Forage Management Practices
Fertility is the capacity of the soil to provide the
nutrients needed for plant growth
Nutrient Management
Background
Soil testing
Hayfield fertility
Pasture fertility
Soil fertilization management is an effort to:
-- modify and sustain the soil chemical
environment:
Reading assignment:
Forages I: Chap. 12,
Web: Pm-1268, Fm-1712, Pm-1688;
Pm-869; Pm-1558 Pm-1811
Alfalfa: pp. 18-23
1) to meet the growing crop needs
2) without exceeding crop needs (economic)
Supplemental Readings:
Forages I: Chapt.14 332-333
Forages II: Chap.24
3) without exceeding the limits of the system
equilibrium (leading to negative affects on
environmental quality)
Nutrient uptake is influenced by:
Root morphology, surface area
Soil pH -- negative logarithm (base 10) of the activity of hydronium ions in a solution
( 10 X difference between whole units ! )
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Taproot system vs. fibrous root system
Amount of nutrient in the soil …………affected by
‘Inherent’ soil fertility (parent material, ‘age
of soil’, weathering & erosion’, & past fertility history)
pH of 7.0
is ‘neutral’;
Nutrient availability
Soil chemistry- pH, solution vs adsorption to clay and OM
< 7 is acidic
>7 is ‘basic’
Soil environment
Soil moisture (deficit or excess), limited O2, physical limitations
Efficiency of uptake
- Old roots vs young; root tips
- Uptake including microrhiza ??
In the Soil
Essential nutrients for plant growth
Sources of ‘Stored’ Nutrients
Macronutrients:
C, H, and O come from the air& water ;
N from the soil and air;
P, K, Ca, Mg, S from the soil
(S is sometimes referred to as a ‘Minor nutrient)’
Micronutrients:
Fe, Mn, Zn, C u, B, Mo, Ni
from the soil
Micronutrients essential to animals, but not
essential to plants:
Si, Na, I, Cr, Co, Se
from the soil
National Research Council, 2001
Long-term
‘storage’
Very slow
availabiliaty
Plant-Available Nutrients
Short-term
‘storage’
Into solution
Soil
Minerals
‘Soil
solution’
Organic Soil MicroMatter organisms,
enzymes etc
Most Legume ‘Adsorbed’
Nutrients (ions)
Nitrogen
Loosely bonded to
clay, colloids
& organic matter
Where does
fertilizer fit
into this ?
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Nutrient mobility in the plant
Cation
Anion
Mobile nutrients: N, P, K, Cu, Zn, S, Mg
‘Soil
Solution’‘
Soil clay particles, colloids and soil organic matter have
both negative and positive charges (usually net neg).
Ions are ‘adsorbed’ to these charges
Cations (Ca++, Mg++, K+, Nh4+ ; and H+) are weakly bonded to (-) sites
Anions (SO4--, NO3-, PO3 -, Cl- ) are weakly bonded to (+) sites
Some states rate soils on their ‘Cation Exchange Capacity’ CEC
(clay type and organic matter greatly influence CEC of a soil
Nitrogen
•
•
•
•
Often the most limiting in forage grass production
Increase in yield
Grasses can take up more than it ‘needs’
N fertilization favors grasses in a legume-grass
mixture
P2O5, phosphate
• Seedling establishment, root growth, winter
survival
• Increase nodulation, nodule mass, earlier
nodulation
• Moves to the sites of growth & metabolic activity
K deficiency
alfalfa
when a deficiency exists
• Deficiency symptoms are found
in older foliage & leaf margins
• Whole plant tissue tests may be best indicator
of the nutrient status of these nutrients
Immobile nutrients : Ca, B, Mn, Mo
Remain in tissue where metabolized
Deficiency symptoms are found
in the newly formed foliage
Boron Deficiency
(tops of the plants)
alfalfa
Calcium
– Higher in legumes, plant structure
– From soil minerals and added limestone
Magnesium
– Necessary for chlorophyll
– Form soil minerals & added dolomitic limestone
K2O, potash
Sulfur
– Not often limiting ( but sometimes ?)
– Sources: manure, soil Organic Matter & air pollution
(coal smoke)
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•
•
•
Boron
– Rarely limiting in legumes
– Sources: manure, soil Organic Matter
‘Luxury consumption’ in grasses and alfalfa
Positive contribution to winter survival in legumes
Longevity, stand persistence
Increase number of nodules
‘Nodulation ‘
N2 Fixation
Legume-Rhizobium symbiotic relationship:
-- Requires a ‘positive energy status’ in the plant
Soil environment:
-- near neutral soil pH
-- adequacy of the major &
several of the minor &
micro nutrients
(molybdenum)
--- not ‘water-logged, etc.
-- Specificity of legume with rhizobial species
It may not be as simple as ‘only one rhizobia
sp. can infect its own forage species’ ;
With improved identification techniques,
more complex cross- inoculation
relationships are being found.
‘infection thread containing bacteria
extends into the root hair & into
deeper root tissue layers
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Nodule forms
(hypertrophic growth of plant tissue and bacterioids)
Moderate – to high levels of available/chemical
soil Nitrogen can limit nodulation and N2
fixation !!
Bacteroids
As little as 50 lbs/A of plant-available N can
slow or completely ‘turn off’ N2 fixation.
Severe defoliation (and plant dependence
on ‘stored carbohydrates’) can cause Nfixation to cease too !
Leghaemoglobin is involved in limiting
O2 in the interior of the nodule
Nitrogen “credit” from plow-down of forage crops:
Contribution of legume N to the production system
FOR ALFALFA – when Iowa suggested ‘N credits’
If previous year’s crop was:
- 50-100% legume in stand: grain crop response the
following year can be equivalent to 100-140 lb N/A of
added N fertilizer
- with 20-50% legume in stand: 50-80 lb N/A
Two years after plow down with 50-100%
legume in stand:
30 lb N/A
Univ of Minn N-credit guidelines for 1st yr corn after alfalfa
2nd yr corn
previous alfalfa stand
1st year corn
4 alfalfa plants/sq ft
150 lb N/acre
75 lb N/acre
2 to 3 plants/sq ft
100 lb N/acre
50 lb N/acre
1 plant/sq ft
40 lb N/acre
0 lb N/acre
– Depends upon the legume
(alfalfa > red clover > BFT)
– Depends upon the age of the stand
( little ‘new or fixed N” during the first half of
the establishment year )
---- Does not take into account the ‘rotation effect’
Now in Iowa, there is less emphasis on ‘N Credit’ amounts,
but on whether the following corn crop is likely to
respond to additional N – and to how much added N
Source: Table 8.3 Forages Vol.1
Amount of Nitrogen fixation for forage legumes and forage legume-grass
combinations during a growing season
Species
Research in several states has indicated little, if
any, response to additional N following alfalfa
Summary of corn grain responses to fertilizer N
following alfalfa
State
Iowa (Morris et al., 1993)
Wisconsin (Bundy & Andraski, 1993)
Minnesota (Schmitt & Randall, 1994)
Pennsylvania (Fox & Piekielek, 1988)
Sites
Total Responsive
29
24
5
3
6
0
1
0
Optimum
N rate
lb/acre
25
0
42
0
Total N2 Fixation
Location
Alfalfa
grown alone
grown alone
in mixture with orchardgrass
in mixture with reed canarygrass
189
102-200
13-131
73-227
KY
MN
IA
MN
Red Clover
grown alone
in mixture with reed canarygrass
62-101
4-136
MN
MN
Birdsfoot trefoil
grown alone
in mixture with reed canarygrass
44-100
27-116
MN
MN
114
KY
White clover
grown alone
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Contribution of legume N to a mixed grass - legume sward
‘sward’ = ‘a population of herbaceous plants,
characterized by a relatively short habit of
growth and relatively continuous ground cover,
including both above- and below-ground parts’
How much legume is enough to make a difference ?
Every little bit is beneficial
1/3 or more legumes in
the sward is the ‘target’
Legume N ‘transfer’ to grass is from:
- death / decomposition of nodules, roots,
leaves/branches
- “root leakage” & possibly ‘root grafts’
( very minor sources / contributions !)
- from manure and urine of animals eating
those legumes
Nutrient balances/imbalances
Ratio of
N : P : K forage crops ?
Some states and soil / plant testing labs make
fertilizer recommendations on the basis of
plant tissue test ratios
With 30% legume --- as much as 50% of the N
in the grass is from legume N,
95% of the N in the legume came from fixation
No N fertilizer is recommended if the mixture contains
at least 1/3 legume (but plants will respond to added N !)
Is there a ‘right’ Ca:MG ratio?
Calcitic lime vs. Dolomitic lime?
Effect of Various Ca : Mg ratios on yields of Alfalfa in Wisconsin
Theresa silt loam
Ca : Mg Ratio
Plainfleld loamy sand
Yield
Tons/Ac
Ca : Mg Ratio
Yield
Tons/Ac
Legumes
10 : 1 : 8.3
Cool-Season Grasses
Warm-season grasses
6.6 : 1 : 7.9
8.2 : 1 : 5.8
2.28
3.3
2.64
4.1
3.61
3.1
3.33
4.1
Corn
5.5 : 1 : 5.7
4.09
5.25
3.6
3.5
4.00
4.81
4.4
4.1
8.44
3.2
8.13
4 .4
Consider these ‘general ratios’
Nutrient balances / imbalances – cont.
High K levels reduce Ca uptake (milk fever ?)
High N & K levels reduce plant Mg (grass tetany ?)
High Ca:Mg (>3:1) reduces plant Mg (grass tetany)
pH affects availability/immobilization
Ca, P, S, Zn, Fe and Al
N utilization more efficient when P and K are
adequate
Forage Persistence
High N (with low K levels) may increase the potential
for winter injury
Forage Quality
In general, fertilization increases the mineral
content of the forage
N fertilization usually lead to rapid vegetative
growth and a corresponding decrease in
water soluble carbohydrates. ( A factor that has
an impact on forage silage/haylage fermentation.)
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