plain talk about clay soils
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Plain Talk About Clay Soils
Serenity, courage, wisdom
Linda J. Brewer
Extension Horticulture Oregon State University
God grant me the serenity to accept the things I cannot change;
9/29/05
courage to change the things I can;
and wisdom to know the difference.
Why Does Clay Crack ?
Shrink-swell clay mineralsPrecise molecular structuresWater out – shrink and crackWater in – swell and close
H2O in or out
Fact #1: Soil includes minerals, organic matter, air and water.
Solid stuff
Minerals 45%
Fluid stuff
Air 20-30%
Water 20-30%
Organic matter 5%
Fact #2: Soil texture is the percentage of sand, silt and clay.
Soil texture cannot be changed.
Fact #3: Soil minerals exist as sand, silt and clay particles.
Sand particles are big. Clay particles are small.
Sand: big particles,
big pores.Clay: little
particles, tiny pores.
SAND - Large
SILT
CLAY - Small
Used with permission: Bradshaw, M and R. Weaver. 1995. Foundations of Physical Geography. W.C. Brown Communications, Inc. Dubuque, IA.
Subduction zone
Fact #4: Same volume, different surface area.
4’ 4’
Surface area = 4X4X6
= 96 sfSurface area = 2X2X6X8
= 192 sf
Do the math and prove it to yourself!
Small particles,
enormous surface area.
Big particles,
small surface area.
Soil Texture and Surface Area
One gram of sand: seven square feet
One gram of clay: 48,000 square feet
Fact #5: Two major forces control water movement through soil.
“capillarity” gravity
•Capillary force draws water from wet to dry zones.
•Soil texture dictates which is in control.
Gravity
Coarse texture<5 minutes
Fine texture20 minutes
Capillarity
and gravity.
60% Clay>90 minutesCapillarity
and gravity.
Serenity Message:
Clay soils gain strength as they dry.Add water slowly to clay soils.Add water regularly to clay soils used for garden beds – don’t let them dry out.
Fine texture over coarse
texture
7 minutes
2 minutes
Coarse texture over fine texture
Fact #6: A plant taking up water is playing tug of war with the soil.
Clayey soils hold water tightly.
Sandy soils have limited storage capacity.
Image used with permission from the BBC’s Schools Website
Evaporation
Transpiration drives water flow toward roots. Roots explore for water.
Plan
t ro
ot
diffusion
root growth
Saturation = Soil Soup
Plant Available Water
capillarity
gravity
evaporation
Fact #7: Unavailable Water As soil dries out, it holds on to water more tightly.
capillarity
not connected
Fact #8: Soil’s water storage depends on texture.
store
d w
ate
r
more
less
claysandy
loam
silt
loam
sand loam clayloam
soup water available
to plants
unavailable water
field capacity
permanent wilting
point
Fact #9: Like diamonds, soil texture is forever.
Serenity Message
Soil texture will not change over the time scale of a human life.Accept this.
Serenity Message:Gypsum may improve clay soils.Sand will not improve clay soils.Sand reduces drainage in clay soils.
Fact #10: Organic matter + clay: sow’s ear = silk purse
Organic matter absorbs and stores water.Organic matter stores plant nutrients.Organic matter feeds soil biology. Organic matter improves soil structure.
Fact #10 continued:
Organic matter acts like a blanket to slow surface water loss.Surface mulch protects soil aggregates.Organic matter does not change soil texture!
Serenity Message:
Organic matter does not change soil texture!Accept this.Organic matter does improve clay soil characteristics.
Working with Organic Matter
Till in organic matter once – or twice.Till in at depth.Renew surface layer annually.
Advantages of Cracking Clays
“Self tilling”Organic matter deposited at depthCracks drain early fall rainsCracks increase soil drying by 50-100%Cracks deposit water at depth
soil surface
More Tips for Succeeding
Alter the micro-climateDon’t dig pots in clay soilsChoose clay tolerant plants: http://oregonstate.edu/dept/ldplants/
Linda J. Brewer
Extension Horticulture
Extension Small Farms
linda.brewer@oregonstate.edu
541-737-1408
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