Cannabis nutrients Part three: Phosphorus and Marijuana.

by Alex Robles

Well everyone today was another beautiful day in the garden.  Fall is on its way or has already arrived in some places.  Cooler temperatures is a good time to talk about phosphorus and why marijuana needs it.

Phosphorus (P) is primary nutrient that is used in large amounts during the cannabis plants life cycle and is known as a mobile element.  It’s needed in photosynthesis because it helps the plant change the energy it gets from the sun into sugars.  These sugars will make the different chemical compounds used in root, leaf and stem growth, but it’s a vital part of dense bud development during flowering.    Phosphorus will also help the plant cope with the stress of drought, frost, pests, and disease easier


The symptoms of a Phosphorus (P) deficiency will start with the lower, older leaves changing color to a dark green, or yellow.  They will begin to develop splotches that are brown, bronze, or purplish in color and look like dead spots. The leaves can also look shiny and feel thick, stiff, rough and will start to curl downward.   The petioles (the stalk that attaches the leaf blade to the stem) as well as the plant’s stems begin turn purple or red.   This change in color will start at the bottom of the plant and work its way up. The plants growth will also slow down and the newer leaves will be smaller than usual and stay small longer.  This happens because phosphorus helps to promote healthy foliage (leaf, stem) growth.   When a phosphorus deficiency is left untreated eventually chlorosis (yellowing of plant leaves caused by a nutrient deficiency) sets in and the leaves will die and fall off.  If too many leaves fall off, your plant will DIE a premature death because they need the leaves for vital functions.

Growers Note:  Some strains will have red or purple stem/petioles as part of their genetics, so it’s a good idea to know what’s normal for that strain.  Make sure other symptoms are noticeable before you call it a phosphorus deficiency on stem color alone.

Warning:  Since phosphorus is essential for photosynthesis, the rate at which it is used up is affected by the intensity of the light.  When there is too much light in an indoor grow the plant will use up phosphorus faster and the symptom of a red, yellow or purple leaf will show up on the part of the plant that gets direct light.  There is such a thing as to much light.

To reverse this deficiency, start by checking your pH.  A safe range across mediums is 5.5 to 6.5 pH.  Anything higher or lower than that and the plant will struggle to take up phosphorus.

Over watering or a constantly wet soil could also cause the plant to take up nutrients at a slower rate.  If the soil becomes compacted it can stunt root growth and cause a weak root system, again the plant will struggle to take up nutrients.  This problem can be solved by making a potting soil mixture that is 25% to 30% perlite/vermiculite to help with drainage and aeration.  Then transplant plant into new mixture.  Remember that transplanting will send your plant into shock, so try the less traumatic options first. 

When temperature gets to be lower than 60 degrees Fahrenheit it could cause the plant to absorb phosphorus at a slower rate.

Nutrient Lockout can also cause a deficiency.  It is caused by high levels of other nutrients in the grow media that block other nutrients from being absorbed.  One remedy for lockout is to flush your plant to break up the nutrient buildup that is on the roots and in the soil.  When you flush the plant, you want to see a lot of runoff.  I like to see the amount of runoff equal to size of the container.   If I’m flushing a 3gal pot, I want to see about 3gal of runoff.   After that 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.  I’m not going to get into ppm.  I want the nutrient so the plant doesn’t starve as it rehabilitates.  When it’s ready (dry enough) for its next feeding, I make sure it is a balanced nutrient mix.

I find that a good source of phosphorus is fish fertilizer.  Bat guano, blood or bone meal, and worm castings are also a good source.  The last three will affect the plant quicker if they are made into a tea.  Most bottle “bloom” nutrients are formulated with high levels of phosphorus to help plants in the flowering cycle.  It’s a good idea to choose one that is formulated with cannabis in mind.

And that my fellow cultivators is all I have to say about phosphorus.  Enjoy your day and remember.

Grow, Learn and Teach.

Click here for pH test kit

Click here for Earthworm Castings

Click here for Digital pH meter

Click here for Fish fertilizer

2 thoughts on “Cannabis nutrients Part three: Phosphorus and Marijuana.

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