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Include last year's nitrogen credits in your fertility plan this year

As you figure out how much fertility to add to your fields this year, make sure you're including nitrogen that is left over from previous cover crops and compost / manure applications. You may need less nitrogen than what is recommended on your soil test.

What is a nitrogen credit?

There are two basic categories of fertilizers: inorganic and organic. Organic in this case does not mean USDA certified organic, but rather any fertility source that is bound in carbon. Organic sources of fertility include things like manure, composted manure, compost, cover crops, fish meal, etc. 

When nitrogen is bound in an organic form, it takes a while to mineralize, or become usable by plants. In some cases, it can take 1-2 years. Farmers should take this into account when making fertility plans to avoid over-fertilizing.

Calculating nitrogen credits from cover crops

If you planted a cover crop in the fall, you are likely going to receive some nitrogen from it this spring. Legume cover crops can provide upwards of 100 pounds of nitrogen per acre; even nonlegumes provide a nitrogen credit because they help to hold nitrogen in the soil and prevent it from leaching. 
The following video provides a step-by-step overview of how to calculate a credit from your cover crop: 

If you are trying to calculate a credit from last fall's cover crop, but you didn't measure the biomass, do your best to estimate. The SARE guide to managing cover crops profitably is free to download, and has a great table on page 67 highlighting average biomass you can expect from a given crop, and an estimated nitrogen credit. If you had a dense stand, lean towards the higher end of the biomass estimate. If it was patchy or terminated early, lean towards the lower end. Keep in mind that these are national estimates, not specific to Minnesota, so it is best to actually measure your biomass and do the calculation highlighted above if you are able to.

Calculating nitrogen credits from compost and manure applications

Nitrogen in  compost and manure can take a long time to become available to your crops. The following guidelines are taken directly from the Nutrient Management for Fruit and Vegetable Crop Production: Maintaining Soil Fertility in an Organic System.
  • Anywhere from 5 to 90 percent of the organic N can be released to usable forms the first year.
  • For dairy manure, 40 to 60 percent of the N is generally considered to be available the first year, and 30 to 40 percent of the N is available the second and third years. 
  • For poultry manure 50 to 75 percent of the N is available the first year and 20 to 25 percent is available the second and third years. 
  • For composted dairy manure, 5 to 20 percent of the N is available the first year.

Dealing with uncertainty

The release of nitrogen from carbon-based sources is hard to predict perfectly; notice the ranges of availability in the list above. As such, it can be hard to develop a perfect nutrient management plan when using organic sources of fertilizer. 
Foliar testing can give you a snapshot of the nutrients that your plants have taken up during the growing season. It's a great way to spot deficiencies before they translate into plant stress symptoms, and to adjust your fertility mid-season. Read more about foliar testing here.
Author: Natalie Hoidal, UMN Extension educator for local foods and vegetable crops

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