Cover Crops Can Help
Maximize Available Soil Moisture
Rachel Stout Evans
Soil Scientist
USDA-NRCS Metcalfe Soil Survey Office
When the earth will…drink up the rain as fast
as it falls.
-H.D. Thoreau, The Journal
What is Available
Soil Water and
Where is it stored?
Available
Water
Available
Water
Ideal Soil Composition
Solids
Pore Space
25% Water
25% Air
45% Mineral
5%
Organic
Matter
MRBI: Managing Cover Crops – A Farmer’s Perspective by David Brandt
Organic Matter
MICROPORES
H2O
MACROPORES
H2O
Soil Minerals
What affects the Available Water Capacity
of a Soil? And can we change those things?
Available
Water
Available
Water
1) Amount and Size of Pore Space
2) Amount of Soil Organic Matter
Organic Matter
MICROPORES
H2O
MACROPORES
H2O
Soil Minerals
Pore Space: Water and Air Movement and Storage
Macropore
Space
Macro and
Micropore
Space
Micropore
Space
Loss of Pore
Space =
Compaction
So, what do Cover Crops do??
Metcalfe, MS 4 Dec 2014
Above Ground…
• Cover Bare Soil
• Decrease Rain Drop
Impact Velocity
• Slow Surface Runoff
• Increase Rain Water
Infiltration into Soil
• Protect from Erosion
Below Ground…
MRBI: Managing Cover Crops – A Farmer’s Perspective by David Brandt
Metcalfe, MS 4 Dec 2014
Living Roots in Your
Soil
• Fall, Winter, and Spring
• Physical, Biological, Chemical
Activity
• Pore Space + Organic Matter =
Available Water Capacity
Long Term Water Storage
New
Roots
Physical
• Roots create macropores - water
and air channels
• Roots grow and regrow along same
paths year after year, creating larger
channels
Old
Roots
Rooting Depth and Root
Size…depends on…
• Plant Species
• Soil Type (texture)
• Soil Density
(compaction)
Eudora, AR
Plant Species:
Tillage Radish
(tuber/taproot)
Triticale
(fibrous grass root)
Plant Species:
Tillage Radish (tuber/tap)
Rape (tuber/tap)
Winter Pea (fibrous)
Clover (fibrous)
Cereal Rye (fibrous)
Tap Roots and Fibrous Roots
in
Sharkey Clay and
Commerce Silty Clay Loam
Metcalfe, MS
Macropores
Soil Density
Compacted layer –
plow pan at 4 inches,
Bruin very fine sandy
loam
First year of planting tillage radish
Fibrous grass (Triticale)
roots penetrating
compacted layer (plow pan)
in Bruin very fine sandy
loam
Eudora, AR
First year of
planting cover
crops
Open
field
Oilseed
Radish
field
Soil compaction
decreased by >40%
Long Term Cover Cropping Penetration Resistance
Fibrous Roots at 30 inches in
Dundee silty clay loam, Yazoo
County
Root Growth and
Pore Space with
Compacted Layer
Plow pan
Ontario Ministry of Ag and Food
Root Growth and Pore
Space without Compacted
Layer
Network
of
biopores
MRBI: Managing Cover Crops – A Farmer’s Perspective by David Brandt
Biological/Chemical
• More and deeper roots
to find and use more
water
• Living roots + little
disturbance
=
Mycorrhizal Fungi Hyphae
AND
Earthworms
(Soil Organisms)
Mycorrhizal Fungal Hyphae and
Plant Roots
MYCORRHIZAL FUNGI
A symbiotic association between fungi and
plant roots
•Fungi serve as a bridge to
bring nutrients (N, P, and
micronutrients) to plant roots
•Under dry conditions, fungi
can bring moisture to the
roots they colonize
Mycorrhizal Fungus
S
Source:
Better Soils for Better Crops
Sticky substance, glomalin, surrounding root heavily
infected with mycorrhizal fungi. Fungi help roots
explore up to 20% of the soil volume. A root by
itself can only explore 1% of the soil volume.
Photo by Sara Wright.
Soil Critters, Bacteria, and Fungi
• Shred organic material
• Stimulate further microbial activity
• Convert dead organic material into useful organic acids
and mineralized nutrients
• Decompose residues and break down nutrients for
plant availability
• Nutrient Cycling: Eat other soil critters and release
nutrients for plants
• Convert NH4+ to NO2- to NO3- (plant available N)
EARTHWORMS
Of all the members of the soil food web,
earthworms need the least introduction
Their presence is
usually an indicator
of a healthy system
Decomposers
Earthworm pulled
plant residue into
burrow
Bury and shred
plant residue
Earthworms eat
residue, digest
and excrete rich
organic matter
EARTHWORMS
IN AGRICULTURE
Increase infiltration.
Earthworms
enhance porosity as
they move through
the soil. Some
species make
permanent burrows
deep into the soil.
Cover crop root mass
• Creates and enlarges pore spaces through the
soil, allowing more air and water to move
through and be stored
• Excretes carbon and sugars for soil critter food
• Hosts beneficial fungi to gather more water and nutrients
for plants
• Hosts earthworms, which create
more pore space and organic matter
• Contribute organic matter as they
decompose
Water Uptake
0”
6”
12”
18”
24”
Increased Water Storage Capacity Equals
less Flooding
50% Void Space
Equals 3-4.5 inches of
additional water
storage capacity.
Illustrated by Cheryl Bolinger-McKirnan & Jim Hoorman
Greenville
Helping People Help the Land
0”
Root Growth and Pore
Space with Compacted
Layer
0”
6”
6”
8”
Root Growth and Pore Space
without Compacted Layer
8”
Plow pan
12”
15”
20”
Ontario Ministry of Ag and Food
12”
Network
of
biopores
15”
20”
MRBI: Managing Cover Crops – A Farmer’s Perspective by David Brandt
Mississippi Alluvial Aquifer
is how irrigate our farmland
Here in the Mississippi Delta
Cover Crops: Use Multiple Species
for different root growth habits and functions
Cover Crops Can Help
Maximize Available Soil Moisture
Thank you