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Soil fertility in organic systems: lessons from long-term studies
William R. Horwath Department Land, Air and Water
Resources
University of California Davis
UC Organic Soil Fertility Management SymposiumThursday, January 15, 2009
Activities & Recreation Center, UC Davis
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Talking points
• Organic agriculture: quality over production• Managing soil fertility in organic systems
– Soil organic matter• Organic and crop rotation effects on soil• Manipulating nutrient availability • Nutrient issues in organic systems
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Last Century Today
Economics -Increasing yield →-Solving resource issues →-Land expansion
-Genetic manipulation-Biological property issues-Food processing
Social -Increasing population →-Food supply →
-Human health-Poverty-Animal rights-Food safetyOrganic production
Environment -Water quality →-Pesticides
-Water quality-Air quality-Greenhouse gas emissions-pesticides
Agricultural drivers
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Converting to organic systems
• Challenge is to use fertilizers with variable nutrient contents
• Nutrient availability is also inconsistent• Key to organic fertilizer management is to
realize that nutrient demand is likely not met on an annual basis.
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Managing long-term nutrient availability
• Key is to manage not only the amount of soil organic matter but also the rate and timing of nutrient release
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Soil Organic Matter • Cation Ion Exchange capacity
• 300 to 700 cmol(+)/kg• Capacity to chelate metals• Enhance soil physical properties
• Water Holding capacity• Source of nutrients
•C/N/S/P = 100/10/1/1• Positive influence on soil properties
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Soil Organic Matter
Stable SOM>1000 years old
Very StableOrganicMatter
Resistant SOM~5 to 100 years old Resistant Organic
Matter
Labile SOMActive fraction~2 year old
Light fraction/Microbial biomass
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Contribution of Soil Organic Matter FractionsTo available soil nitrogen
Stable SOM
Resistant SOM
Labile SOMActive fraction
Available nutrients
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Organic and crop rotation effects on soil
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Soil C and N in Sustainable Agriculture Farming System project under different management. Soil %C Soil %N System Fall
1988 Fall 1996
Fall 2000
Fall 1996
Fall 2000
Organic 0.83 1.08 1.13 0.117 0.116 Low-input 0.83 1.03 1.04 0.111 0.107 Conv-4 0.83 0.90 0.92 0.094 0.095 Conv-2 0.83 0.84 0.88 0.092 0.094
Carbon
Organic 5.3 t C ha-1
Cover crop 3.4 t C ha-1
Nitrogen
Organic 462 kg N ha-1
Cover crop 273 kg N ha-1
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Microbial NutrientsMicrobial Biomass after 10 years of management at
SAFS g
N g
-1so
il
Soil depth (cm)0-15 15-30 30-60
20
40
60
80
orga
nic
Low
-inpu
tC
onv-
4
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Nutrient availability
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Time (Y)
% S
oil C
arbo
n
1 5 100
1.0
1.5
0.5
OrganicLow-inputConventional
Soil Carbon Change over 10 years
80 to 90% of 10 year accumulation
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Fertilizer & Soil N availabilityand synchronyN
Upt
ake
May June July SeptAug
maximumuptake
reallocation
seed/fruit
leaves
N amendment
Mar/Apr
Cover crop/organic amendment
incorporate availability
• Timing of incorporation• decomposition• quality (C/N)• amount
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0
10
20
30
40
50
60
70
0 20 40 60 80
Days of incubation
Cum
ulat
ive
N m
iner
aliz
e(m
g/kg
dry
soi
l)
conventionalorganiccover crop
I = std. dev.
Nitrogen mineralization potential in different Farming Systems
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N u
ptak
e (k
g N
ha-1
)N requirement varies depending on cropping system
SAFS
GrainYield (kg ha-1)
11,930a 11,500a
Total Nuptake (kg ha-1)
270a 180b
Yield/N uptake
44.2 63.9
CONV ORG
Critical N concentration varies by cropping system
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Mineralizable N over growing season
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63 68 62 73
No significant difference in N
recovery
Systems recovery of N using stable N isotope methodsSAFS
0
10
20
30
40
50
60
70
80
CONV LI-Fert LI-Vetch ORG
Tot
al %
N r
ecov
ery
System
Plant
Soil(2 m) 0
10
20
30
40
50% Recovery of Fertilizer N
in Corn
Fert
ilize
r U
se E
ffic
ienc
y (%
)
Treatment
Low-inputFertilizer
Low-inputFertilizer
+Vetch
ConventionalFertilizer
Fertilizer use highest in conventional management
N allocation differs by system
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Manipulating nutrient availability
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Namendment
Fertilizer and Soil N Availability
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0
5
10
15
20
25
Organic RotationUptake of Vetch N
Treatment
Vetch
Vetch+
Manure
SAFS
Uptake of additionalVetch N
Upt
ake
of v
etch
N k
g /h
a
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Fertilizer N
Fertilizer and Soil N Availability
Labile Soil N Pool
ANI
How does cropping system management change pathways and allocation?
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Uptake of vetch N compared to fertilizer N
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Nutrient issues in organic systems
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Average yield (ton ha-1) of tomato among different cropping systems.
Cropping System
Marketable Yield
Unmarketable Yield
Total Yield
Conventional
72.2 19.7 91.9
Low-input
72.6 25.4 98.0
Organic 69.0 26.9 95.9
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Point N2 O Emissions from SDI and FI (μg m-2 h-1) Compared by Cover Crop Treatment
0
150
300
450
600
0 200 400 600
NCC (ug N2O m2- hr1-)
WLC
C (u
g N 2
O m
2- h
r1-) SDI FI
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Runoff Water Quality Seasonal Volume-Weighted Average Concentration
Standard TillageStandard Tillage Conservation TillageConservation Tillage
All Other Constituents of Concern (COC)
• No substantial difference between treatments
• Orthophosphate, NH4-N, NO3-N all below 2mg/L
• TSS CT > ST
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Summary• With appropriate combination of amendments sufficient
amount and synchrony of nutrient delivery can be achieved
– Limiting factor is the soil can only store finite N– Key is to manipulate the size of mineralizable N pool
• Interactions of amendments with other amendments and soil nutrient pools needs further research to fine tune nutrient delivery
• Organic management can impact the environment