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. Here's a rough guide to calculating nutrients. While manure can be highly variable, we have estimates for an average amount of nitrogen, phosphorus, and potassium available from different types of manure, reported in pounds of nutrient per ton (English ton - so 2000 lbs) of manure. These estimates are for solid manure, not liquid, see the webpage linked above for more info on liquid manures.
- Poultry manure: 52 pounds of nitrogen, 47 pounds P2O5, and 38 pounds K2O per ton of manure.
- Beef manure: 18 pounds of nitrogen, 12 pounds P2O5, and 16 pounds K2O per ton of manure.
- Dairy manure: 14 pounds of nitrogen, 7 pounds P2O5, and 16 pounds K2O per ton of manure.
In the first year that it's applied, availability really depends on how quickly it gets incorporated into the soil. For example, if you broadcast dairy manure and then incorporate it within 12 hours, you'll get about 60% of that nitrogen. If you wait more than 96 hours to incorporate it, you'll only get 25%. Regardless of how long you waited in year 1, you'll get about 25% of the nitrogen from beef, dairy, or poultry manure in year 2.
1 cubic yard of beef manure is somewhere around 660 lbs. So, let's say you spread around 4 cubic yards per acre last year. That's about 1.2 tons. 18 pounds of N per ton is 21.6 pounds of nitrogen applied to your field total. 25% of that will be available this year, so you'll have just over 5 pounds extra nitrogen, which you can subtract from your total. This isn't a huge amount, but if you're spreading large quantities of manure it can add up quickly.
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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