Four Steps to Manage Nitrogen (Organically) for Cannabis
Plants can uptake various sources of nitrogen…
Nitrate
Ammonium
Organic Nitrogen Compounds (mostly amino acids)
In conventional (“salt-based”) cannabis production, most growers use exclusively nitrate and ammonium. In these systems, plants prefer 70-90% nitrate and 10-30% ammonium. Growers have the ability to hold nitrogen concentrations constant through an entire round using soluble fertilizers like calcium nitrate or potassium nitrate.
Nitrogen levels may look like this…
Or this…
This is great when it comes to control. BUT there are always trade-off’s.
There is increasing evidence that organic nitrogen can play a major role in improved secondary metabolism (ex: terpenes), abiotic stress resistance (ex: disease), and nitrogen use efficiency. This is all due to an improved C:N in the soil and plant, and synergistic metabolic effects of multiple nitrogen forms. (One way for salt-based growers to capture these benefits is to foliar feed amino nitrogen in Veg!)
At any given time in organic cannabis production, the plant has access to several “pools” of nitrogen in the soil. These pools are transforming all the time as microbes convert organic nitrogen from proteins to amino acids to ammonium to nitrate.
So in organic systems, nitrogen levels may look more like this…
Luckily, in organic production it’s not necessary to hold a constant concentration of nitrogen. Having enough nitrogen at the right time (and ideally in diverse forms) for metabolic functions is all that matters.
In organic production, we simply apply various organic forms of nitrogen (ex: amino acids, protein meals, compost) and the microbes convert it into various “pools” available for the plant. Your plants will be consuming amino acids, ammonium, and nitrate in any living soil system.
But here’s a confusing twist. Even though the organic forms of nitrogen are best for plant health and nitrogen use efficiency, one of the best way to manage nitrogen is still to focus on the levels of ammonium and nitrate on a soil test. Why?
There is no good way to test organic forms of nitrogen. So we’re stuck with ammonium and nitrate when soil testing.
Ammonium and nitrate give a sense of whether organic nitrogen is sufficient, as organic nitrogen is the predecessor to both of these forms.
Therefore, the goal should be to maintain nitrate and ammonium in an acceptable range using soil testing, and follow a few simple steps to optimize from there.
Step 1: Amend your soil with nitrogen amendments and compost before planting
Test your soil, plug the results into the Soil Dr. Tool, and apply the organic nitrogen amendments and compost recommended in the prescription.
After the soil sits (a.k.a. “cooks”) for a few weeks, much of the nitrogen in those amendments will be converted by microbes into ammonium, and then nitrate. Having healthy levels of nitrate in your soil ensures your plants won’t get hungry, and also indicates that you have enough organic nitrogen mineralizing.
The point of this step is to build nitrate levels up to about 100-125ppm before transplanting, ensuring your plants have enough available nitrogen—but not too much—in the reservoir of soil to grow quickly for weeks in any pot or bed size.
Note: with the right nutrition and environmental conditions, most strains do fine in Veg up to 200ppm, and deficiencies usually don’t appear until below about 30ppm. 100-125ppm is a healthy quantity that still leaves room for additional N feeding (explained below).
The remaining organic amendments and compost that haven’t already broken down will continue to release amino acids, ammonium, and nitrate through the rest of the round.
We suggest diversifying your nitrogen amendments. The Soil Dr. default is to use three nitrogen sources (alfalfa meal, blood meal, and granular poultry) that theoretically optimize nitrogen release rate, microbial diversity, and low heavy metals. Each contributes 33% of the necessary pre-plant nitrogen, so growers can always replace any one of these products by doubling another (ex: skip granular poultry and double soybean meal). Depending on phosphorus requirements and pH, growers may also see Crustacean Meal or Bone Meal on their Soil Dr. recommendation. These provide a small amount of nitrogen, which is accounted for in the recommendation.
The Tool also adjusts the quantities based on whether you plan to apply compost, and what kind of compost is used. For example, a composted chicken manure will provide much more nitrogen than a composted yard waste.
Organic Nitrogen Amendment Options:
Alfalfa Meal
Soybean Meal
Granulated Poultry Manure
Blood Meal
Fish Meal
Insect Frass
Neem Meal
Karanja Meal
Feather Meal
Crab/Crustacean Meal
Fish Bone Meal
Slow-release fungal biomass
Step 2: Use tissue testing and visual identification in Veg and early Flower
After planting, use tissue testing and plant visual symptoms to determine if and when your plant needs more nitrogen.
Tissue testing at least once in Veg and again at week two of Flower is suggested. 4-5.5% Nitrogen in the tissue before week two of flower is sufficient.
As the plants flower, they will move nitrogen from their leaves into their flowers. If you tissue test every week through Flower, you’ll see your nitrogen levels drop. This is why we don’t believe tissue nitrogen is accurate after week two of Flower.
The other low-tech way to determine if your plants need more nitrogen is visually! Nitrogen is a highly visual nutrient, and your plants will tell you they need more as they yellow. This will almost always align with tissue results and soil nitrogen levels, so calibrate your eye as a grower.
Extra Credit: Sap Testing
One of the only advantages to sap testing over tissue testing is you get to see all the forms of nitrogen in the plant, not just total nitrogen. This allows you to zoom in on metabolic bottlenecks in the nitrogen fixation pathways.
Once ammonium and nitrate is taken up by the plant, it should be converted into proteins very quickly. Therefore, you should always have low ammonium and low nitrate in the plant.
High ammonium? Double check your magnesium and manganese.
High nitrate? Double check your molybdenum, sulfur, and iron.
(Phosphorus, sulfur, magnesium, manganese, boron, potassium, and zinc are all needed to get aminos into complete proteins!)
Don’t sap test until you’ve followed all the other steps first! Only sap test if you’ve captured the low hanging fruit and are hungry for more. Also, don’t sap test until you read my “Guide to Sap Testing”.
Step 3: Feed
When you determine your plants need more nitrogen, or predict they will need more, feed!
Feeding gives you control. It’s like having your hand on the throttle.
Here are all the variables that may influence how much you need to feed, how often, and how soon after amending your soil.
Round Length
Soil volume
Strain
Seed/clone
Indoor/outdoor
Compost Additions
Compost Quality
Size of Transplant
Light Intensity
Temperature
Cannabis plants are very responsive to the human touch and respond very well to continual feeding.
We believe feeding creates a more consistent nitrogen supply. Feeding the right things also maintains diversity of organic and inorganic forms of nitrogen. Again, diversity of nitrogen form is metabolically superior!
Feeding Options:
Topdressing Amendments. Any of the nitrogen amendments can be top dressed. This is the slowest approach, but helps build slow-releasing, carbon-based nitrogen sources in a living soil.
Liquid Fish Hydrolysate. This is the most nutrient-diverse form of liquid nitrogen. It’s packed with organic nitrogen forms, ammonium, phosphorus, and micronutrients.
Soluble Amino Acids. This is the most targeted form of liquid nitrogen, produced from soybeans or fish.
Teas. Any teas that have been made with nitrogen amendments will be high in nitrogen. If done correctly, this is one way to convert organic forms of nitrogen into nitrate.
Foliars. Amino acids are the more efficient source of foliar nitrogen.
Step 4: Adjust
At the end of the round, decide if you applied too little or too much nitrogen.
Did your plants fade too early? Were they too green?
Did you hit your yield goals?
Were the terpenes good?
Now make adjustments!
Either increase/reduce your pre-plant amendments, or increase/reduce your feeding regimen.
Repeat.