Understanding N-P-K

gore December 7, 2011 15
Understanding N-P-K

Understanding how and when to fertilize is an important part of growing marijuana.  Each bag or bottle of fertilizer is labeled with a set of numbers representing three of the macro-nutrients used by plants.  These numbers are always in the same order and correspond to the specific plant nutrients nitrogen, phosphorous, and potassium (N-P-K).  If you hadn’t guessed, the symbols N, P, and K are the same as used on the Periodic Table of Elements. An exception to this method of labeling is found in Australia where a fourth letter is assigned for Sulfur (S).

N-P-K value labeling is highly regulated by the country or state in which the fertilizer is intended to be sold.  The value listed on fertilizer packaging for N is calculated by the percentage by weight.  For example if the N-P-K reads 12-1-1, then the product is 12% nitrogen by weight.  The labeling methods used for phosphorous and potassium are more complicated and do not represent the raw elemental values.  Phosphorus is measured by its content of oxides while potassium is measured by its soluble compounds. This is important to understand because some fertilizers may contain much more of these nutrients than their manufacturers are allowed to print on the packaging.  It should be noted that some fertilizer companies circumvent regulations by printing soluble and insoluble nutrient percentages on their labels.

Nitrogen (N)

Most of what you breathe is nitrogen, an odorless, colorless gas that constitutes over 75% of Earth’s atmosphere.  Despite the overwhelming presence of atmospheric nitrogen, there are only a few kinds of plants in the legume family that can make use of it.  Marijuana is not a legume, so nitrogen must be supplemented. 

Synthetically-produced nitrogen fertilizer is made in the form of anhydrous ammonia from natural gas. Anhydrous ammonia is then usually converted into ammonium nitrate for storage and ease of application.  Once granulated, the ammonium nitrate is ready for use as a cost-effective and easy-to-use plant food.  Natural sources of nitrogen are numerous and include; blood meal, manure, bird and bat guano, among many others. 

Because of its crucial role in photosynthesis, fertilizers high in nitrogen are used during the ‘vegetative’ phase of growing marijuana. Nitrogen is key to both chlorophyll and protein production and used throughout photosynthesis for the conversion of carbon into sugars and other organic compounds.

  • Grow fertilizers are high in nitrogen
  • Used for rapid growth
  • Used to build chlorophyll and promote a healthy green color
  • Improves volume and health of plant foliage

Phosphorus (P)

Phosphorous is a volatile and reactive element, which is why it is probably never found in its pure elemental form in nature.  Because of this volatility, fertilizer packaging is actually showing a measure of its oxide (P2O5).  Phosphorous is commonly refined from stone called phosphotite or mined from ‘guano islands’ comprised primarily of bird and bat dropping. A common biomass source of phosphorus is found in ground bone (bonemeal).

Marijuana plants use phosphorus throughout their growing cycles but at elevated levels when flowering. As the plants’ nutrient requirements change, the feeding schedule is adjusted to include ‘bloom foods’ with higher levels of phosphorus and lower levels of nitrogen.  Because plants also use Phosphorus in root formation, low doses of it are often found in cloning solutions.

  • Bloom foods are higher in phosphorous
  • Improves bud production 
  • Aids root development
  • Involved in oil and glucose production

Potassium (K)

Potassium is an alkali metal that reacts violently with water and oxidizes quite rapidly when exposed to air.  This unstable nature requires that it be added to fertilizers in the form of potassium compounds known as potash.  Not all forms of potash are suitable for plants; K2SO4 , K2SO4, 2MgSO4, and K2Mg2(SO4)3 are the three that seem the best suited for a plant’s needs. It should be noted that potassium oxide (K2O) is never used for fertilizer as it is caustic and hygroscopic. Like phosphorus, potassium is mined from mineral-laden ore.

Like N and P, potassium (K) is a key component in the metabolic functions of plants and plays a large role in the production of proteins and photosynthesis.  Potassium also helps strengthen the marijuana plant’s immune system, which helps the plant cope with environmental stresses, including insects and diseases.  Bloom formulated fertilizers often include increased levels of potassium because it helps improve flower quality.

  • Found in both ‘Grow’ and ‘Bloom’ fertilizers
  • Helps plants immune system
  • Increases flower quality

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  1. miguel March 23, 2012 at 11:42 am -

    Im growing in a dwc system 6 plant 10 gal indiviual connected. 6 weeks in to flower 80% of tops are leaning over supported by trelis net. is this normal?or something i did wrong

  2. gore March 27, 2012 at 10:28 pm -

    This is a strange place to post this sort of question, Please take this question to our forum. I would love to engage in this discussion but this is not the place.

    link to indoor discussion;

  3. craigs1361 September 8, 2012 at 5:56 am -

    … whats the best combo of NPK to use in vegetative state …??? … and for the flowering state …??? … Does using a blooming fertilizer in the veg state of a plant hurt its production of foliage…???

    • Rambo September 16, 2012 at 3:57 pm -

      This is a really great question and I think there is probably more then one correct answer. In part it will depend on your soil or growing medium. For outdoor, I usually use a 30-10-10 as a base nutrient for veg and then add supplements as needed. Who knows what it ends up being. For flowering, you want to drop to something like a N level of 5-10 and up the PK for a 20 or so. I will depend on what you soil can handle without throwing off the PH to much. Post your question in the forum and lets see what the others have to say.

    • gore September 16, 2012 at 4:56 pm -

      Some how people have gotten the notion that there is a perfect N-P-K or ratio thereof that is ideal for marijuana, this is simply no the case. As Rambo stated in his comment, there are simple guidelines that can help generalize nutritional requirements.

      Furthermore, it should be noted that the values Rambo offered reflect typical N-P-K values one might expect to see in a synthetic fertilizer feeding program. Of course, if one where growing organically the numbers would be much lower.

      That said, different strains have different nutrient requirements and the only real way to find perfect feeding rations is to work with any given strain for several years of generations.

      In general terms, cannibals is an extremely heavy feeder and can survive, even excel in conditions that other plants may find toxic. It is not uncommon to see outdoor growers feeding at 200-300% recommended strength.

    • gore September 16, 2012 at 5:01 pm -

      Big Slugger

      Big slugger grows some of the largest marijuana plants we have ever seen. To find out what he’s feeding watch the video. One detail omitted however is how much he’s feeding. I believe, by the time he’s ready to harvest he will have administered 200-250lbs of fertilizer over 25 plants.

      Just thought this might help give an idea.

  4. jerry November 23, 2012 at 10:37 am -

    I applied npk fertilizers to my 3 month marijuana farm and the next day the plants started to wither and die.what could be the cause?

    • Rambo November 25, 2012 at 9:07 am -

      I’m guessing that you burnt the plants by using more fertilizer then they could handle. When trying new fertilizers, always start with half the recommended dose to make sure the plants don’t burn. Then, gradually increase the strength until just the very end of the leaf tips begin to burn, then back it off just a hair. Try flushing your soil with water only for a few days and see if they start to improve.

  5. Michael August 10, 2013 at 4:14 pm -

    I grow outdoors and have been taking a tip from the Japanese of using “night water” (pee) mixed with powdered non-fat milk as a fertilizer. I also add molasses to up the sugar content. So far so good.

  6. GoJohnny September 12, 2013 at 5:43 am -

    I have a two part fert with 3-0-0 and 1-2-6 meant to be used in conjunction with each other during Veg stage. Would it be possible to stop using the first part and only use the 1-2-6 for flowering and NOT do any significant harm to plants? Keep in mind the bottles do say to use IN CONJUNCTION with each other.

    • Rambo October 2, 2013 at 9:04 pm -

      When in doubt. Follow the directions

  7. BelowMe January 1, 2014 at 8:14 pm -

    I was wondering if anyone knew why most of the leaves die so quickly in the bloom? What would I use to cure it? Also I had a problem trying to register on this site. can i have a little help there too please?

  8. Jay May 24, 2014 at 3:11 pm -

    Hi I was wondering I’m using happy frogg soil and was gonna us grow more 30-10-10 what ph should I have it at ? Doesn’t the grow more have a lot salt built up if I use a lot can u email me back thank u

  9. Lewis June 23, 2014 at 2:08 am -

    Where can I get fertiliser from npk

  10. JAX July 2, 2014 at 9:35 am -

    Hello sir,
    I wanted to know if you could Please explain how 2MgSO4 has potassium. The best sources of potassium for hydroponics are potassium nitrate, mono potassium phosphate, and potassium hydroxide.

    Potassium sulfate really isn’t a great source of potassium as it will provide too much sulfate.