N-P-K is a common term when it comes to feeding plants, but what does it mean? It’s quite simple, actually.

N-P-K is an acronym that stands for nitrogen (N), phosphorus (P) and potassium (K). These elements get most of the attention when it comes to plant nutrition since all three are used in large quantities by all plants.

Additionally, exactly when they were discovered historically also affects their popularity as they were the first three elements to be derived and used on plants. It makes sense they get most of the spotlight.

The N-P-K of a fertilizer represents the ratio of the minerals in the mix, or how much of each element is contained by percent of total weight or volume.

For example, a 10-5-14 nutrient formulation contains 10% nitrogen, 5% phosphorus and 14% potassium.

If you have spent any time in a gardening center, you probably have seen three numbers printed on bottles and bags of plant nutrients. This is the N-P-K ratio.

What N-P-K is Right for Most Plants?

The good news is that there are many high-quality plant nutrients available at your local indoor gardening center to choose from.

Gone are the days where indoor gardeners had to mix up their own hydroponic nutrient solution from things bought at a chemical supply store.

Choosing a nutrient formula with the correct ratio for your garden from this crowded field is the key.

What’s the correct ratio for your garden? That depends on what you’re growing and the developmental stages of the plants.

Generally, there are two life stages that drive a plant’s nutritional needs: vegetative (growth stage), and fruit/flowering (bloom stage).

NPK Formulas for Growth

The earlier example of 10-5-14 is a popular N-P-K ratio for powder-based grow nutrients. Plants in the vegetative stage require quite a bit of nitrogen, less phosphorus and a bunch of potassium to grow up into large plants.

During this phase, plants are growing their stems and pushing out leaves as fast as they can. Extra nitrogen is necessary to support these activities. Potassium is used in large quantities during a plant’s full life cycle, both vegetative and fruit/flowering.

NPK Formulas for Bloom

The corresponding bloom formula’s N-P-K ratio is 5-15-14. During the fruit/flowering phase of a plant’s life cycle, plants have finished building their support scaffolding and solar cells (leaves) and focus instead on reproducing the next generation.

The amount of nitrogen needed by the plant usually decreases due to less leaf and stem production, plus the plants have built up nitrogen stores that are used during this stage.

The amount of phosphorus needed increases to support the production of flowers and fruits. As with the grow formula, the potassium level is high in bloom formulas compared to the others.

Another use for bloom formulas is feeding clones. Feed plain water to fresh-cut clones until roots appear.

After that, feed clones a weak bloom formula. Bloom formula is better than grow formula for clones because the extra phosphorus will encourage root growth. Once fully rooted, switch to a grow formula at the same strength as used for the donor plant.

General Purpose NPK

The third main category of plant nutrients is general purpose. These can be recognized by N-P-K values that are about the same for all three, such as 10-10-10.

They are fantastic for creating charged soils or soils that already have some or all the nutrients for the entire growth cycle mixed into them.

General-purpose nutrients are most often used outdoors, but they are worth mentioning because they often wind up on the shelves of indoor gardening centers.

What is a Bloom Booster?

Bloom boosters have become popular. When used correctly, they can offer explosive growth, increased final yield and increased quality.

When used improperly, they can spell disaster—decreased final harvest size and poor quality. The N-P-K of a typical boom booster is something like 0-50-30.

Bloom boosters are a way to add additional amounts of phosphorus and potassium without increasing the nitrogen load.

Watch your overall nutrient strength when boosting—many gardeners cut the strength of their primary nutrients in half when incorporating a full-strength bloom booster. Also, watch for leaf tip burn, which indicates an overdose, and reduce the strength of the nutrient solution if this is noticed.

Transitioning NPK Formulas

When transitioning the garden to the fruiting stage, which usually involves decreasing the light cycle from 18 to 12 hours, the plants begin to stop vertical growth and shift into producing fruits and flowers.

It’s often advised to use half-and-half grow/bloom formula during the transition period, which generally lasts two to three weeks.

When vertical height growth ceases, the transition period is over. Drop the grow formula and switch to full bloom formula.

How Much NPK Do I Use?

This is my favorite question, and I usually respond with, “as much as it takes and no more.” This is where experience comes into play.

What should you do if you don’t have experience?

My best advice is to start at half of the manufacturer’s recommended strength until you learn what works best for your garden.

It’s easier to feed a hungry plant a little more nutrient than to try and rescue over-fed ones.

Remember, research is essential. Friends, family, the Internet and garden supply retailers make it easy to compare notes with others.

Just be wary of whose advice you choose. Compare overall ideas and don’t try to live up to one person’s opinion. What works for them may not apply to your situation, though overall trends should point you in the correct direction.

A bit of common sense needs to be used when selecting and using different nutrients. When you see large N-P-K numbers (greater than 20), this signifies that you need to be careful when using that product—it should be diluted heavily (think 1 tsp. per multiple gallons of water).

Many bloom boosters are diluted by 1 or 2 tsp. per 5 gal. On the other hand, when feeding hungry plants with small N-P-K numbers (such as two or less) you will need to use quite a bit (think multiple teaspoons or tablespoons per 1 gal.).

This is especially true when using many of the popular organic lines. For those who like to use charged media and feed with compost teas, be aware you might end up having to use two to three times the recommended amount of compost tea to get the results you want.

The Truth About Potassium and Phosphorus Fertilizers

Both phosphorus and potassium have a secret to tell in regards to what’s on the label. Nitrogen gets left out of this story because it behaves properly—what’s on the label is what’s in the bottle. The story is a bit different for both phosphorus and potassium—both of them are labeled as their oxide, not as their element.

This means there is less of the actual element (what the plant truly eats) in the nutrient bottle than what is listed on the label.

Why is this? It comes from labeling requirements imposed by government agencies, which are not straightforward. It might be helpful to grab your favorite nutrient bottle and take a look.

The label probably reads, “Available phosphate (P2O5)” for the amount of phosphorus and “Soluble potash (K2O)” for potassium.

The problem is that the N-P-K labeling for the product does not show the actual concentration of these elements themselves (P or K only), but instead gives credit to the attached oxygen molecules.

For reference, the elemental concentration of phosphorus is 43.6% of what’s labeled and potassium is 83%.

While this information may not have a direct impact on how much you feed your plants, it helps explain why bloom boosters actually have a small margin of error when it comes to accidentally overusing them.