Nitrogen: why and when do cannabis plants need it?

Cannabis plants need nitrogen through their entire life cycle, but they need the most during the vegetative stage, when they're building stems, branches and leaves; in flower, demand drops but doesn't disappear. A healthy cannabis plant carries around 3–4% nitrogen in its above-ground plant matter, which is why nitrogen counts as a "macronutrient," one of the big three alongside phosphorus and potassium.

Cannabis uses nitrogen in lots of ways, especially to build stems, branches and leaves. Once flowering kicks in, the plant stops putting out new leaves and branches and focuses on growing buds, relying mainly on phosphorus and potassium.

Nitrogen is also needed to make chlorophyll. Without nitrogen, plants can't produce the food they need or get the building blocks they need to grow. So even though most of it gets used during veg, plants still need a decent amount during flowering.

When you shop for nutrients, you'll usually see a "grow" formula and a "bloom" formula. Grow (or veg) nutrients are higher in nitrogen than bloom nutrients.

Nitrogen deficiency is one of the most common problems you'll see in a cannabis plant. The plant uses so much of it that the nitrogen available in the growing medium (soil, water) can run out fast.

Nitrogen is a "mobile" nutrient. That means if the plant needs nitrogen in one specific spot, it can move it from one leaf to another.

A cannabis plant with a nitrogen deficiency shows the first signs on its lower leaves. When part of the plant needs nitrogen, the plant pulls it from somewhere that has more. So if the older leaves still have nitrogen and the new growth isn't getting enough, the plant moves nitrogen from the old leaves to the new ones, and the leaves giving it up turn yellow.

Since the plant also uses nitrogen to make chlorophyll, its leaves lose their green color: with less nitrogen, less chlorophyll gets made and the color fades. Without enough chlorophyll, the plant can't photosynthesize and will eventually start to wilt.

Whenever you're treating a deficiency in a cannabis plant, the first thing to do is check the pH of the growing medium. For soil grows, aim for a pH of 6.5; for hydro and coco, 5.8. If the pH is in range, the problem is probably a real lack of nitrogen. To fix a nitrogen deficiency, add more nitrogen: check how much you've been feeding, compare it with the dose on the product label and adjust.

How do you get rid of excess nitrogen in cannabis plants?

If you've noticed your weed plants are getting too much nitrogen, the first step is to cut back on the fertilizer you're feeding them. Once you've done that, water with clean, pH-balanced water to flush the roots and wash the extra nitrogen out of the soil.

Another option is to switch to a fertilizer that's low in nitrogen and higher in phosphorus and potassium. You can also work carbon-rich organic matter into the soil, like mature compost, to help soak up the excess nitrogen.

Keep in mind that once leaves are damaged, they won't go back to their original color. If excess nitrogen has already hurt some leaves, remove them so the plant can put its energy into growing new, healthy ones.

What happens when cannabis plants get too much nitrogen?

Too much nitrogen can have several negative effects on a cannabis plant's growth and health. First, it can cause excessive leafy growth at the expense of flowers. The plants may get big and bushy, but they won't produce high-quality buds.

Excess nitrogen can also make plants more vulnerable to diseases and pests, since soft, lush green leaves are more attractive to insects and fungi. It can also interfere with the plant's ability to take up other important nutrients, which can lead to other deficiencies.

Another telltale sign of excess nitrogen is leaves turning a deep, glossy dark green, often called "nitrogen toxicity" or nitrogen burn. The leaves may also look shiny and feel almost waxy to the touch, and the tips often claw downward.

How do you fix yellow leaves on a cannabis plant?

Yellow leaves on a cannabis plant can come from a bunch of problems, but one of the most common causes is a nutrient deficiency, like a lack of nitrogen. It can also be a sign of too many nutrients, especially if the rest of the leaves look dark green and glossy.

If the cause is a deficiency, the fix is to add the missing nutrients. If it's an excess, cut back on how much you're feeding.

It's also essential to check the pH of your growing medium (soil, water and so on), since the wrong pH can make it hard for the plant to absorb essential nutrients. The ideal pH is around 6.5 for soil and around 5.8 for hydro and coco.

Finally, make sure your plants are getting the right amount of light, water and airflow. Overwatering or underwatering can also cause yellow leaves.

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