 Bar = 1023 cm Water Column

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Soil Moisture Tension Relationship Height Water Colm. in cm. Atmosphere (Bars) pF (Schofield) 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)

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Classification of Soil Water Gravitational water – Excess water in soil pores – drains out due to gravitational force – Not available for plant growth Capillary water – Water left out in capillary pores after excess water has drained – Held by surface tension – cohesive force 1/3-15 atmp. – Available to plants Hygroscopic water – Water absorbed by a oven dry soil when exposed to a moist air – Held at high tension - tightly held by adhesion force – water of adhesion 10000-31 atmp., water not available – permanent wilting point

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Soil water constants Soil water proportions which dictate whether the water is available or not for plant growth. Saturation capacity: W ater content of the soil when all the pores of the soil are filled with water. (Maximum water holding capacity) So il moisture tension almost equal to zero. Field capacity: W ater retained by an initially saturated soil against the force of gravity. At field capacity, the macro-pores of the soil are drained off, but water is retained in the micropores. Soil Moisture tension at field capacity varies from 1/10 (for clayey soils) to 1/3 (for sandy soils) atmospheres.

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Field Capacity (FC or  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  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)

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Permanent wilting point As the Plants extract water, the moisture content diminishes and the negative (gauge) pressure increases. At one point, the plant cannot extract any further water and thus wilts. Temporary wilting point: this denotes the soil water content at which the plant wilts at day time, but recovers during night or when water is added to the soil. Ultimate wilting point: The plant wilts and fails to regain life even after addition of water to soil.

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Available Water Definition Water held in the soil between field capacity and permanent wilting point “Available” for plant use Available Water Capacity (AWC) AWC =  fc -  wp Units: depth of available water per unit depth of soil, “unitless” (in/in, or mm/mm) Measured using field or laboratory methods

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Field capacity - 

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Permanent wilting point -  pwp

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Fraction available water depleted (f d ) (  fc -  v ) = soil water deficit (SWD)  v = current soil volumetric water content Fraction available water remaining (f r ) (  v -  wp ) = soil water balance (SWB)

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Total Available Water (TAW) TAW = (AWC) (R d ) TAW = total available water capacity within the plant root zone, (inches) AWC = available water capacity of the soil, (inches of H 2 O/inch of soil) R d = depth of the plant root zone, (inches) If different soil layers have different AWC’s, need to sum up the layer-by-layer TAW’s TAW = (AWC 1 ) (L 1 ) + (AWC 2 ) (L 2 ) + . . . (AWC N ) (L N ) - L = thickness of soil layer, (inches) - 1, 2, N : subscripts represent each successive soil layer

Range of available water holding capacity of soil:

Soil texture % moisture based on dry wt. of soil Depth of available water FC PWP cm per meter depth of soil Sand Sandy loam Loam Clay loam Silty clay Clay 6-12(9) 10-18(14) 18-28(22) 23-31(27) 27-35(31) 31-39(35) 2-6 (4) 4-8 (6) 8-12 (10) 11-15 (13) 13-17 (15) 15-19 (17) 6-10(8) 9-15(12) 14-20(17) 17-22(19) 18-23(21) 20-25(23) Range of available water holding capacity of soil

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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 ) Factors that change AWC

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Horizontal movement due to capillarity Vertical movement due largely to gravity Gravity vs. Capillarity

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Water Infiltration Def’n.: the entry of water into the soil Influencing 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

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Cumulative Infiltration Depth vs. Time For Different Soil Textures

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Infiltration Rate vs. Time For Different Soil Textures

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Water Infiltration Rates and Soil Texture

Infiltration rate for different soil textures:

Soil Texture Basic infiltration rate (cm/hr) Sand Sandy loam Loam Clay loam Sandy clay Clay 2.5-25 1.3-7.6 0.8-2.0 0.25-1.5 0.03-0.5 0.01-0.1 Infiltration rate for different soil textures

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Soil Infiltration Rate vs. Constant Irrigation Application Rate

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Soil Infiltration Rate vs. Variable Irrigation Application Rate

Rooting Characteristic of Plants :

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 Sunflower Cauliflower Chilli Pearl millet Tree crops

Water requirements of crops:

Sl. No. Crop Duration (days) Water Req. (mm) 1 2 3 4 5 6 7 Rice Groundnut Sorghum Maize Sugarcane Ragi Cotton 135 105 100 110 365 100 165 1200 500 500 500 2000 400 600 Water requirements of crops

Points to remember:

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

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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

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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

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Soil Water Measurement Gravimetric Measures mass water content (  m ) 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

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Neutron scattering (attenuation) Measures volumetric water content (  v ) 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 Measurement

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Soil Water Measurement Neutron Attenuation

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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 Soil Water Measurement

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Tensiometer for Measuring Soil Water Potential Porous Ceramic Tip Vacuum Gauge (0-100 centibar) Water Reservoir Variable Tube Length (12 in- 48 in) Based on Root Zone Depth

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Electrical Resistance Blocks & Meters

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Thank you

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