SOIL WATER
STRUCTURE AND PROPERTIES OF WATER
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POLAR MOLECULE: NEGATIVE CHARGE ON ONE END, POSITIVE
ON THE OTHER
COHESION AND ADHESION
ATTRACTION TO CLAY SURFACES
HYDRATION OF IONS
SURFACE TENSION
IMPORTANCE OF WATER IN SOIL
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SUPPLY WATER TO PLANTS
SOLVENT FOR SOIL SOLUTION
MODIFIES TEMPERATURE AND AERATION
CAPILLARITY => ADDITIVE EFFECTS OF ADHESION AND
COHESION
SOIL WATER MOVEMENT
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MOVEMENT AFFECTED GRAVITY IN SATURATED SOIL
MOVEMENT AFFECTED BY TENSION GRADIENT IN UNSATURATED
SOIL
MOVEMENT OF WATER GOES FROM HIGH TO LOW FREE ENERGY
SATURATED SOIL=HIGH FREE ENERGY
DRY SOIL=LOW FREE ENERGY
MOVEMENT ALWAYS FROM WETTER SOIL TO DRYER SOIL
TEXTURE EFFECTS
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SATURATED FLOW GREATER IN SANDY SOILS
UNSATURATED FLOW GREATER IN MEDIUM AND FINE TEXTURED
SOILS
RELATED TO POROSITY/PORE SIZE
INFILTRATION PATTERN AFFECTED BY TEXTURE
MOVEMENT AFFECTED BY LAYERS WHICH GENERALLY IMPEDE
FLOW
AVAILABLE WATER
THE PORTION OF THE SOIL WATER THAT IS USABLE BY PLANTS.
DIFFERENCE BETWEEN FIELD CAPACITY AND WILTING POINT.
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FIELD CAPACITY=SOIL MOISTURE CONTENT WHEN
GRAVITATIONAL DRAINAGE CEASES IN A SOIL THAT WAS
SATURATED
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WILTING POINT=SOIL MOISTURE CONTENT AT WHICH THE PLANT
CAN NO LONGER ABSORB WATER
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AVAILABLE WATER=FIELD CAPACITY - WILTING POINT
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AVAILABLE WATER RELATED TO TEXTURE
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SILT LOAM HAS THE HIGHEST AVAILABLE WATER
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SOIL CHARACTERISTICS, NOT ROOT CHARACTERISTICS
CONTROL WATER UPTAKE BY PLANTS
SOIL TEXTURE AND WATER CONTENT (IN/FT)
TEXTURE
SAND
SANDY LOAM
LOAM
SILT LOAM
CLAY LOAM
CLAY
FC
1.4
1.9
3.1
3.4
3.7
3.9
WP
0.4
0.6
1.2
1.4
1.8
2.5
MEASURING SOIL WATER
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GRAVIMETRIC METHOD
GRAVIMETRIC WATER CONTENT
% MOISTURE = (WET WT. – DRY WT.) / DRY WT. X 100
EXAMPLE: (100g – 80g) / 80g x 100 = 25%
VOLUMETRIC WATER CONTENT
GRAVIMETRIC WATER CONTENT x BULK DENSITY
EXAMPLE: SILT LOAM -- 25 % x 1.3 = 32.5 %
SAND – 25 % x 1.5 = 37.5 %
AW
1.1
1.4
1.9
2.0
1.9
1.4
ESTIMATE WATER AVAILABLE FOR IRRIGATION
VOLUMETRIC WATER CONTENT x 12 in.
EXAMPLE: 0.325 x 12 in. = 3.9 in./ft.
THIS EXAMPLE CALCULATES THE TOTAL WATER,
NEED TO KNOW AVAILABLE WATER (AW)
FIELD CAPACITY – WILTING POINT = AW
32.5 % - 15.0 % = 17.5 %
0.175 x 12 in. = 2.1 in. AVAILABLE WATER/ft.
WHEN SCHEDULING IRRIGATION MAINTAIN >45 % AVAILABLE
WATER
0.45 x 2.1 = 0.95 in.
USE CHECKBOOK METHOD TO ADJUST FOR
EVAPOTRANSPIRATION LOSS
WATER MANAGEMENT AND CONSERVATION
HYDROLOGIC CYCLE
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97 % OF EARTH’S WATER IN OCEANS
CYCLE FUELED BY ENERGY OF SUN
FATE OF PRECIPITATION
o RUNOFF
o PERCOLATES THROUGH SOIL
o EVAPORATES FROM SURFACE
o ABSORBED BY PLANTS
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IN WISCONSIN
o ANNUAL PRECIPITATION ~ 31 in/yr
o 21 in. EVAPOTRANSPIRATION
o 4 in. RUNOFF
o 6 in. PEROCOLATION
CONSERVING WATER IN AGRICULTURE
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REDUCE RUNOFF
o TERRACES, DIVERSIONS, ETC.
o CONTOUR TILLAGE
o STRIP CROPPING
o CONSERVATION TILLAGE
o CROP ROTATION
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REDUCE CONSUMPTIVE USE
o REDUCE EVAPORATION WITH MULCHES, CROP RESIDUE
o REDUCE TRANSPIRATION WITH WIND BREAKS, WEED
CONTROL
o IRRIGATION SCHEDULING
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MANAGE PLANT USE EFFICIENCY
o PROPER FERTILIZATION INCREASES WATER USE
EFFICIENCY
o LIMIT COMPACTION
TRANSPIRATION RATIO=WATER USE/DRY MATTER
CROP
TRAN. RATIO
ALFALFA
858
OAT
635
POTATO
575
CORN
372
SORGHUM
271
LAMBQUARTER
658
PIGWEED
300
PURSLANE
281
lb WATER USED/lb DRY MATTER PRODUCED
DRAINAGE
PROBLEMS WITH POOR DRAINAGE
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REDUCE AERATION TO ROOTS
DENITRIFICATION
NUTRIENT UPTAKE REDUCED
DELAYS TILLAGE AND OTHER OPERATIONS
COOLER SOIL DELAYS GROWTH
WETLANDS vs WET SOILS
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WILDLIFE AREAS (1/3 ENDANGERED SPECIES)
NATURE’S SPONGE AND FILTER
CRANBERRIES
ARTIFICIAL DRAINAGE
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SURFACE
o LAND LEVELING ON SLOWLY PERMEABLE SOILS OF NC
WISCONSIN
o DIVERSIONS ABOVE FIELDS, BARNYARDS
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SUBSURFACE
o BURIED DRAIN TILE
o REQUIRES AN OUTLET
o WATER ONLY ENTERS IF SOIL IS SATURATED
o SPACING DEPENDS ON SOIL TYPE, PLACEMENT DEPTH
IRRIGATION
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SUBSURFACE
o UTILIZES CAPILLARY RISE
o PERMANENT
o INEFFICIENT IN SANDY SOILS
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SURFACE - FURROW
o WESTERN US
o RELATIVELY INEFFICIENT
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TRICKLE
o MOST EFFICIENT
o USUALLY PERMANENTLY SET
o HORTICULTURE
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SPRINKLER
o HAND MOVED=LABOR INTENSIVE
o SOLID SET=GOLF COURSES, BERRIES
o TRAVELING GUN
o CENTER PIVOT
o LINEAR
IMPROVING IRRIGATION EFFICIENCY
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MAINTAIN EQUIPMENT
BASE RATE ON CROP NEED (SCHEDULING)
RATE SHOULD NOT EXCEED INFILTRATION