A Guide to Choosing Organic Nitrogen Amendments
All organic nitrogen amendments work. Because they’re carbon-based, they all feed soil biology and release available nitrogen to your plants in the process. Don’t over-think it.
The major differences are in the cost, release rate of nitrogen, and other beneficial nutrients provided.
Two Quick Notes:
If you’re wondering about using liquids like ferments, amino acids, or teas for nitrogen, jump to the bottom of this post. ⬇️
Compost and worm castings are excellent nitrogen sources—possibly the best—if they're high quality and your soil needs the other nutrients they provide. For example, if your soil is already high in potassium and your compost is super high in potassium, this is NOT the best option. The Soil Dr. Tool will recommend compost as often as possible when it’s a good fit.
Considerations
Use What You Have
First and foremost, what do you already have? Half a bag of feather meal? A few bags of alfalfa meal? Use those first! That’s the most sustainable and cost effective option.
Cost Per Unit of Nitrogen
Here is the approximate price per pound of nitrogen provided by each amendment:
Granular Poultry - $9.50
Feather Meal - $9.79
Blood Meal - $13.65
Fish Meal - $15.60
Soybean Meal - $16.57
Alfalfa Meal - $22.32
Fish Bone Meal - $22.50
Crustacean Meal - $23.75
Bone Meal - $28.67
Neem Meal - $35.35
Karanja Meal - $39.77
Release Rate of Nitrogen
Each amendment will release plant available nitrogen at a different rate. Assuming your soil moisture is good and the temperatures are warm enough, biology will mineralize the nitrogen amendments are convert these products into various forms of available nitrogen—amino acids, ammonium, and nitrate.
The release rate depends on the particle size of the amendment and the C:N of the amendment.
Fine particles with a low C:N release FAST. Think blood meal or feather meal or fish bone flour. Blood meal and feather meal have a high nitrogen content (12% and 13% respectively) and are small particles. They release quickly, which is why you can often smell the ammonium off-gassing if you topdress them.
Larger particles with a high C:N release SLOWER. Think alfalfa meal, neem meal, or insect frass.
Use release rate to your advantage…
➡️Acute deficiency? Or need to significantly raise N levels? Go with blood meal, feather meal, or fish meal.
➡️Just trying to maintain a steady release rate of N? Diversify your sources and get the majority of nitrogen through slow-releasing products like alfalfa meal.
Secondary Benefits
None of these amendments are JUST nitrogen. They are organic and contain every nutrient in differing quantities. Nitrogen amendments will supply other nutrients and biostimulating compounds to your soil. Keep that in mind and follow these tips…
If you’re trying to build potassium in your soil use alfalfa meal or granular poultry manure. The potassium content plus the large quantity applied will start to add up over time.
If you want to build phosphorus levels over time, nothing beats granular poultry manure.
If you want the biostimulant triacontanol, use alfalfa meal.
If you want the biostimulant chitinase, use insect frass or crustacean meal.
If sodium or chloride is already high in your soil avoid ocean-based products like fish meal or fish bone meal.
Flavor/Aroma Differences
A handful of cannabis growers have reported “off flavors” in their flowers resembling ammonium or metallic flavors. I attribute this in part to low C:N products such as blood meal, which create a flush on ammonium into the soil and ammonia into the surrounding air. Especially in an indoor environment, this can find it’s way into the expression of the flowers. This problem is made worse when amendments are thoroughly tilled into the soil. This difference is more art than soil and is really only a consideration for high-value crops where flavor and aroma are everything.
Ferments, Amino Acids, and Teas
The Soil Dr. Tool and this review of dry nitrogen sources is focused on DRY PRE-PLANT amendments. This is the simplest and least expensive way to amend your soil.
However, many growers prefer to feed nitrogen through the season in the form of liquid products drenched or fertigated. This is a great approach to improve the precision and timing of feeding. It also allows you to use various DIY products like ferments and teas that have various biological and biostimulating benefits.
I believe you should do both! Amend your soil before planting to get a baseline amount of nitrogen then feed homemade biologically-rich liquid products through the season as needed. A soil test isn’t going to tell you how much to feed through the season. It’s most valuable for determining pre-plant amendments.