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Profitable nitrogen fertilizer use requires crediting or accounting for non-fertilizer nitrogen sources that may become available to the crop during the growing season. This Crop File provides some general considerations for adjusting nitrogen fertilizer rates to optimize yield and profit.
- Agronomic nitrogen rate (ANR)
- Is estimate of total nitrogen required from fertilizer and non-fertilizer sources for specific production situation
- ANR should be adjusted for nitrogen contributions (N credits) from various sources
- Two initial approaches
- Calculated need
- ANR = (Vm * Nv) + (Yg * Ng)
- ANR: total requirement, lb N/ac
- Vm: vegetative material, lb/ac
- Nv: N content of vegetative material, lb N/lb
- Yg: yield goal, yield_unit/ac
- Ng: lb N/yield_unit
- Response data from university research
- Rate studies compare yield with no nitrogen to yields at increasing nitrogen rates
- Objective is to determine point at which yield no longer increases with increasing N fertilizer rate
- Set realistic yield goal
- Considerations:
- Production history
- Soil productivity class
- Expected weather conditions
- Soil moisture availability
- Precipitation probabilities and/or supplemental irrigation
- Adjust initial rate for economic return?
- EONR = economic optimum nitrogen rate
- Nitrogen rates for maximum profit are always lower than nitrogen rates for maximum yield.
- Based on grain-to-fertilizer price ratios.
- Cost for one bushel of grain compared to cost of one pound of nitrogen
- Nitrate-nitrogen (NO3-N) soil test
- Nitrate is primary form taken up by plant root systems
- Crop uptake depends on adequate soil moisture
- Preplant nitrate test (PPNT)
- Common soil test in western U.S.
- Not typically used in eastern U.S.
- Subsoil samples to include active root zone will improve accuracy
- Usually collected to 24-inch or 36-inch depths
- Credits range from 50% to 100% of nitrogen identified by soil test
- Most soil nitrate credit calculations assume soil sample was taken at planting or shortly before planting i. Credits range from 50% to 100% of NO3-N/ac identified by soil analysis
- Soil nitrate credit becomes more uncertain as interval between sampling and planting increases
- Potential loss from leaching and/or denitrification
- Loss potential affected by excess soil moisture and/or soil characteristics
- Leaching loss potential.
- Denitrification loss potential
- Loss is difficult to predict
- Depends on specific field conditions,
- Soil texture
- Field topography
- Precipitation amounts, duration and intensity
- Other factors
- Pre-sidedress nitrate test (PSNT)
- Diagnostic test found useful for corn
- Particularly useful where production system includes legumes and/or manure
- Applicable across wide geographic area
- Results can be influenced by early season soil temperatures, especially in northern areas
- Soil samples collected when corn plants are 6 to 12 inches tall
- Ammonium-nitrogen (NH4-N) soil test
- Soil concentrations generally low
- Typical concentrations about 2 to 5 ppm NH4-N
- Concentrations highest shortly after applying ammoniacal fertilizers or manures
- NH4-N quickly converted to NO3-N in warm soils
- Adding NH4-N soil test results to NO3-N results can improve accuracy of nitrogen credit
- Soil organic nitrogen
- Microbial mineralization of soil organic matter (SOM) can supply nitrogen to growing crops i. Is difficult to estimate nitrogen credit
- About 1% of SOM nitrogen can be converted (mineralized) to nitrate over one growing season i. SOM is about 58% carbon; about 5% to 6% nitrogen
- N credits of 10 to 20 lb N/ac for each percent of SOM are common
- Calculations may include factor for yield goal
- Assumes more productive soils may have comparatively higher mineralization rates than soils with lower yield potential
- Amount of mineralized nitrogen varies greatly
- Depends on:
- SOM characteristics
- SOM ranges from highly degradable to highly stable materials
- Soil microorganism activity
- Activity affected by soil moisture, soil temperature, soil aeration, and other factors
- Several laboratory soil tests have been developed to predict nitrogen mineralization rate and amounts of PAN
- None are acceptably accurate
- Variable, unpredictable field and weather conditions affect actual rates and amounts
- Legume in rotation provides nitrogen credit
- Likely due to increased mineralization of plant material rather than direct nitrogen credit
- Credit only applies to summer crop[1] following legume in rotation
- Winter or spring crops don’t have enough time for adequate mineralization to credit nitrogen
- Microbial activity requires warm soil
- Greatest credit is from “green manure”
- Top growth is soil incorporated and decomposed by microbes
- Typically involve biennial or perennial forage legumes; e.g., alfalfa, sweet clover
- About half of credit from top growth, half from root system
- Grain or cover crop legumes can provide smaller, but significant amounts of nitrogen
- Nitrogen credit is primarily from decomposition of root system
- Final nitrogen credit affected by:
- crop species
- nodulation success
- stand density
- amount of top growth remaining after harvest or termination
- Total nitrogen
- Manure nitrogen content affected by:
- Animal species and type
- Monogastric vs. ruminant
- Young vs. mature animals
- Animal diet
- Proportion of grain and forage
- Protein level (protein = organic nitrogen)
- Additives or supplements
- Manure form: solid, slurry, or liquid
- Storage type: open or confined
- Length of storage
- Biosolids nitrogen content affected by:
- Digestion method; aerobically or anaerobically
- Stabilization method: e.g., lime stabilization or composting, other
- Plant available nitrogen (PAN) is substantially less than total N content
- PAN affected by nitrogen composition
- Organic-nitrogen must be mineralized to inorganic nitrogen
- Rate of microbial activity for decomposition process delays nitrogen release
- Ammonia/ammonium must be converted to nitrate or may be lost by volatilization
- Nitrate is immediately plant-available, but may be lost from leaching or denitrification
- Carbon-to-nitrogen ratio (C:N ratio)
- Refers to proportion of carbon compounds (e.g., cellulose, lignin, etc.) to organicnitrogen fraction
- Indicates relative “degradability”
- Increasing C:N ratio indicates increasing difficulty for microbes to decompose material
- Affects time required to release PAN
- Potential for nitrogen loss and lower nitrogen credit
- Application method:
- Surface broadcast or incorporated,
- Sprinkled or injected
- Manure type: moisture %
- Higher moisture more conducive to ammonia loss
- Weather conditions at application
- Soil temperatures affect microbial activity
- Temperature, amount of wind affects ammonia volatilization rate
- Soil characteristics: pH, texture, etc
- pH can affect ammonia volatilization
- Texture can affect nitrate leaching
- Residue management affects soil temperature
- Nitrogen can become immobilized in high residue system
- Microbes utilize soil nitrogen during decomposition; covert it to microbial proteins
- Not available to seedling roots until decomposition is complete
- Lower soil temperature was main cause of lower net soil nitrogen mineralization rate
- Research using polypropylene as artificial residue affected soil temperature and mineralization similar to corn residue
- Can be nitrogen “debit” in high residue systems, such as no-till
- No-till corn-on-corn systems
- May require higher nitrogen rates than tilled systems
- Often more susceptible to slow early growth and nitrogen deficiency
- Often problem in northern production areas
- Strip-till or ridge-till systems
- Typically require intermediate ANR
- Young seedlings grow in tilled zone
- Significant amount of residue remains outside the zone.
- Tilled zone is warmer; has higher mineralization potential than residue zone
- Nitrogen placement may
- Placement can help improve nitrogen use efficiency
- Objective is to minimize contact between nitrogen fertilizer and residue
- Place fertilizer nitrogen below the residue in concentrated band
- Dribble nitrogen solution in concentrated band on the soil surface
- Extra 20 to 30 lb N/ac may benefit in some years
- May help compensate for lag time of nitrogen release
- Factors other than application rate influence N use efficiency
- Includes application time, fertilizer placement, and fertilizer material(s)
- Timing
- Matching application and availability to crop uptake patterns improves efficiency
- Additives
- Typically delay release of PAN
- Can affect time-frame for nitrogen uptake
- Can help avoid conditions conducive to nitrogen loss
- Examples
- Using delayed or split nitrogen applications on sandy soils
- Applying series of smaller nitrogen amounts through fertigation
- Using urease inhibitor or nitrification inhibitor
- Some irrigation waters contain nitrate
- Not desirable situation from water quality stand point
- Nitrate is readily available and is moved into root zone by irrigation water
- lb N/ac = ppm NO3-N in water * 0.226 * inches of irrigation water applied
- Nitrogen is removed as forage protein during grazing
- Must be replaced to avoid yield loss in successive crop
- Example: grazing wheat in winter and harvest grain in summer
- Apply 35 to 40 lb N for each 100 lb of liveweight beef gain during grazing period
- Grazing adjustment affected by:
- Crop type
- Grain vs. grass ii. Annual vs. perennial
- Animal type
- Maturity, body size
- Stocking rate (animals per acre)
- Length of grazing period
Bundy, L.G. 2009. Making sense of nitrogen credits for corn production. Crop & Soils, March 2009. 42: 4-7.
James R. Brown, J. R., et. al. 2004. Soil test interpretation and recommendations handbook. Univ.
of Missouri Coop. Ext., Columbia, MO. pg. 6-12.
Leikam, D.F., et. al. 2003. Pub. #MF-2586: Soil test interpretations and fertilizer recommendations. Kansas State Univ. Coop. Ext., Manhattan, KS. pg. 2-6.
Shaver, T. 2014. Pub. #EC155: Nutrient management for agronomic crops in Nebraska. Univ. of Nebraska Ext., Lincoln, NE. pg. 3-16.
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