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Nitrogen Management & Crop Performance

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Overview

Nitrogen is the second most crucial element for plants after carbon and plays a key role in plant growth. Conventional agriculture traditionally uses crop removal estimates to determine the amount of synthetic nitrogen fertilizers to apply each crop season. Other nitrogen sources such as soil organic matter, water, and air are often not considered. This approach, combined with the ready availability of inexpensive synthetic nitrogen fertilizers, can result in the overapplication of synthetic fertilizers. Excess nutrients can lead to nutrient leaching and runoff that has caused a number of environmental problems. Modern conventional and organic agricultural practices improve nitrogen use efficiency to reduce environmental risks. Regular soil and plant tissue analyses are used to determine pre-plant and in-season fertilizer application rates, timing, and placement. Conventional agriculture relies on synthetic nitrogen fertilizers to replace the nitrogen removed by the previous crop. Organic nitrogen management considers all organic nitrogen sources, such as soil organic matter, air, water, and type of organic fertilizer, to supply nitrogen needs that are determined by soil and plant tissue analyses. Organic fertilizers have variable nitrogen release rates that can be efficiently managed by implementing the Four R’s (Right rate, Right time, Right source, and Right place (Farzadfaar 2021).

Part 1: Sources of Organic Nitrogen

Organic nitrogen can come from the atmosphere, Soil Organic Matter (SOM), crop residue, irrigation water, manures, compost, organic fertilizer residue, and residual soil nitrates and ammonium (M. D. Lloyd 2020). The nitrogen from crop residues, soil organic matter, and some organic fertilizers such as poultry litter, contain nitrogen that is not immediately available to the plant. Plant secretions and microorganisms are needed to breakdown these complex organic sources of nitrogen in into small peptides, amino acids, or mineralized into plant-available nitrate (NO3-) and ammonium (NH4+). Nitrogen is immediately available in the form of amino acids from organic fertilizers made from hydrolyzed soy meal. Nitrogen from the air requires free-living or symbiotic microbes to facilitate nitrogen fixation and incorporation into plant tissues. Some nitrogen such as soil ammonia, nitrate, nitrite, amino acids, and small peptides that remain are in soil, are immediately available to the plant. The key to successful organic nitrogen management depends on knowing total and available nitrogen, crop nitrogen needs, and crop production targets.

Part 2: Modern Models for Plant Nitrogen Nutrition

Conventional agriculture’s prior reliance on inorganic fertilizers assumed that plants take up nitrogen primarily as nitrates (NO3-) and ammonium (NH4+) (Figure 1). Nutrients sequestered in soil organic matter had to be mineralized into plant available inorganic nitrogen NO3- and NH4+.  Since the efficiency of this process was unclear, emphasis centered on synthetic fertilizers to supply a crop’s nitrogen requirement needs.  Today, both conventional and organic crop management programs recognize the contribution of soil organic matter (Figure 2) (Farzadfaar 2021).

 

 

The model includes nitrogen contributions from various sources other than synthetic nitrogen.  In doing so, modern practices reduce the need for nitrogen inputs, reduce loss of nitrogen to the environment, and increase nitrogen use efficiency.  Since organic fertilizers are variable in their nitrogen release, it is important to remember the Four R’s of nitrogen use efficiency:  Right rate, Right time, Right source, and Right placement, to optimize the released nitrogen.  The Four R’s recognizes that excess application of any input, synthetic or organic, contributes to the inorganic nitrogen pool and can lead to nutrient leaching from the soil.

Part 3: Nitrogen from Soil Organic Matter and Practical Applications

Soil Organic Matter (SOM) can majorly contribute to available nitrogen.  The amount released depends on the amount of SOM, soil temperature, soil moisture, and soil texture.  Additional nitrogen can come from cover crops, manures, and composts.  There are a few generalizations.  More available nitrogen is released under warm, moist conditions.  Usually, soils with higher clay content have higher SOM and higher nitrogen mineralization rates.  Soils with a longer soil-building history also have higher nitrogen release since specific conditions vary between locations. A soil test is the best way to determine available nitrogen at a given time.

Crop residue and cover crops can contribute considerable amounts of nitrogen depending on tissue nitrogen, C:N ratio, soil moisture, and if residues are on the surface or tilled into the soil.  Nitrogen concentration in vegetables varies from 2.5% to 5% (Lloyd M. D., 2020), similar to that of a leguminous cover crop.  The lower the C:N ratio, the faster nitrogen becomes available.  Cover crops have a C:N ratio of 15 to 20, so nitrogen mineralization will occur more slowly.  Higher soil moisture and tilled residues also release more nitrogen.

The next crop is estimated to directly use 4-30% of a cover crop’s nitrogen.  When terminated, young cover crops release more of their nitrogen than older crops.  The residue of some crops, like broccoli, maintains a significant amount of nitrogen after harvest and may provide 178-255 pounds of nitrogen per acre.  Table 1 lists a few Internet sites that have crop nutrient removal information.  

Table 1.  Nutrient removal rates for crops.

Website 

URL

Crops

Nutrient Removal Rates by Grain Crops

https://www.canr.msu.edu/news/nutrient_removal_rates_by_grain_crops 

Grain:  Corn, soy, wheat

California Crop Fertilization Guidelines

https://www.cdfa.ca.gov/is/ffldrs/frep/FertilizationGuidelines/ 

Field Crops and Vegetables

Tree Crops

University of Minnesota Extension Crop-specific nutrient needs

https://extension.umn.edu/nutrient-management/crop-specific-needs  

Corn

Soybean

Small grains

Beans peas, sweet corn

Grasses and legumes

Oilseed

Sugar beet

Potato

Wildlife food plots

UT Fertilizer Recommendations

https://utcrops.com/soil/ut-fertilizer-recommendations/  

Agronomic

Pasture/Hay/Silage

Vegetable/Root Crop

Fruit and Nut

Home Garden, Nut, Vegetable, Lawn, Ornamental

Fertilizer Recommendations Guide

https://www.nrcs.usda.gov/sites/default/files/2023-06/EC750_2023.pdf 

Field and Forage Crops

University of Georgia Forages

https://georgiaforages.caes.uga.edu/management/fertilization-guidelines.html 

Forage Crops

Summary

The following example utilizes key concepts for organic nitrogen management:

Example: If you have 5% OM soil, you could have up to 100 pounds of organic nitrogen available for the crop.  Dryland wheat requires 100 pounds of N per acre.  Under favorable environmental conditions, additional pre-plant nitrogen may not be needed.  However, local conditions vary, and additional nitrogen may be needed.  Wetland wheat requires up to 150 pounds of N per season.  Additional nitrogen would be needed.  This can be supplied pre-plant with poultry litter using an estimate of 60% of the nitrogen in the poultry litter becoming available for the crop during the season.  In-season foliar or drip applications of fast-release nitrogen could be used for other crops.  The remaining nitrogen from poultry litter may be released over the next two years.

The essential points from the example are:

  1. Soil organic matter can provide a significant amount of nitrogen for the crop.
  2. Some slow-release organic fertilizers can contribute nitrogen beyond the current crop season.  For example, poultry litter can contribute nitrogen for another two years.
  3. Organic nitrogen sources are mineralized into NO3—and NH4+. Soil tests are essential to maintain proper levels since nitrates and ammonium, regardless of their origin, are still subject to leaching and runoff.
  4. The application timing of organic fertilizers depends on the type. Fast-release organic nitrogen can be used in season to address nitrogen deficiencies.
  5. The amount of organic fertilizer to use depends on crop needs, the type of fertilizer, and whether it is slow- or quick-release.

Thus, understanding the crop, the environment, and inputs can increase nitrogen use efficiency.  This will result in significant returns while maintaining sustainable stewardship of our agricultural lands.

Work Cited

Farzadfaar, S., J. D. Knight, K. A. Congreves. 2021. "Soil organic nitrogen: an overlooked but potentially significant contribution to crop nutrition." Plant Soil 462: 7-23. https://link.springer.com/article/10.1007/s11104-021-04860-w.

Lloyd, M., D. Geissleler, P. Lazicki, J. Muramoto, and R. Smith. 2020. Estimating nitrogen availability in organic annual production: For N budgeting and other purposes. https://ucanr.edu/sites/SFA/files/322312.pdf.

Lloyd, M., P. Lazicki, D. Geisseler, J. Muramoto, R. Smith, E. Smith. n.d. Estimating nitrogen (N) for organic crop production: worksheet. https://ucanr.edu/sites/SFA/files/322313.pdf

Gaskell, M. a. (2007, October - December). Nitrogen Sources for Organic Vegetable Crops. Retrieved from https://greatbigplants.com/wp-content/uploads/nitrogen-sources-for-organic-vegetable-crops.pdf

Grubinger, V. (2005, May). Managing Nitrogen on Organic Farms. Retrieved from https://www.uvm.edu/vtvegandberry/factsheets/managingNorganic.html

Tei, F. S. (2020, June). Nitrogen management of vegetable crops. Retrieved from https://library.wur.nl/WebQuery/wurpubs/fulltext/525190

Cameron, K. H. (2013). Nitrogen losses from the soil/plant system: a review. Ann. Appl. Biol, 162, 145-173.

 

 

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