Nitrogen Optimization

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Nitrogen optimization involves a balance of providing the appropriate amount of nutrient to the crop at the right time while preserving profit margin and reducing N loss back into the environment. Timing, rate, source, method of application, and nitrogen (N) loss mechanisms play a role in N optimization. Consider the advantages and disadvantages for each N source before deciding which product, rate, and application method to use. The N loss mechanisms of leaching, denitrification, and volatilization may affect N availability.

Nitrogen Application Timing

Application timing can be influenced by factors such as weather and workload. It is desirable to apply N as close to the period of rapid plant uptake as possible to reduce risk of N loss. Sidedress applications are preferred over preplant; however, preplant applications are preferred over fall applications.1

Fall N application has a greater risk of N loss. If applying N in the fall, anhydrous ammonia is recommended because it has the lowest risk of loss compared to other N sources.1,2 Anhydrous ammonia should be applied with a nitrogen inhibitor and only when the soil temperature is less than 50 °F.

Spring pre-plant, sidedress, or split pre-plant/sidedress are preferred N application due to less potential N loss and improved timing in relation to plant uptake. If applying N more than two weeks prior to planting, anhydrous ammonia is recommended to help reduce N loss.1 Due to increased N loss potential, early applications are not recommended on sandy or poorly-drained, coarse soils.

When considering in-season N applications, injection of ammonia or urea ammonium nitrate (UAN) is the preferred method. Broadcast UAN post-emergence have the potential to cause severe leaf burn.

Application Rate

The rate of N applied is an important variable because of economic and environmental issues. Before determining N rate farmers must:

  • Estimate the total amount of N available from soil.
  • Estimate total N required by the crop.

This can be a difficult decision because of factors, such as temperature and precipitation that affect the release of N (mineralization) from the soil. Therefore, N application may vary from year-to-year and from field-to-field on the same farm.

Applying N based on yield goals has been a common method for determining N rate.1 Variable rate technology can be an important tool to aid in the N application to specific areas where there is greater potential for yield response. The N source should be considered when determining application rates. Previous legume crops and manure application should be a factor in calculating the amount of N available in the soil.

Sources and Application Methods

There are two forms of N used by plants: ammonium and nitrate. Other N forms must be converted to ammonium or nitrate for plant utilization. There are several N fertilizers available, each has advantages in different situations (Table 1).

All other forms of N fertilizer are derived from anhydrous ammonia. They are less N dense and have a higher price per pound of N, but are generally easier to store and apply, and are safer to handle.3

N Loss Mechanisms

The efficient use of N is affected by not only the crop, but also by opportunities for N loss in the N cycle. The primary mechanism for N loss include leaching, denitrification and volatilization:

Leaching is the process by which nitrate moves downward in the soil profile out of the root zone from excessive rainwater. UAN and nitrate-containing fertilizers are highly susceptible to leaching, especially on coarse-textured soils.

Denitrification is the process by which soil bacteria that thrive in water saturated (anaerobic) soils convert nitrate into a gaseous form of N. Volatilization of N gas can result in as much as 5% nitrate loss per day.4 This primarily occurs in heavy, poorly drained and those soils that are compacted and restricting drainage.

Volatilization occurs in urea-based fertilizers if they are surface applied and not incorporated into the soil. Urease enzymes in the soil and plant residue convert urea to free ammonia gas. On warm days, up to 15 to 20% urea-based N can volatize within a week of application. There are products that can be used with urea to inhibit volatilization and reduce N loss risk.

Sampling for Residual Nitrogen

Agronomists looking at soil surveys across cotton fields in Texas have determined that many fields have residual nitrogen (N) deep in the soil profile. Dr. Gaylon Morgan, AgriLife Extension state cotton specialist, stated that “crediting this soil residual nitrogen will equate to significant cost savings. However, soil sampling is required to know how much residual nitrogen exsits.”5 This residual N has largely been unaccounted for, and can lead to several problems throughout the growing season like excessive growth that demands the use of a plant growth regulator, increased incidence of Verticillium wilt and aphids, and potential harvest delays.6

To account for residual N, soil should be sampled at a depth of 24 inches, every year for irrigated fields and every other year for dryland production. Once you have your soil test results, you can calculate how much N will be required based on the yield goal by multiplying the soil test results (provided in ppm) by 8 to convert to lb N/acre. Then subtract the value by the N requirement listed for the specified yield goal in Table 1.7 Accounting for any residual N in the soil can help reduce production costs by potentially reducing fertilizer cost, the frequency of plant growth regulator application, and harvest delays.


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Table 1. Nitrogen (N) sources and application.


Understanding and Using Soil Test Results

Soil tests help measure plant-available soil nutrients, not the total quantities of nutrients in the soil. They also cannot directly measure inputs of nitrogen and other nutrients from the mineralization (breakdown and release of nutrients) of organic matter. A soil test can be as general as phosphorus (P), potassium (K), and soil pH, but can include nitrogen (N), secondary, and micronutrients as well.

Fertilizer recommendations provided with soil test reports are specific to the crop identified on the soil sample submission form and are based off of locally conducted nutrient response tests with representative soils of the region. In these nutrient response tests, the specific nutrient is added in increments, such as 20, 40, 60, and 80 lb/acre, to soils with known nutrient levels ranging from deficient to adequate for each nutrient of concern in order to determine the response of the crop to the fertilizer inputs. These tests are repeated at numerous locations to account for climatic and soil variations (organic matter content, texture). Yield comparisons from these test plots indicate if, and at what soil test level, a response to added fertilizer will occur based on the soils and climate of the region.

Sources: 1 Scharf, P. and Lory, J. 2006. Best management practices for nitrogen fertilizer in Missouri. MU Extension, University of Missouri-Columbia Publication M1027. https://plantsciences.missouri.edu/. 2 Sawyer, J. and Creswell, J. Nitrogen applications. Iowa State University Extension. NMEP7. 3 Dorn, T. Nitrogen sources. University of Nebraska Lincoln Ext. 288-01. 4 Nielsen R.L. 2006. N loss mechanisms and nitrogen use efficiency. 2006 Purdue Nitrogen Management Workshops. https://www.agry.purdue.edu. 5 Fannin, B. 2015. Experts offer some key considerations for Texas cotton farmers preparing to plant. Texas A&M AgriLife. http://today.agrilife.org/. 6 Kelly, M and Keys, K. 2014. Texas cotton: planting the right variety in good soil conditions crucial for yields. Texas AgriLife Extension. http://agfax.com/. 7 Bronson, K. and Boman, R. 2009. Nutrient management for Texas High Plains cotton production. http://lubbock.tamu.edu/.