Cannabis nutrients Part Two: Why does marijuana need nitrogen (N)

by Alex Robles

What’s going on everyone, I hope everyone’s day was spectacular.  Today I want to talk about the macro nutrient nitrogen (N) and marijuana.  There’s a whole lot of information to go over so lets  get started.

The cannabis plant uses a lot of nitrogen (N) during its life cycle.  It is used to make the chlorophyll that give leaves their green color, which is an essential part of photosynthesis.  The marijuana plant will use this macro nutrient in large amounts during vegetative growth to help develop a strong and healthy system of roots, stems and branches that will support the dense, heavy flowers (buds).   During the flowering stage the cannabis plant needs less nitrogen because by then the only thing that it is needed for is photosynthesis and plant maintenance.  At this point the plant is using other nutrients to make buds.  Plants can’t pull nitrogen from the air and use it, it has to be changed into compounds like nitrate ions (NO3-) or ammonium ions (NH+4) by microbial life. 

Nitrogen is known as a mobile element and the easiest way that I can explain it is like this:

  Think of the leaves as a savings account for nutrients.  All the nutrients are deposited into the leaves but not all of the nutrients can be withdrawn if they’re needed in other parts of the plant when there is a deficiency in the soil.  The ones that can be withdrawn and moved around are known as mobile.

Healthy Levels

A plant with a healthy level of nitrogen will have an even green color and the leaves will be soft and almost moist.  Of course genetics will always play a part in how a plant looks and fells, but a healthy plant looks happy.  The leaves should only start to turn yellow during the last two weeks before harvest because that’s when I would typically stop feeding it nutrients and just give it water.  The yellowing happens because the plant is being starved and it has to use any nutrients that are left in the soil and in the fan/water leaves.

Nitrogen Deficiency

When a plant is nitrogen deficient, that means it needs nitrogen and is probably the most common deficiency that happens.  If that’s the problem, then the plant will start to take the nitrogen from the lower older leaves in order to feed the newer, upper leaves.  This is why the yellowing and wilting begins with the older leaves during a nitrogen deficiency.  The plant leaves will begin to show signs of chlorosis (yellowing of plant leaves caused by a nutrient deficiency) at the leaf tip and will move to the center.  The petioles (the stem that attaches the leaf blade to the stalk) will appear red and the leaf veins will look green at first but will eventually turn yellow also.  The yellow leaf will begin to turn brown and get soft before getting crisp, curling in and falling off.   The plant will also look pale and the yellowing will spread up the plant stalk.  The deficiency will also make the plant grow slower and the flowers will be smaller than average because photosynthesis slows down.  The plant will also be too weak to fight off pests and disease if it doesn’t get enough nitrogen.  If it is left untreated the plant will quickly start to turn completely yellow and lose its leaves.

These are some of things that could cause and fix a nitrogen deficiency:

  • The most obvious one is an actual lack of nitrogen. To solve this problem I add a nitrogen-enriched fertilizer to the nutrient mix as soon as possible so the plants will have plenty of nutrients while it rehabilitates.  Beneficial microbes will also help.  Fish Emulsion, fish meal and worm castings are great organic nitrogen-enriched fertilizers.  They are readily available at most hardware/garden stores/nurseries.  The bonus you get with fish fertilizers is that they contain micro-nutrients that help prevent other nutrient deficiencies.   When I add liquid fertilizers or bottled nutrients to the watering cycle, I always feed them half the amount that is recommended on the bottle because I don’t want to over feed and burn the cannabis plants.
  • A PH value higher than 7.0 in the soil/coco coir could cause a deficiency. At these levels the plant struggles to absorb certain nutrients through its roots.  You can fix this by making sure your water and nutrient mix has a PH value between 5.5 and 6.4.
  • Nutrient Lockout will also cause a deficiency.  It happens when there are high levels of other nutrients in the soil that block other nutrients from being absorbed.  One remedy for lockout is to flush the plant to brake up the nutrient buildup that is on the roots and in the soil.

Growers Note:   The leaves will start to yellow during the last two weeks before harvest.  Also, if the yellowing of the leaves are at the top of your plant or the yellow leaves are in areas of new growth, then it probably isn’t a nitrogen deficiency.

Nitrogen Toxicity

Nitrogen toxicity happens when a cannabis plant has too much nitrogen.  The plant will have a dark green color because it uses nitrogen to make chlorophyll.  If the plant has more nitrogen than it can use, the only thing it knows what to do with the extra N is make more green color.  The leaves will also begin to cup and claw downward.   If left untreated the leaves will turn yellow and die, just like in a deficiency.  The difference is that during toxicity the leaves will yellow but the stems will stay dark green.  Too much nitrogen is also going to get in the way of proper bud growth during flowering.

For me the toxicity would usually show up during flowering because I wasn’t paying attention to my nutrient levels.  To solve this problem I would have to flush the plants soil.   When I flush a plant I over water it until I see water coming out of bottom of the container.  I like to see the amount of runoff equal to size of the container.   If I’m flushing a 3 gallon pot I want to see about 3 gals of runoff.   I’ll wait until the soil is dry before I feed it, usually about 4-5 days depending on the size of the container. For the first feeding after the flush I would give it a light feeding, about a 1/8 to a ¼ of the usual amount of nutrient mixture with the usual amount of water.  If you’re running a deep water culture system (DWC), I suggestion you throw out the reservoir water and replace with fresh water with a light feeding mix: use the same ratios as the suggestions for soil for 24hrs.  After that replace that reservoir with a fresh water and a balanced mixture of nutrients.

Well ladies and gentlemen as Porky Pig once said ” that’s all I have to say about nitrogen”.  Go forth and enjoy your garden.

Grow Learn Teach

Fish Emulsion

Earthworm Castings

pH test kit

3 thoughts on “Cannabis nutrients Part Two: Why does marijuana need nitrogen (N)

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