Unit 9: Soil Fertility
Management
Chapter 10
Objectives
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Understand objectives of soil fertility
management
Philosophies/techniques of precision farming
Using & obtaining valid soil samples
Considerations in making/following fertilizer
recommendations
Knowledge of fertilizer quality
How to calculate fertilizer blends
Fertilizer application methods
Benefits/limitations of manure use
Introduction
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Fertilizer is one management option
used almost universally
Must replace soil nutrients lost by
harvest
Over-fertilization can result in
dangerous pollution
Technology has increased fertilizer
efficiency
Goals & Concerns in
Fertility Management
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Goals regarding fertility
– Increase yield
– Reduce costs/unit production
– Improve product quality
– Avoid environmental pollution
– Improve environmental health &
aesthetics
Goals & Concerns in
Fertility Management
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Efficient land managers: spend <20%
of production costs on fertilizers,
expect >50% increase in yields
Fertilizers may not be profitable if:
– Water is the most limiting factor
– Other growth hindrances – insects,
diseases, acidity, extreme cold
– Increased yield has less market value
than the cost of buying/app of fertilizer
Goals & Concerns in
Fertility Management
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Fertilizers – generally most profitable
farm input
Soil fertility problems usually the
easiest to solve
Soil nutrients typically present in finite
amounts, don’t replenish themselves
Crops typically contain: (in rank of
amount found in the plant) N, K, Ca, P,
Mg, S
Goals & Concerns in
Fertility Management
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Utilizing fertilizers may help cut unit
cost of production by maximizing yield
– Improved fertility = improved yields,
improved aesthetic appeal
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Environmental concerns abound
– Fertilizer laws viewed as lax by some
– Farmers may be the primary cause of
non-point-source pollution
Goals & Concerns in
Fertility Management
– Three common pollutants:
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Nitrates
– Percolate through to groundwater
– Not safe to drink
– Cause “Blue-baby” syndrome – inhibits
oxygenation of blood
– Becoming common near heavily fertilized fields,
feedlots, dairies
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Phosphates
– Pollute surface waters by runoff
– Promotes algae growth in rivers/ponds
– Depletes available oxygen in the water for fish
Goals & Concerns in
Fertility Management
– Wise use of fertilizers must be
encouraged, actually improve the
environment
Crops, trees, etc. - remove more CO2,
decrease sediment, dust, erosion
 Plays important role for future of the planet
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Scale of Land
Management
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Large- & Medium-Scale Management
– Large-Scale
Low levels of operational precision, little
reliance on sophisticated technology
 May be most feasible/profitable for some
 Simple & low-tech
 Some shy away from high tech for other
reasons
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Scale of Land
Management
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Disadvantages
– Some parts of field may receive too much/little
fertilizer or pesticide
– Less than optimal yields
– Inefficient use of fertilizers & pesticides
– Higher cost of production/unit
– Environmental pollution due to over application
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Advantages
– Minimal technological training & instrumentation
needed
– Field operations can be performed w/ standard,
readily available, cheaper equipment
Scale of Land
Management
– Medium-Scale
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Subdivide field into two+ management units
– Delineation may be based on:
 Soil types
 Past management differences
 Farmer’s observations
Ex. High, medium, low N application areas in
the field
 Same equipment/technology needs as for
large-scale management farmers
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Scale of Land
Management
Does improve efficiency of farm inputs
 Can reduce excessive applications of
chemicals/fertilizers
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– May do spot treatments/applications w/in a field
due to field observations
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Small-Scale Management (Precision
Farming)
– Global Positioning System (GPS) –
network of U.S. satellites w/ a signal
detection system used to locate positions
on the ground
Scale of Land
Management
– Soil sample fields on a grid
– Data collection points no more than a few
feet apart
– Each sample site mapped using GPS
– Custom applicators can custom apply
fertilizers at variable rates that change
constantly as the applicator travels the
field – variable rate application, site-
specific management, precision farming
Scale of Land
Management
– Potential to substantially decrease
fertilizer/chemical application rates
– Potential to substantially decrease input
costs
– Does require expensive technology,
equipment & extensive technical
knowledge
Soil Sampling
Standard method for determining soil
fertility
Use w/ precision farming to minimize
inputs
Accuracy of sample is key!!!!
Soil Sampling
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Depth & Number of Samples
– Sampling depth – 7-12” for typical soil
analysis
Shallower depth for no-till/sod crops – acidlayer can form at very top of soil structure
 For accurate N analysis – 24-36” depth
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– For composite sampling – fewer #
samples decreases accuracy of analysis
Soil Sampling
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Sampling Frequency, Time, & Location
– New land, land new to you – yearly for 1st
few yrs until you understand the soil
– Every 2-3 yrs, unless concern for
environmental problems
– Analysis – determines which nutrients can
be made available in the soil & which will
need to be supplied
– Samples often pulled in fall to provide
enough time for analysis/amendments
Soil Sampling
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Spring sampling is more accurate, but
conditions may not be favorable, or not
sufficient time
– Sampling row crops problematic
Can hit a fertilizer zone
 Hard to get enough representative samples
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Soil Sampling
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Uniformity of Sampling Areas
– Examine field for differences in soil
characteristics, past treatments
– Consider:
Uniformity of productivity
 Topography
 Soil texture
 Soil structure
 Drainage
 Depth/color topsoil
 Past management
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Soil Sampling
– Sampling area
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Each composite sample should represent
<12.5 ac
– Grid sampling can be as small as you need
– 5-10 ac grids are common
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Providing Detailed Soil & Cropping
Background
– Helps to provide w/ soil analysis to
increase accuracy of fertilizer
recommendations
Soil Sampling
– Include:
Previous crop
 Crop (s)) to be grown
 Realistic yield goal
 Last liming & fertilization rates
 Manure applications
 Soil series (if known)
 Drainage info
 If irrigation used
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Soil Sampling
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Other problems:
– Temp, geographic location, elevation, farming
practices, etc.
Soil Tests
Law of the Minimum: growth of the
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plant is limited most by the essential
plant nutrient present in the least
relative amount (first-limiting)
Soil Acidity Evaluation
– pH measured w/ electrode & solution
– Lime requirement – amount of lime
required to achieve desired pH
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Reported as buffer pH
Soil Tests
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Soil Test for N
– No good tests for soil available N
– Most states provide N recommendations based
on yrs of field plots trials on various crops, soils,
management, fertilizers
– N recommendations consider:
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Previous crops
Estimates N carryover
N needed to decompose residues
Projected yields
Climate
Soil Tests
– Lab N tests accurate, but nearly
impossible to interpret
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Some will discourage N testing
– Behavior of carryover N unpredictable –
can make analyses invalid
Leaching
 Denitrification
 Mineralization
 Climate
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Soil Tests
– N recommendations based on yield goals
rather than soil reserves
– Corn Rule – 1.2-1.4#N/bu of yield goal
How much N should be recommended for
corn following corn, expected yield 120
bu/ac?
 How much N should be recommended for
corn following soybeans, expected yield 195
bu/ac?
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Soil Tests
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Soil Tests for P & K
– Widely used to predict probability of crop
response to fertilization
– Survey:
47% soil tested medium to low for P
 43% soil tested medium to low for K
 P & K soil levels declining in many states
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– P testing
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Quite reliable – soil P is very stable from yr to
yr
Soil Tests
Most soil P unavailable to crops
 Soil test extracts & measures what may
actually be available
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– K testing
Tests both exchangeable & soluble reserves
 Conflicting testing procedures over which is
most accurate
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– Some estimate upper threshold needs ~159246#/ac (above which no response to K fertilizer)
– Others - 335#/ac on clay soils (calculated based
on soil CEC – higher CEC = decreased available K)
– Some experimentation w/ soil probes
checking K, NO3, PO4, SO4
Soil Tests
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Soil Test for Ca & Mg
– Related to need for lime
– Well-limed soils rarely Ca & Mg deficient
– Mg deficiency more common than Ca
Coarse-textured or acidic soils
 Many yrs using non-Mg containing lime
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– Mg testing for:
Exchangeable soil Mg
 % Mg saturation of soil colloids
 Ratio of K:Mg
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Soil Tests
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Soil Test for S & B
– S testing inaccurate – acts much like N
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Can test – but must take variability into
account
– Boron level recommendations
<1.0 ppm – deficient for plant growth
 1-5.0 ppm – adequate
 >5.0 ppm – excess/toxicity risks
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Soil Tests
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Soil Test for Micronutrient Needs
– Difficult to develop accurate tests due to
relatively infrequent need for field
supplementation
– Can be done, if requested for a specific
need
– Adds expense to soil analysis
Soil Tests
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How Good Is Soil Testing?
– Analyses recalibrated regularly based on
field trial studies
– Validity of analysis related directly to
accuracy of sample, information provided
to the lab
– Soil analyses generally very valid for: P,
K, soluble salts, pH, lime
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Other tests should only be used on as-needed
basis
– Extra cost
– Less accurate
Analysis of Plants
Only way to be sure of soil nutrient
availability
 Plant Analysis vs. Soil Testing
– Plant most accurate report on what
nutrients are actually available
– Plant analysis leaves little to no room for
amendments to the soil
– When deficiencies are acknowledged,
yield usually already affected
Analysis of Plants
– When is plant analysis most helpful?
Treatment of an easily-corrected deficiency
 Long-growing crops: turf, tree fruits, forests,
sugar cane
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Quick Tests in the Field
– Can test for N, K status in plants
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Collect ~20 leaves for sample
– Must be random from different locations
– Don’t select only affected-looking leaves
Analysis of Plants
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Chop/mix, squeeze sap & test
Most effective for greenhouse/nursery growers
– Amendments can easily be made
– High possible economic losses
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Total Plant Analysis
– Done in a lab
– Should be tested by stage of development
– Random sampling key
Analysis of Plants
– Indicate part of plant sampled & be
consistent
– Dry to prevent spoilage (confounds
results)
– Wrap in paper and mail w/ complete
report – complete history, information
critical
Analysis of Plants
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Interpreting Plant Analyses
– Accurate interpretation difficult if not all
critical information provided
– Element classified as deficient if below
threshold nutrient levels
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Levels change through season, stage of
development, etc.
– Some general disagreement from
scientists on what threshold levels are
Analysis of Plants
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Critical Nutrient Range
– CNR – ranges at which nutrients are:
Visually deficient
 Hidden deficient
 Slightly deficient
 Sufficient supply
 Toxic
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Analysis of Plants
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Visual Nutrient Deficiency Symptoms
– Chlorosis – yellowish to whitish
appearance to foliage, stem
– Necrosis – dead tissue
– Causes: disease, insect damage, salt
accumulation, stress, nutrient deficiencies
– Some visual symptoms same for many
diseases/deficiencies
Analysis of Plants
– Nutrients are relocated in the plant by
two pathways
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Xylem – water-carrying vessels
– All nutrients can pass through
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Phloem – sugar-carrying vessels
– Not all nutrients can relocate
– Mobile nutrients – travel freely
– Immobile nutrients – can’t be moved from their
location in the plant
– Mobile nutrient deficiencies tend to occur
on older leaves – plant sacrifices old for
new tissue
Analysis of Plants
– Immobile nutrient deficiencies –
symptoms on shoot/root tips, fruits
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Can’t be treated from the soil w/ fertilizer –
plant can’t send Ca (ex) to the ripening fruit
– Mobile nutrients:
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N, P, K, Cl, Mg, S
– Immobile nutrients:
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Cu, Mn, Zn, Fe, Mo, S
– Very immobile nutrients:
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B, Ca
Fertilizer
Recommendations
Different labs make different
recommendations
Traditional philosophies being challenged
P application rates
 Yield-based N recommendations
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Fertilizer
Recommendations
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Developing a Fertilizer
Recommendation
– Must have sufficient plot data to correlate
yields & nutrient needs
– Once a general amount of fertilizer is
known:
Subtract for manure application
 Subtract for residual P or N
 Add/subtract for N, P, S because of soil
organic matter levels – can count on them
supplying some
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Fertilizer
Recommendations
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Test Reports
– Labs usually full-service
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Soil, plant, manure, irrigation water testing
– See soil test report
Fertilizer Quality
Fertilizer grade – amounts of N, P, K in a
fertilizer required by law to be listed
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Also required:
– Weight of material, manufacturer
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Optional:
– Filler composition, acidity in soil potential
Calculating fertilizer N, P, K amounts
10-20-10
 15-12-18
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Fertilizer Quality
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Amounts listed as: elemental N, phosphate,
potash (not direct indication of elemental P, K
supplied)
Acidity & Basicity of Fertilizers
– Most affect soil acidity in some regard
Superphosphate, Triplesuperphosphate,
Potash – neutral
 MAP, DAP, all N fertilizers – acidifiers
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Fertilizer Quality
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Solubility & Mobility in Soil
– Function of:
Elemental charge
 Tendency to form insoluble compounds
 Adsorption ability
 Soil texture
 Water movement
 Concentration of other ions
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Fertilizer Quality
– Examples
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P may only move a few cm
– Must be place in/near root zone
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N can move w/ extent of water movement
Fertilizer Calculations
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Calculating Fertilizer Mixtures
– Mixing 34-0-0 ammonium nitrate & 0-460 TSP to get 1 ton mixture of 15-10-0
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How much of each do we need?
– How about if we needed a 12-14-6
fertilizer for a customer?
What might we use for each ingredient?
 How much of each would we need?
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Fertilizer Calculations
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Weights of Fertilizer to Apply
– Planting corn expected to yield 125 bu/ac
How much N do we need?
 Soil analysis recommended 88#/ac phosphate
 How much ammonium nitrate & TSP do we
need?
 What is our final application rate?
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Fertilizer Calculations
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Calculations Involving Liquid Fertilizers
– Use dry fertilizer calculation if sold by
weight
– If sold by volume, usually applied by
volume
– See example pg. 336
Techniques of Fertilizer
Application
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Starter (Pop-Up) Fertilizers
– Addition of fertilizer w/ the seed during
planting, dribbled in a strip near the see,
banding w/in 2” of seed
– Most beneficial for P, K – some for N, but
not as necessary
– Advantages:
Cold soils
 Low nutrient levels in the root zone
 Fast-growing plants
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Techniques of Fertilizer
Application
– Disadvantages:
Slows planting
 Can burn seedling, if placed too close
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Broadcast Application
– Uniform application across entire surface
– Left on surface, or incorporated
– Somewhat less efficiency of fertilizer
Especially when not incorporated quickly
 Why?
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Techniques of Fertilizer
Application
– Reasons to broadcast:
Only practical method of application –
pastures, turf, etc.
 Low-fertility soils needing high fertilizer rates
 Easy, cheap, personal preference
 Flexible – split applications, ability to add
after crop is growing
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Techniques of Fertilizer
Application
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Deep Banding
– Application of strips into the soil
– Either between/side of row, where the
seed may be planted
– Typically 4-12” depth
– Knifing in anhydrous most common
Gas able to dissolve in soil water before it
escapes
 Losses can be high if dry, sandy
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Techniques of Fertilizer
Application
– Disadvantages:
Strong equipment needed
 High fuel costs
 Danger of dealing w/ anhydrous
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– Advantages:
High yield response potential
 Puts fertilizer where most roots are, very
efficient use
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Techniques of Fertilizer
Application
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Split Application
– Divided total fertilizer rates delivered in
2+ applications
– Reasons to split applications
If large applications are needed – increase
efficiency of nutrient use
 Soil conditions dictate – risk for high nutrient
losses
 Control vegetative growth in early stages
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Techniques of Fertilizer
Application
– Advantages:
Increased efficiency of N utilization
 Provide a “boost” to the plant during growth
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– Disadvantages:
Extra pass through field
 Not effective for P, K because of immobility
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Techniques of Fertilizer
Application
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Side-Dressing or Topdressing
– Side-dressing – surface or shallow band
application put on after crop is growing
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Broadcast, surface stripped, sprayed, knifed
– Principles to consider:
Decreases potential N losses
 Added in the furrow to allow water to help w/
infiltration
 Not effective for P, K
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Techniques of Fertilizer
Application
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Point Injector Application – place P, K
into soil in the root zone w/out
significant root damage
– Used more in small plots, gardens
– Push stick, rod into soil, fill w/ fertilizer,
cover
– Effective for: fruit trees, grapes, shrubs,
etc.
– Not common in field use
Techniques of Fertilizer
Application
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Fertigation – application of fertilizer w/
irrigation water
– Can apply large quantities of nutrients
– Very effective for N
Some see 30-50% more efficient use of N
 Cut of 50% in N rates w/ same/better yield
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– Must be careful of potential problem w/
salts
Techniques of Fertilizer
Application
– Able to apply when need is highest
– Immediate/convenient application
– Most effective on soils w/ poor nutrient
retention & for mobile nutrients
– Chemigation also possible – not discussed
in depth here
Techniques of Fertilizer
Application
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Foliar Application – foliage wetted to
maximize nutrient absorption through
leaf stomata & epidermis
– Feasible for: N supplementation,
pesticides, micronutrients, etc.
– Guidelines:
Only suited for applications of small amount
(can burn plant)
 Decreased rates can be used
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Techniques of Fertilizer
Application
Need wetting agent to help the spray to
distribute evenly across surface
 Helpful when root conditions restrict nutrient
uptake
 Quick response/remedy to deficiency (also
short residual)
 Wind must be calm, humidity >70%, temp
<85° F
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Techniques of Fertilizer
Application
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Fertilizing in Paddy & Other
Waterlogged Soils
– Paddy rice – production on water covered
soils
Water 2-6” deep
 One of very few crops that tolerate anaerobic
conditions
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– Difficult to fertilize due to high nutrient
loss risks
Fertilizer Efficiency
Great focus on increasing efficiency of
fertilizer use
Research
 Real-time sensors in soils that immediately
detect nutrient deficiency
 Transgenic plants
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Fertilizer Efficiency – fraction/percentage
of added fertilizer that is actually used
by the plant
Fertilizer Efficiency
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Typical fertilizer efficiencies:
– 30-70% for N
– 5-30% for P
– 50-80% for K
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Maximum profits rarely at maximum yields
– Last amounts of fertilizer to produce more yield
cost more than yield increase
– Management also key

Use of BMP’s increasing
– Encourage environmental protection
– Couple w/ agronomic success
– Increase economic yields, leading to sustainable ag
Fertilizer Efficiency
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Plant Root Systems
– Some plants better scavengers than
others
– Absorption greatly affected by fertilizer
distribution
– Smaller root system = shorter growing
season = >dependence on fertilizer
– Growth rates & size also effect amount of
nutrients demanded
Fertilizer Efficiency

Weeds
– Response to fertilizer much like crops
– N fertilization may increase weed growth
> crop growth
– Application method can also affect weed
growth

Ex – broadcast fertilizer can tend to help
weeds get good start
Fertilizer Efficiency

Fertilizer-Water Interactions
– Availability of nutrients directed impacted
by soil water content
– Drip fertigation may be most efficient use
of water & fertilizer
Common in greenhouses
 Can be effective in field use

– Israeli farming uses drip irrigation
Fertilizer Efficiency

Fertilizing for High Efficiency
– Guides to optimal fertilization:
Avoid large additions of N or K (50#/ac +) on
sandy soils – use split application
 Avoid broadcast applications of urea &
ammonia on warm/moist soils – volatilizes
easily – incorporate
 Avoid N losses on poorly drained soils by
using ammonium
 Band P
 Use starter fertilizer

Fertilizer Efficiency
Keep N & K fertilizers out of seedling zone to
avoid burn
 Reduce leaching by avoiding application
before rain or irrigation
 Foliar apply, if feasible/appropriate
 Know nutrient demands of crop
 Improve management
 Remember law of minimum
 Soil test

Livestock Manure as
Fertilizer
Many benefits of using manure:
Recycles nutrients
 Potential to reduce pollution
 Adds C to soil
 Improve aggregation, infiltration, microbial
vigor

Risks:
Increased weed pressure
 High cost of obtaining/applying if you don’t
own it

Livestock Manure as
Fertilizer
Not as convenient as commercial fertilizer
 Pollution anxiety


Nutrient Production & Recovery
– Production rates predictable &
measurable
– Ration has heavy influence on nutrients in
manure
Livestock Manure as
Fertilizer

Manure & Nutrient Budgets
– Generous applications of manure no
longer norm

Some states require & enforce strict manure
management guidelines
– Restricted application due to soil P levels
instead of N
– Manure still can’t meet plant needs alone

Crops remove much higher levels of
nutrients/ac
Livestock Manure as
Fertilizer

Using Manure
– Most recognize advantages of using
manure
– Manure production unevenly distributed
in farmland
– Expensive to transport very far
– Too abundant in areas, not enough land
for application
Livestock Manure as
Fertilizer
– Must balance three factors
Supply crop nutrients
 Dispose of waste
 Protect environment

– More focus on manure later
Assignment