Comparing high and low input systems to
manage soil organic carbon
Frances Hoyle, Natalie Hogg, Justin Laycock, Liam Ryan (Department of Agriculture
and Food WA) and North-Stirling Natural Resources Group staff
Key messages

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

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Increasing fertiliser rates did not result in measurable yield responses and at
high rates decreased grain protein in canola.
There were no agronomic benefits measured in response to the application of
biochar.
No measureable change in soil organic carbon or other soil properties was
determined from imposed treatments.
Biochar application may be decreasing mineralisation (breakdown) of
background SOC in surface soil.
No agronomic response = no economic benefit observed as a result of
imposed treatments.
Aim
To identify what, if any, effect altering inputs to increase plant biomass would have
on soil carbon, and whether amending soils with biochar would increase soil carbon
storage or provide agronomic benefits.
Background
Growers are constantly assessing the long term profitability and sustainability of their
farming systems. Often growers look to target an optimum gross margin rather than
highest yield. This demonstration trial was established in 2013 and carried on into
2014 to determine whether measureable changes in (SOC) and productivity could be
associated with higher amounts of organic inputs. The question being posed at this
site was whether changes in net primary productivity (NPP) or the application of
biochar would influence short term soil organic carbon levels by manipulating the
organic inputs entering soil. In this instance three rates of fertiliser were used, and
the biochar assessed in comparison to a control that had been ripped and harrowed.
Supporting your success
Trial details
Property
Plot size and replications
Soil type
Paddock rotation
Treatments
Clear Valley, Gnowangerup
100m x 18.2m x 3 replicates
Shallow duplex - gravelly sand over clay to shallow
sand over clay, sloping site
2012: Canola
2011: Pasture
2010: Wheat
2009: Pasture
1. Farmer practice (55 kg/ha NPK crop + 15 kg/ha
sulphate of ammonia)
2. Low Fertiliser (35 kg/ha NPK crop + 10 kg/ha
sulphate of ammonia)
3. High Fertiliser (79 kg/ha NPK crop + 21 kg/ha
sulphate of ammonia)
4. Rip and harrow (Plus low fertiliser)
5. Biochar 1.5 t/ha (Plus rip and harrow, low
fertiliser)
Activity
2013
2014
Crop type
Canola (Hyola 559)
Wheat (Mace)
Sowing date
3 May
26 May
Seeding rate
2.6 kg/ha
60 kg/ha
Growing season
rainfall
209 mm
314 mm
550 g of Atrazine
500 g Propyzamide
1.2L glyphosate
100ml Alfa-cypermethrin
100ml Diametholote
Herbicide
pre-emergent
500 g of Propyzamide
200 ml Bifenthrin
300 ml Chlorpyifos
1 L gromoxone
300 gms diuron
118 gms sakura
Herbicide
post-emergent
Atrazine 1.1 kg
Select 500 ml
440ml LVE MCPA 570
Herbicide
knockdown
8 June 19 L/ha Black Urea
26 June 21L/ha Black Urea
Fertiliser
30L Hi Tech Black Urea (post-em)
7 August
30L Hi Tech BluS
1L Hi Potash
2L Poly PK
500ml fulvic
200ml fulva wet
1 September 25L Black Urea
500ml fulvic
2L Poly PK
300ml boron
1L Hi Pot
1L Hi 3 (Traces blend)
2L Hi Calcium
200ml Fulva wet
*fertiliser is from Australian Mineral Fertilisers.
Results
2013
Soil
Baseline soil sampling was conducted in March 2013 (Table 1). There was low to
moderate water repellence measured in the surface soil (0-10 cm) at this site using
the molarity of ethanol droplet test, but none measured for soils below 10 cm.
Soil pH increased with depth and was considered above the critical target of pH 4.8
in the subsoil but below the critical target of pH 5.5 in the surface (Table 1).
Soil was marginal for nitrogen and low in potassium (Table 1). Water holding
capacity of the soil was 21% (average all depths).
Table 1 Baseline soil test results from the trial site prior to the implementation of
treatments in March 2013.
Depth
(cm)
BD
(g/cm3)
pH
(CaCl2)
Total
SOC
(%)
SOC
stock
(t C/ha)
0-10
1.48
4.9
1.1
16.8
13
7
20
31
28
26
10-20
1.65
4.8
0.5
7.7
12
4
5
23
18
8
20-30
1.61
5.1
0.3
2.4
12
3
4
21
22
10
C/N
NH4
ratio (mg/kg)
P
K
NO3
S
(Colwell, (Colwell,
(mg/kg)
(mg/kg)
mg/kg) mg/kg)
Grain yield
The site averaged 0.86 t/ha canola and 49.8% oil but there were no treatment
differences in grain yield or oil measured. Grain moisture averaged 4.6%.
Grain protein was significantly (p < 0.05) higher on the farmer practice and low
fertiliser treatments (average 18.5%) and lowest on the biochar treatment (average
17.5%). However this difference was not large enough to influence the total amount
of nitrogen (N) taken up by the plant which averaged 26 kg/ha N across all
treatments.
Site influences were also evident in higher amounts of admixture in plots at the top
end of the trial area where there was a greater depth to clay in the subsoil. However
this was not evident in any one treatment.
2014
Soil
The carbon to nitrogen ratio (16:1) of the soil was typical of the range found in WA
soils. Moderate water repellence was evident for surface soil in soils that had been
ripped. Other treatments showed low risk of water repellence. Soil pH was nominally
higher (pH 5.4 decreasing to 5.0 below 10cm) than when measured in 2013 but
showed no treatment responses.
Soil ammonium levels averaged 3 mg/kg and though there was slightly higher levels
for the topsoil (0-10cm) in low input and biochar treatments this change was <1.5
mg/kg and was not considered further. Nitrate concentration was also low at this site
(<10 mg/kg). Sulphur was highest in the surface (15 mg/kg) decreasing to 5 mg/kg
below 10cm.
Soluble and small particulate organic carbon (DOC) was approximately 20% higher
in ripped treatments than in un-ripped treatments with either normal or high fertiliser
rates, suggesting a release of ‘new’ labile carbon previously protected by the soil
structure (data not presented). The concentration of DOC - which is a suitable food
substrate for soil organisms - is highly correlated to both the microbial biomass
(MBC) and greater biological soil nitrogen supply (potentially mineralisable nitrogen;
PMN; Table 2).
Table 2 Soil testing results from the trial site after the implementation of treatments in
March 2014.
Soil
depth
(cm)
0-10
10-20
20-30
LSD
BD
(g/cm3)
Clay
(%)
1.7
1.8
1.8
p<0.05
4
4
7
2
SOC
stock
(t C/ha)
16
6
5
1
SOC
CEC
DOC
MBC
PMN
P
K
(%) (meq/100g) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg)
0.99
0.35
0.27
0.06
3.3
1.4
1.5
0.3
106
45
42
10
141
35
22
12
11
1
<1
1
28
21
16
6
40
21
18
4
Soil organic carbon stocks
(t C/ha)
No significant changes in soil organic carbon stocks were measured at this site. Soil
organic carbon stocks were variable across the site and while a slightly higher
carbon stock is associated with the biochar treatment this is not considered different
to the rip and harrow treatment (Figure 1). High variability in carbon together with a
large carbon background (25 t C/ha) make it difficult to measure the small change in
carbon associated with the biochar. At an application rate of 1.5 t/ha biochar and
assuming 40% carbon content, this treatment only added 0.6 tonnes carbon per
hectare - so there is insufficient confidence to suggest the ‘difference’ indicated
below is real.
40
30
20
10
0
High fert
Control
(100kg/ha) (70 kg/ha)
Low fert
Biochar
(40 kg/ha) (rip+harrow)
Rip and
harrow
Figure 1 Soil organic carbon (t C/ha, 0-30cm) in 2014 under treatments imposed in
2013. Bar is the standard error of the mean.
Grain yield
The site averaged 2.45 t/ha of wheat with no treatment differences measured (Figure
2) and no signficant change in grain protein yield (nitrogen uptake in grain; Figure 2).
There were no differences in grain quality receival standards (hectolitre weight = 77
kg/hl; small grain screenings = 6%; protein at 11% moisture = 8.1%) .
3.5
Grain yield (t/ha)
3.0
2.5
2.0
1.5
1.0
29
32
35
31
39
High fert
(100kg/ha)
Control
(70 kg/ha)
Low fert
(40 kg/ha)
Biochar
(rip+harrow)
Rip and
harrow
0.5
0.0
Figure 2 Grain yield (bars) and nitrogen uptake (numbers in bars, kg N/ha) of wheat
in 2014 under treatments imposed in 2013. Bar is the standard error of the mean.
Economic analysis
While no extensive analysis has been done on the economics of these applied
treatments, the application of biochar at 1.5 t/ha did not result in any measureable
agronomic benefit and thus to date has been an expense with no associated return.
Ripping and harrowing in this instance did not return any further gains in terms of
yield or quality at this site in 2013 compared to the control and again did not appear
to address a significant constraint at this site in this year.
Higher fertiliser rates did not affect grain yield or quality. Thus in this instance the
‘best’ treatment (i.e. ‘most profitable’) would have been the low fertiliser input
treatment with lower associated input costs and achieving the same grain yield and
quality.
Comments
The site is situated on a slope and has a gradual change in depth to clay moving
down slope.
In 2013 and 2014, despite low soil nitrogen levels there was likely sufficient nitrogen
applied in basal application of black urea to supply crop requirements and it is not
surprising that the small change in nutrient addition - equivalent to 3 kg/ha nitrogen,
2 kg/ha phosphorous, 1 kg/ha potassium and 1.5 kg/ha sulphur between applied
fertiliser treatments, combined with dry periods during crop growth was not evident in
terms of measureable plant responses.
Changes in soil organic carbon take time and are difficult to measure due to large
spatial (and temporal) variability. The application of organic or carbon based inputs
must be put in context to a large background stock and sampling induced errors.
A good question to ask in the case of considering why you might try a new product is
whether it provides a specific function or role that is missing from your soil, or
whether it is applied at a rate sufficient to actually make a measureable change. A
great approach is to ‘strip’ test treatments in a paddock making sure that you also
represent your ‘normal’ farming practice – measure a change in the attribute of
interest - and go from there.
Abbreviation Meaning
kg/ha
t/ha
L/ha
kg/hl
mg/kg
g/cm3
mm
Kg
C/ha
N/ha
LSD
CEC
N
C
NH4
NO3
P
K
S
SOC
CaCl2
BD
MCPA LVE
MBC
PMN
C/N ratio
N/ha
DOC
Kilogram per hectare
Tonne per hectare
Litre per hectare
Kilogram per hectolitre
Milligram per kilogram
Gram per cubic centimetre
Millimetre
Kilogram
Carbon per hectare
Nitrogen per hectare
Least significant difference
Cation exchange capacity
Nitrogen
Carbon
Ammonium
Nitrate
Phosphorus
Potassium
Sulphur
Soil organic carbon
Calcium chloride
Bulk density
MCPA herbicide (present as the ethylhexyl ester) low volatile ester
Microbial biomass-carbon
Potentially mineralisable nitrogen
Carbon nitrogen ratio
Nitrogen per hectare
Soluble and small particulate organic carbon in solution
Acknowledgements
Thanks to the Facey group and their growers for continued interest and support for
this project and management of the trial site. A special thanks to Kevin Wise for
access to the trial site and assistance for the duration of the trial. Also thanks to the
North-Stirlings Pallinup Natural Resource Group staff for help in trial site sampling
and Richard Bowles (UWA) for analyses.
This project is led by the Department of Agriculture & Food WA in collaboration with
the North-Stirlings Natural Resources Group and is supported by funding from the
Australian Government Department of Agriculture - Action on the Ground program
and the GRDC.
Contact: Frances Hoyle frances.hoyle@agric.wa.gov.au