In a PK Panic? A Breakdown of Potassium and Phosphorus

By Rich Hamilton
Published: August 1, 2016 | Last updated: May 4, 2021 06:00:53
Key Takeaways

What is PK? What does it do? How does it work? When is it used, and for how long? Why are there different PK ratios? Bloom boost PKs? What’s all that about? Help! OK, slow down. Don’t panic. PK simply stands for potassium and phosphorus, and Rich Hamilton calmly breaks down when to use it and how it works.

Source: Nabokv/


P is the chemical symbol for phosphorus. Phosphorus is a part of the vital nutrients needed by all plants and is classed as a macronutrient. Sometimes macronutrients are referred to as the primary nutrients. The key role of phosphorus in this form is to aid energy transfer in the flowering stage and strengthen cell formation. As a result, it improves quality and increases flower size, weight and potency.



K is the chemical symbol for potassium. Potassium, like phosphorus, is one of the vital nutrients that is used by all plants and is classed as a macronutrient. Nutrients for plants are split into four different classifications: macronutrients, secondary nutrients, micronutrients and non-fertilizer elements.

Differing from some of the other plant nutrients, potassium doesn’t constitute in any plant structures as its role is more of regulation. It helps with the regulation of nearly all of the plants growth phases including the plant’s breathing, rate of photosynthesis, the activation of enzymes and many other functions. Basically, if your plant has deficiencies in potassium it will never reach its full potential. It’s vital to use potassium in the right quantities at the right times to get the most out of your plant.


What Do PK Boosters Do?

The simple and uncomplicated answer is that giving the plant a dose of PK will simulate a natural chemical change within it. This chemical change tells the plant that all the energy being used to grow the plant should be directed to the ends of the stems, thus encouraging growth in those areas, whether it be flowers or fruit. By chemically leap-frogging this stage earlier than it would naturally happen means that the flowers/fruit have a longer time to develop, and with a supercharged push.

The plant would normally do this naturally by building up the PK to a certain level whereby the chemical change can take place and the plant can go into its next stage of its life. By feeding the plant directly with the correct levels of potassium and phosphorus it chemically induces this process and makes PK readily available to the plant to help accelerate the flowering process.

Off-the-shelf PK derivatives are normally found in four groups when it comes to the indoor gardening market:

  • Straight PK (liquid and concentrate)
  • Super PK (liquid and concentrate)
  • PK booster/all in one (liquid and concentrate)
  • Powder PK (soluble concentrate)

Straight PKs

Straight PK is the most commonly known and used PK application on the market. Its ratios are PK 13/14 and as a result it’s communally know in the market as such. Most nutrient brands have a PK 13/14 —13% potassium, 14% phosphorus.

Straight PK or PK 13/14 works best when it is used as a small blast in the standard feeding schedule. In order to get the most out of it you will have to use it alongside a flowering booster. Some growers use PK 13/14 for a week only, some growers use it in just one feed, usually at week four or five of flower. It is a common misconception that you should use PK 13/14 throughout the entire flowering period. The straight PKs are normally at the lower end of the price range and are suitable for any medium and growing method.


Super PKs

Super PKs are becoming more and more common. These products come in a liquid form ready to dilute and have a much higher ratio of PK, some as high as 18% and 20% respectively. The idea of super PK is simple. Where standard PK 13/14 gives the plants the chemical inducement to make both potassium and phosphorus readily available to the plant, super PKs and their ratios deliver the maximum amount of PK the plant can absorb, maximizing the effects and the flower production. Does it work? Yes, it does! The big caveat, however, is that I find it works better if you use a little less than the suggested dosage ratios. Super PKs are high concentrates, and it is all too easy to accidentally overdose your plants. Just like straight PK 13/14, to get the most out of super PK you should use it alongside a flowering booster/additive.

Super PK also differs from the straight PK 13/14s in its application. Some of the super PK companies suggest to start implementing them from week two to four in flower all the way through until the plants are to be flushed. Like PK 13/14, most super PKs are suitable for any medium type and growing method.

PK Boosters

PK boosters/all-in-ones are a mix of a PK and plant/flowering booster/accelerators that tend not to advertise their PK ratios as much as the other variations. Sometimes they are referred to as ‘miracle bottles’ as they have pretty much everything you could need in one bottle.

It’s no surprise that these are normally at the higher, if not the top, of the price range with two, three or sometimes four products in one bottle. They can be cost-effective, though. When all the math is done, some of them actually work out to be less expensive than buying all the different products separately. PK boosters/all-in-ones are normally used throughout the entire flowering period at a lower rate of dilution.

They do work, and they work well, however, I always like to additionally use a straight PK 13/14 on top of the PK booster. I would normally add the PK 13/14 in at the end of week four of flower depending on the plants maturity. If you consider this, please proceed with caution because as I mentioned earlier, it is very easy to overfeed your plants. All-in-ones/boosters are suitable for any medium type and growing method.

Powder PK

Powder/granular PK is a dry powder mix of potassium, phosphorus and other chemicals that you mix in directly with your feed solution by weight (i.e., one gram for every 10 liters). Unlike the other PKs, the powder form is normally administered at the start of the flowering process for the first two weeks. The belief is that using PK like this shortens the flowering cycle and increases yields.

There are PK flowering enhancers that can also be used in the last few weeks of the flowering cycle to boost crops. While these are technically PK powders, they contain so many other chemicals they are more machine than man, so to speak, and are known more as a flower booster/enhancer than a PK.

Using powders can be tricky as it is very easy to overfeed your plants. If you are going to have a go at using these, my advice is to take your time and follow the dosage rates on the packaging, ideally reducing them to begin with. Most powders are suitable for any medium type and growing method.

So What Types of PK Supplements Work Best in the Garden?

It all depends on what feeds you are using and what you want your plants to do and when. Here are the effects of PK in a nutshell:

  • PK used early tends to increase the amount of flower sites.
  • PK used at the mid to later stages tends to concentrate on increasing what’s already there.
  • PK used at the midpoint gives you the best of both worlds (but by definition also the best of none).

There is no clear ‘this is the best way’, although everyone will tell you their way is the best. With so many different types of plants, people and methods of growing, it’s whatever works best for you. If you’re a PK straight shooter, adapt your feed range and try a PK booster all in one. If you’re a super PKer, try a straight PK and so on. Sometimes the grass is actually greener on the other side. If you don’t try, you won’t know.


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Written by Rich Hamilton | Writer, Consultant, Author of The Growers Guide

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Rich Hamilton has been in the hydroponics industry for more than 20 years, working originally as a general manager in a hydroponics retail outlet before becoming an account manager at Century Growsystems. He enjoys working on a daily basis with shop owners, manufacturers, distributors, and end users to develop premium products.

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