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Calculating Delta Yield is equal parts art and science
Calculating Delta Yield is equal parts art and science

If your soil is not likely to respond to added fertilizer, the question of "how much yield will you get if you add fertilizer" does not arise. However, if the probability calculated in the previous step suggests that added fertilizer will have some benefit, then we need to estimate how much benefit might we expect, in terms of additional bushels per acre. This additional production is called Delta Yield.

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Response to added fertilizer is a function of N:C ratio as well as the water availability in the root zone.
Response to added fertilizer is a function of N:C ratio as well as the water availability in the root zone.

Microbes either uptake nitrogen when their environment is carbon rich in a process called immobilization, or release nitrogen in the form of plant-available nitrate (NO3- in the process of mineralization. The carbon to nitrogen ratio and the soil pH are important factors in this metabolic switch.

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A screen capture from the MRTN calculator for Central IL showing no relationship between optimal corn yields and optimal fertilization rates.
A screen capture from the MRTN calculator for Central IL showing no relationship between optimal corn yields and optimal fertilization rates.

An economically optimum nitrogen rate (EONR) is a nitrogen rate that maximizes the returns on investments in nitrogen application. This rate depends on how responsive your soil is, how weather affected the growing season, current costs for various forms and applications of nitrogen and projected revenues from the yield. Calculating a true EONR prospectively is very difficult. However a simpler form of the EONR is often referenced in calculators like the Maximum Return to Nitrogen (MRTN).

Key takeaways

Making a recommendation for optimal nitrogen application to maximize profitability involves the following steps:

  1. Determining if your soil will respond to added nitrogen
  2. Estimating how much additional yield you could obtain from the soil
  3. Determining how much nitrogen is required for this additional yield
  4. Making corrections based on historic data, tillage and other relevant management practices

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Making nitrogen rate recommendations requires several pieces of information, including historic yields, management practices, soil types and of course current soil tests. In this second part we look at the three key soil tests needed to make good recommendations.

Key takeaways

Three soil tests are important to make a good recommendation:

  • The FertiSaver-N test measures the amount of mineralizable nitrogen in the soil
  • The Loss on Ignition (LoI) test measures the amount of carbon in the soil
  • The pH test measures how conducive the soil is to optimal biological activity.

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This is the first of a multipart series on how to make site-specific, nitrogen rate recommendations using the SoilDx FertiSaver-N system.

Key takeaways
  • Measuring the organic N fraction of soil is a useful approach to determine soil N availability.
  • The Illinois Soil Nitrogen Test is a simple chemical test to measure the amino sugar N fraction in organic matter.
  • The results of the test need to be interpreted in a context dependent manner: It is not sufficient to look for simple thresholds in the ISNT value to make nitrogen rate predictions. Other factors such as soil type, depth, available water, and soil pH are critical to accurate recommendations
  • The ISNT has been evaluated independently by several groups and on several crops beyond corn in the Midwest.

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The SoilDx Dashboard is a Web-based tool that allows you to operate your SoilDx hardware. The FertiSaver-N hardware connects to a Google Chrome browser over USB. You open up a web browser and log in to the Dashboardwith your username and password.

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The nitrogen cycle, simplified
The nitrogen cycle, simplified

Q: Why are some soils more fertile than others?

Dr. Khan:

Most soils of the Midwestern USA are very fertile because they are moderate to high in organic matter, developed on deep loess deposits and have good nutrient and water supplying potential. These soils need less nutrient inputs from fertilizers, as opposed to those soils that are lower in organic matter with a limited rooting depth, and thus lower in overall nutrient supplying capacity.

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Q: How is nitrogen gas (N2) converted to plant-available nitrogen?

Dr. Khan:

The ultimate source of all nitrogen used by plants is N2 gas, which accounts for 78% of air. In order for plants to use it, N2 must first be converted to ammonium (NH4+) through biological or chemical fixation.

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