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Soil-Water-Plant Relationships A. Background

1. Holdridge Life Zones

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2. Average Annual Precipitation

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3. Arable Land - land that can be used for growing crops

4. Irrigation System Design Factors

a. Water-holding capacity of a soil (root zone of the plant)b. Water-intake rate of the soilc. Root system of the cropd. Amount of water that the crop usese. Rainfall amount and distribution throughout the growing season

Understanding soil-plant-water relationships is necessary in order to plan and manage efficiently irrigation for specific crops grown on particular soils and in order to adjust the design to various conditions.

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B. Soil Properties

1. Soil Bulk Density, ρb, g/cm3, typical values: 1.1 - 1.6 g/cm3

Ms = dry soil mass, gVb = soil sample volume, cm3

2. Soil Particle Density, ρp, g/cm3, typical values: 2.6 - 2.7 g/cm3

Vs = solids volume, cm3

3. Porosity, Φ, typical values: 30 - 60%

𝛒𝐛=𝐌𝐬

𝐕𝐛

𝛒𝐩=𝐌𝐬

𝐕 𝐬

*100%

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C. Soil Water

1. Terminology

a. Water Content, W

1) Dry Weight Basis2) also called gravimetric water content3) oven dry means 105 oC until constant weight (≈ 24 hours)

b. Volumetric Water Content, θ

ρb = soil bulk density, g/cm3

ρw = water density, g/cm3

𝐖=𝐖𝐞𝐢𝐠𝐡𝐭𝐨𝐟 𝐖𝐚𝐭𝐞𝐫

𝐖𝐞𝐢𝐠𝐡𝐭𝐨𝐟 𝐎𝐯𝐞𝐧𝐃𝐫𝐲𝐒𝐨𝐢𝐥

𝛉=𝐕𝐨𝐥𝐮𝐦𝐞𝐨𝐟 𝐖𝐚𝐭𝐞𝐫𝐕𝐨𝐥𝐮𝐦𝐞𝐨𝐟 𝐒𝐨𝐢𝐥 =𝐖 𝝆𝒃

𝝆𝒘

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c. Equivalent Depth of Water

D = soil depth, ind = water depth, in3/in2 or in

𝒅=θ𝐃

Dd

𝒅=0.25𝐃

Water

For a volumetric water content, =0.25

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2. Soil Water Potentiala. Description

1) Measure of the energy status of the soil water2) Reflects how hard plants must work to extract water3) Units: bars or atmospheres4) Negative pressures (tension or suction)5) Water flows from a higher (less negative) potential to a lower (more negative) potential

b. Components

ψt = total soil water potentialψg = gravitational potentialψm = soil matric potential (soil water "tension")ψo = osmotic potential

Matric potential, ψm, usually has the greatest effect on release of water from soil to plants

ψ𝒕=ψ𝒈+ψ𝒎+ψ𝒐

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c. Matric Potential and Soil Texture

1) Tension or suction created by small capillary tubes

2) Small pores create higher suction than large pores

3) For a given matric potential, coarse texture soils (sands) hold LESS water than fine texture soils (silts and clays).

Height of capillary rise, h, inversely related to tube diameter

h1

h2

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e. Effect of soil texture on water holding capacity

Coarse Sand SiltyClay Loam

Dry Soil

Gravitational Water

Available Water

Unavailable Water

Water Holding Capacity

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f. Available Water Holding Capacity (AWHC)

1) Amount of water the soil can hold between field capacity and wilting point. Usually in/ft of soil or inches over entire root zone depth.

2) Field Capacitya) Approximation of the amount of water retained by the soil after the initial stage of drainage. b) Less than saturation, ≈ 1/3 barc) Usually < 48 hours for most soils

3) Wilting Pointa) Moisture level in soil where plant cannot remove waterb) Function of crop and stage of growth

4) Permanent Wilting Point - plant dies

PermanentWilting Point

Wilting Point

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d. Soil Water Characteristic Curve

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g. Available Water Percentage (AWP)

1) function of soil texture2) available water remaining in the root zone at any time3) % of AWHC4) Field Capacity: AWP = 100%5) Wilting Point: AWP = 0%

h. Maximum Allowable Deficiency (MAD)

6) Maximum soil water depletion allowed Expressed as % of AWHC

7) When AWP = 100 - MAD, it is time to irrigate8) 50% often used, less for vegetables, more for drought

tolerant crops

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i. Readily Available Moisture (RAM)

1) portion of available water over which irrigation scheduling occurs, % of MAD

2) RAM = AWHC * MAD when soil is at field capacity

MAD% of AWHC

AWHC(in or in/ft)

RAM% of MAD

MAD

AWHC(in or in/ft)

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j. Another look at soil moisture characteristic curves

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4. Wetting Patterns

1) Vertical movement due primarily to gravity

2) Horizontal movement due primarily to capillarity

CoarseTextured

Soil

FineTextured

Soil

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D. Soil Water Measurement

1. Gravimetrica. Measures mass water content ( m)b. Take field samples, weigh, oven dry, weigh

Advantages: accurate; multiple locationsDisadvantages: labor; time delay

2. Feel and Appearancea. Take field samplesb. Feel them by hand

Advantages: low cost; multiple locationsDisadvantages: experience required; not highly accurate

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Appearance at Different Moisture Contents Sand Clay Loam Loam Silt Loam

ftp://ftp-fc.sc.egov.usda.gov/MT/www/technical/soilmoist.pdf

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3. Neutron scattering (attenuation)a. Measures volumetric water contentb. Attenuation of high-energy neutrons by hydrogen

nucleus

Advantages: - samples a relatively large soil sphere

- repeatedly sample same site and several depths

- accurateDisadvantages:

- high cost instrument - radioactive licensing and safety - not reliable for shallow measurements near

surface

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4. Dielectric Constanta. Soil's dielectric constant is

dependent on soil moistureb. Time domain reflectometry (TDR)c. Frequency domain reflectometry

(FDR)d. Primarily used for research

purposes

5. Tensiometerse. Measure soil water potential

(tension)f. Practical operating range is

about 0 to 0.75 bar of tensiong. Limitation on medium- and fine-

textured soilsPorous 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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6. Electrical resistance blocksa. Measure soil water potential (tension)b. Tend to work better at higher tensions (lower water

contents)

7. Thermal dissipation blocksc. Measure soil water potential (tension)d. Require individual calibration

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40%

30%

20%

10%

RootDepth

Water Extraction by Root Systemin Moist Soil as a Function of Root Depth

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E. Plant Root Systems

http://www.upperbigblue.org

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F. Irrigation Scheduling

1. Evaporation Methods

a. Atmometer

b. Pan Evaporation

c. ET Equation using weather data

d. Using the Agweather Site on MESONET

2. Scheduling Methods

ET Using the Oklahoma MESONEThttp://www.mesonet.org/index.php/agriculture/map/agriculture_essentials/evapotranspiration/short_crop_etos_et_map

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Irrigation Planner: Oklahoma MESONEThttp://www.mesonet.org/index.php/agriculture/irrigation_planner

25http://www.extension.umn.edu/distribution/cropsystems/components/DC1322b.pdf

Irrigation Scheduling:

Checkbook Method