We’ve all seen it on our nutrient bottles or on the ads for different nutrient additives. Most of these companies claim that their ratio is the best for maximizing yields and speeding plant growth. There are even different ones depending on the stage of life your plants are in. This can sure make choosing the right nutrient formula confusing.

NPK Ratio

NPK stands for Nitrogen, Phosphorous, and Potassium.

These are a plant's macronutrients, which means they are the nutrients a plant uses in the largest amounts. This is aside from carbon of course which is extracted from the carbon dioxide in the air during photosynthesis.


NPK is Simply a Ratio

The NPK ratios are not an indication of the amount of any given nutrient, but rather a relationship between them. For example, an NPK ratio of 10:5:8 would indicate that for every 10 molecules of nitrogen (the first one) there are 5 molecules of phosphorous, and 8 potassium. This shows an emphasis on the plants need for nitrogen in this case and offers less emphasis on a need for phosphorous. Potassium falls in the middle of the 2 in this formula.


What Nutrients are For Different Growth Stages

Plants use nitrogen mainly for its structure and therefore is needed heavily in the vegetative or “general growing stage” of its life. Phosphorous is used heavily for the roots and during the development of flowers and buds and therefore is often used in rooting or flowering formulas in higher amounts. Potassium is needed for general maintenance of all of the various plant processes and helps with the plant's immune system and general structural strengthening. This means that it is required throughout the plant's life in fairly regular amounts.


Why NPK Ratios Matter

Now that you understand what NPK means, and why it really is not as confusing as it is made out to be, let's look at the reasons behind modifying this ratio. There are 5 main categories that can be considered for creating a nutrient formula, they are:

  1. Rooting formulas

  2. Vegetative formulas

  3. Flowering formulas

  4. All purpose

  5. Plant specific


1. Rooting Formulas

When a plant is developing roots, the main macronutrient needed is potassium, as well as phosphorous. This means that a good rooting nutrient will contain higher amounts of potassium, and may or may not have higher amounts of phosphorous as well. This delivers the nutrition needed most during this stage of plant development.

The main addition to rooting blends is not actually the macronutrients (NPK), but generally, includes micronutrients and hormones that tell the plant to send out more roots. This has nothing to do with the NPK ratio but is important to remember when trying to stimulate root development.


2. Vegetative Formulas

As a plant develops and grows throughout the vegetative stage, it tends to use large amounts of nitrogen and potassium as it creates more leaves and structures. This is why most vegetative nutrients have a higher level of nitrogen and potassium than phosphorus.


3. Flowering/Budding Formulas

These formulas tend to have higher amounts of phosphorous than anything else because as a plant flowers it requires a lot of this macronutrient to drive flower production. Potassium is still needed during this stage, and many formulas include higher concentrations of this macronutrient as well. This can vary greatly, however.


4. All purpose

An all-purpose nutrient formula generally follows a 20:20:20 or 10:10:10 ratio. The idea is that all nutrients are in equal portions, so it can be used all year round to make the growing process easier. This is great for houseplants or for those looking for the simplest nutrient regimen possible. It will not maximize your yields or growth rate, however because it is not going to offer the most efficient nutrients needed by your plants.

Imagine your plants are in the vegetative stage of growth and as such do not need as much phosphorus as nitrogen. The all-purpose formula given will include all macronutrients in the same supply. The plant will use all of the nitrogen up and leave the remaining phosphorous behind. In many cases, this is not a problem, but when you are trying to maximize growth and yields, it will be most beneficial to ensure the plant always has access to this extra nitrogen it might have used if it was present. Unfortunately, you cannot simply just add more fertilizer to achieve this since adding more fertilizer may exceed the limit and burn your plants. Instead, the solution is to increase the ratio of nitrogen. This does not affect the total dissolved fertilizer in the water but will give the plant the additional nitrogen it needs to thrive.


5. Plant Specific

Some fertilizers are intended for only one species of plant. This is simply to maximise the nutrients this plant is known to use most. Some examples of this are orchid food, cactus food, or tomato food. These fertilizers take into account the specific requirements an individual category or genus of plants use and optimize the formula for them. This can get confusing with some plants since their nutrient requirement will still change over the different life stages of the plant. In effect, these formulas are simply an optimized all purpose fertilizer.