Organic fertility management
Organic fertility management is much more than adding nutrients into the soil.
Overall goal is to balance nutrient inputs and outputs and ensure a good balance of nutrients for the crop
to achieve this requires a complex mix of soil management activities including tillage, irrigation, residue management, weed management and crop rotation planning
Neglecting any of these components can compromise crop performance.
What is meant by soil fertility and soil quality?
Soil fertility is the capacity of a soil to provide nutrients required by plants for growth, and is one component of soil quality.
Soil quality is a broader concept that can be defined as the capacity of the soil to: Accept, hold, release and mineralize nutrients and other chemical
constituents Accept, hold and release water to plants, streams, and
groundwater Promote good root growth and maintain good biotic habitat for
soil organisms Resist degradation
Requirements
good soil structure to provide adequate aeration (oxygen for respiration)
good water infiltration (movement of water into the soil), moderate pH ( ideally between 6.0 to 7.5), low salinity (dissolved salts in soil water) low levels of potentially toxic elements such as boron,
manganese and aluminum. balanced fertility that provides adequate levels of macro
and micronutrients that plants and microbes require.
Goals of a sustainable fertility/soil management program
To sustain good productivity and crop quality. Provide a balanced nutrient supply for the crop. Time seasonal nutrient availability to correspond with crop demand. Minimize disease/pest susceptibility. Build soil OM as a long term reserve of nutrients and to maintain good soil
structure and habitat for soil organisms
To sustain environmental quality. Maintain or improve soil quality Minimize off-farm impacts, for example:
Avoid non-point source pollution via surface runoff, erosion & leaching. Prevent soil erosion and sedimentation of waterways. Close nutrient cycles as much as possible: within the field, the farm, or within a
watershed, and even at regional and national scales.
It all starts with the soil and understanding how nutrients cycle in agroecosystems.
Soil Development and Agroecosystems
Soils = Climate, Organisms, Relief, Parent Material, Time. Soils=clorpt
Agroecosystems alter soil processes! Practices modify soil properties Farmers manage soil chemistry and
fertility
Processes in the Soil Profile
Source: The Nature of Soils, Brady 1999
AdditionsLossesTranslocations -
movementTransformations
chemical changes
Soil Texture
Soils can be separated into different particles size fractions, e.g.Sand 0.05 mm – 2 mmSilt 0.002 mm – 0.05 mmClay<0.002 mm
Soils are a mixture of different soil particle sizes.
Soil Physical Properties
Texture--particle size distribution.
Structure--aggregate properties.
Tilth--porosity and workability.
Soil Chemistry and Fertility
Soil pH
Cation exchange capacity (CEC)
Organic Matter
Nutrient availability
Cation Exchange Capacity
Source: Brady and Weil, 1996
In many soils, mineral particles are negatively charged, which repel negatively charged ions (anions) and attract positively charged ions (cations).
Soil pH and Nutrients
Source: Brady and Weil, 1996
Farmers try manage soil pH carefully because it:
Affects plant growthaffects nutrient availability
For example, Nitrification (NH4
+ --> NO3-) can reduce soil
pH. Many growers will add lime to increase pH.
Soil Microbial Processes
Decomposition of plant and animal material.
Mobilize (release into the soil) and Immobilize (assimilate) nutrients.
Create Soil Structure by providing the “glue” to hold aggregates together, and creating pore spaces for air and water movement.
Constituents of Soil Organic Matter
Source: Brady and Weil, 1996
Soil food web
Plant macro-nutrients•C, H, O Basic constituents of organic material
•N Proteins, chlorophyll, enzymes etc
•Ca Cell walls , cellular signals
• P Energy transfer - ATP etc
•Mg Chlorophyll, enzymes, protein synthesis
•S Proteins
•Cl Light reaction, ionic balance, stomatal movements
•K Ionic balance, osmosis, enzyme activator
•Micronutrients – Zn, Mo, B, Mn, Cu
Nutrient deficiencies in Tomato
Nutrient Cycles:How nutrients move through the environment
Simple N-cycle
Lightning, pollution
INPUT LOSSCOMPONENT
Nitrogen cycle characteristics
Inputs: fertilizer manures & other organic
materials N2 fixation
atmospheric deposition
Main stores: atmosphere N2 gas
soil OM (>90% soil N)
Outputs/losses crop harvest denitrification leaching erosion volatilization
Microbes rule!!!!!!
Key microbial processes & N transformations Mineralization:
organic N inorganic N (many forms) (ammonium, NH4
+)
Immobilization: inorganic N Organic N (ammonium, NH4
+) (many forms)
(nitrate NO3-)
Nitrification: ammonium nitrite nitrate
Denitrification: nitrate gaseous forms - nitrogen oxides and N2 gas
Ammonia volatization: ammonium, NH4
+ ammonia gas NH3
N2 - Fixation: Conversion of N2 gas into organic forms of N
Root nodules on clover root
Root nodules:symbiosis between legume (plant) & rhizobium(bacterium)
N2 fixation:
•organisms in symbiotic relationships e.g. rhizobium and legumes,frankia and coeanothus, alder
•free living organisms
•N2 NH4+
1. Ammonia release from soils increases as pH increases
2. Denitrification increases in wet soils
3. Both processes increase in warm soils
Gaseous N Losses
INPUT LOSSCOMPONENT
Phosphorous cycle characteristics Inputs:
fertilizer manures & other organic
materials plant residue atmospheric deposition
(small) weathering of rocks
Main stores: soil minerals & rocks soil OM much smaller % of total
soil P than for N
Outputs/losses crop harvest erosion leaching only if soil P
exceedingly high
Soil chemistry and mineralogy rule! - with microbes playing a greater role in high OM soils
Role of mycorrhizae in Plant P uptake
Known to be critical in low P natural ecosystems Some crops are partly dependent on mycorrhizal fungi:
citrus, grapes, avocados, and bananas, Others that benefit from having them include:
melons, tomatoes, peppers, squash, corn, millet, sorghum. Benefit of mycorrhizae highest at lowmoderate P
favored when P is more limiting than C supply, not favored when P less limiting than C supply
Roots colonized by mycorrhizae reduce penetration by root-feeding nematodes
pest cannot pierce the thick fungal network. Can also improve drought tolerance, soil aggregation
and N nutrition
Types of mycorrhizae
VAM or vesicular-arbuscular –found on diverse set of plants except many trees
EctomycorrhizaeTypically on woody plants
VAM
spores
vesicle arbuscule
Ectomycorrhizae on beech tree roots
Root covered withfungal sheath
X-section showing sheath
Hyphae of sheath
Managing Nitrogen
Issue of synchrony between N mineralization and crop demand
Timing of release depends onMoisture, temperatureQuality of organic material being added
What controls net mineralization of N Balance of mineralization vs immobilization
C:N ratio microbes need about 25x as much C as N to grow If C:N ratio of organic amendment is <20-25, then excess
N is released, ---mineralization>immobilization If C:N ratio is around 25, then
---mineralization = immobilization If C:N ratio is >25 then N limits growth so microbes
scavenge nitrogen --- mineralization<immobilization
Presence of resistant or inhibitory compounds slows mineralization
Lignin, polyphenols etc.
I n p u t C :N P e a k N O 3
( p p m )T im in g o f p e a k *
( D a y s a f t e rin c o r p o r a t io n )
L e g u m e +c o m p o s te d m a n u r e 9 - 1 2 3 0 - 4 5 7 - 3 5 ,2 1 - 4 9 , 7
L e g u m e o n ly 1 0 - 1 5 2 2 7 - 5 0 , 2 1 - 4 9 , 7
L e g u m e + s t r a w 1 9 - 2 5 1 5 - 2 0 7 - 3 5 , 2 1 - 4 9 , 7
S t r a w 4 6 - 9 4 5 - 1 0 7 - 7 7 , 2 1 - 7 7 ,7 - 2 1
N o a d d i t io n s - 1 0 - 1 5 7
D a t a f r o m M . V o l a t e t a l , u n p u b l i s h e d . * d a t a f o r 3 s e p a r a t e y e a r s f r o m t r i a l a t U C D a v i s .
T i m e
N r e l e a s e b ym i c r o b e s
C r o p Nd e m a n d
V u l n e r a b l et o l o s s
T i m i n g o f N r e l e a s e w i t h c r o p d e m a n d
FIELD NITROGEN BALANCE
Inputs = Imported fertilizer + atmospheric deposition
+ N2 fixation
Outputs = N exported in crop + N leached into ground water + N in eroded material + N lost by denitrification .
Amounts of nutrients removed by crops (kg/ha)
N P K
Alfalfa hay
Corn – grain
Rice-grain
Tomatoes
cotton
500
200
80
150
55
45
40
15
25
13
350
40
15
200
17
CASFS Farm Nitrogen Budget
% N %P %K
Compost 1992 1.13 0.57 1.18
Compost 2001 0.93 0.45 0.79
Inputs 1
Inputs 2Atmospheric deposition: Less than 1.0 kg/ha/year, of N, P and K, according to EPA data.
Inputs 3Biological nitrogen fixation – legumes
hard to measure – major source of uncertainty in budget
OUTPUTS
1.Leaching – likely to occur in the fall and spring (difficult to measure)
2.Gaseous losses – quantitatively unlikely to be an important component.
3.Erosion – unlikely – CASFS farm fields generate little runoff.
SOM
Soil solution
Fields studied
Apples
Tipi
Pears
Potatoes
Strawberries
Plums
Main Field
Onions+GarlicMixed vegetables
RyegrassCSA
Corn+Beans
Mixed vegetables
Garden
N P KVegetables 45.2 7.2 81.6
Onion + garlic 48.2 7.6 46.7
COMPOST* 16.3 7.9 13.9COVER CROP ? BNF 0.0 0.0
Budget balance -77.1 -6.9 -114.4
Output
Input
Main North Field – 1 year budget
* 1/6 of amount applied every 6 years
Simulated budget for one rotation cycle
N P K N P K1 0.0 0.0 0.0 0.0 0.0 0.0
2 108.8 53.8 103.9 0.0 0.0 0.0
3 0.0 0.0 0.0 48.2 7.6 46.7
4 BNF 0.0 0.0 67.5 10.5 92.4
5 BNF 0.0 0.0 115.7 18.2 133.8
6 BNF 0.0 0.0 46.7 12.0 45.5
7 BNF 0.0 0.0 24.6 3.6 30.1
8 BNF 0.0 0.0 67.5 10.5 92.4
TOTAL 108.8 53.8 103.9 370.1 62.4 440.9
BUDGET -261.3 -8.6 -336.9
INPUT CROP EXPORTYEAR
1. Biological N fixation (BNF) is crucial to compensate for the N exports. Cover crops need to fix 52 kg N/ha/year.
2. Do not know how much N lost by leaching
3. Estimation of BNF is needed to allow us to get an estimate of losses (leaching + gaseous)
4. Potassium export is exceeding input - use higher K compost or other sources of K
5. Phosphorus appears to be in balance
Conclusions
Combine information from budgets with soil testing to refine fertility management
SOM
0
1
2
3
4
5
6
7
8
9
%
Up garden
main field
apples
tipi field
soil ex-P
0
50
100
150
200
250
ppm
Up garden
main field
apples
tipi field
Exchangeable K
0
100
200
300
400
500
600
700
800
1984 1986 1988 1990 1992 1994 1996 1998 2000 2002
mg
K/kg
soi
l
main field apples down garden
research up garden meadow
soil pH
4
4.5
5
5.5
6
6.5
7
7.5
pH
Up garden
main field
apples
tipi field
