Managing
soil carbon
Dr Bill Cotching
www.billcotching.com
How Does Carbon Enter The Soil?
Plant residues
Soil organic carbon
Potential
Attainable
Actual
Clay content
& type
Rainfall &
Soil organic carbon
temperature
Management
Actual
Attainable
Potential
Soil carbon level controlled by:
Clay content
Rainfall (+ irrigation)
Land use / management
(Topography in Tasmania)
Relationship between carbon and the
clay + silt content for Tasmanian soils
Sparrow, Belbin & Doyle
SCaRP
Carbon stock (0- 30 cm, MgC/ha)
200
Maximum
150
100
50
Minimum
0
Texture Contrast
Dermosol
Ferrosol
Vertosol
Relationship between soil carbon and
Nov-March rainfall for Dermosols under pasture
SCaRP
Rainfall (30 yr Nov-Mar)
400
300
200
100
0
20
40
60
TOC (mg/g)
80
100
120
Relationship between soil carbon and
Land use in Tasmania SCaRP
80
0 - 0.1 m
Total organic carbon (mgC/g)
70
0.1 - 0.2 m
35%
60
50
0.2 - 0.3 m
35%
35%
40
30
20
10
0
Pasture (46)
Cropping
(47)
Dermosol
Pasture (27)
Cropping
(54)
Ferrosol
Pasture (27)
Cropping
(29)
Texture contrast
Pasture (26)
Cropping
(26)
Vertosol
Effect of long term cropping on soil carbon in Ferrosols
Sparrow , Cotching, Parry-Jones, Oliver, White, Doyle 2013
Equilibrium
Hierarchy of influence of variables on SOC (SCaRP) :
Soil Order >
mean annual rainfall + temp >
land use >
cropping frequency >
tillage type
The test !
Organic matter
Organic carbon
Total soil carbon
Organic matter = 1.72 X organic carbon
OM = carbon + nutrients
(58 %)
1 tonne of humus-C will “lock up”
50 kg N, 20 kg K, 3 kg P and 4 kg S
0.1 % carbon on your soil test
= 1 T organic carbon /ha
= $140 / ha of N, P, K, S
Coal Valley cropping
Coal Valley cropping
Coal Valley cropping
Coal Valley cropping
Oatlands - pasture
Oatlands - pasture
Oatlands - pasture
Soil organic carbon contribution by crop
from BlackMagic carbon model
Pasture (Grass)
Green Manure
(Ryegrass)
Triticale
Wheat
Peas
Beans Poppies
Broccoli
Potatoes
Onions
So what for your farm? Grow more biomass
So what’s good
?
and what’s not so good
for soil organic matter
Next test !
Perennial pastures
Contribution to soil organic carbon
+
-
Green manure crops
Recycled amendments
Stubble/crop
residue retention
Fertilisers
No tillage
Tillage
Stubble burning
Fallow
Erosion
Points to remember
• Introducing perennial plants into crop rotations has the
greatest potential to increase soil organic matter
• Perennial systems don’t sequester soil C after equilibrium
• Drought often results in soil organic matter decline
• Difficult to measure soil carbon accurately
Key messages
• Different soils have different capacities to store soil carbon
• Growing more plant material is the best way to boost soil C
Soil organic matter (carbon) is part of a
sustainable farming operation,
not a get rich quick scheme.