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