The ABC of N-P-K



A fertilizer, also called plant food, is composed of minerals plants need for their growth. In a soil where compost or manure is added regularly (both are also rich in minerals), a fertilizer isn’t always necessary. That may also be the case in areas where the previous year’s leaves are left to decompose on the spot. However, in gardening situations where you regularly take minerals away (by harvesting crops, by cutting back perennials in the fall, by raking up and removing fall leaves and lawn clippings, etc.), fertilizers are usually required. That’s why “take-away” cultures, like vegetable gardens and lawns, are usually needier of fertilizer than plantings that are mostly allowed to grow on their own, like shrub borders and groundcovers.

Potted plants probably need the most fertilizer of all, because their confined roots are simply not able to “reach out and grab” minerals in their surroundings the way they’re designed to; plus they are also prone to leaching due to rain and watering, meaning minerals you do add are often quickly washed away.

What is N-P-K?

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N-P-K: the big three of fertilization! Ill.:

In most countries, by law all commercial fertilizers must display in large type three figures that represent the percentage of nitrogen (N), phosphorus (P)and potassium (K)*, called macronutrients or major nutrients, that they contain. An 11-12-9 fertilizer would therefore contain 11% nitrogen, 12% phosphorus and 9% potassium.

*In fact, the percentages are of N, P2O5 and K2O, but by convention, NPK is used.

It is usually said that nitrogen is used to stimulate green growth. Thus, a fertilizer rich in nitrogen would be useful for seedlings and leafy plants (foliage house plants, conifers, leafy vegetables, etc.). But that simplifies things far too much: nitrogen is simply the most vital nutrient in plant growth and is used by plants in practically everything stage of their development.

Add too much nitrogen, though, and you could end up with lush, abundant leaf growth, few to no flowers, disease infestations and insect invasions (aphids, for example, are renowned for their love of nitrogen-boosted plants).

The fact remains, though, that nitrogen is the element that is most often missing in garden soils and pretty much every gardener will have to return nitrogen to the soil in one form or another.

Phosphorus is said to promote root development and also stimulate flowering and fruiting. This isn’t precisely true (phosphorous is in fact used in all sorts of ways by plants), but neither is it entirely false. Based on this belief, phosphorus-rich fertilizers are often sold as flower fertilizers, vegetable fertilizers or rooting fertilizers.

On the downside, fertilizers too rich in phosphorus (more than 15%) can negatively affect the establishment of mycorrhizal fungi, beneficial fungi that whose role is also to seek out and absorb phosphorous from the soil. And excess phosphorous flowing into bodies of water is causing environmental problems worldwide. For those reasons, you’ll be seeing more and more fertilizers with little to no phosphorous … and that’s fine where phosphorous is abundant in the soil, as indeed it often is.

Finally, the role of potassium seems primarily to promoting strong stem growth, flowering and fruiting. The commonly held belief that potassium is important in helping in plants resist bad weather, diseases and parasites is probably a bit exaggerated, but, like phosphorous and nitrogen, potassium is so important to healthy plant growth that it is very hard to draw a clear line as to its specific effects.

The belief that potassium boosts resistance has led to potassium-rich fertilizer often being recommended for vegetables and as a fall fertilizer to help plants better tolerate winter conditions.

Complete Fertilizer

A fertilizer that includes all three macronutrients (nitrogen, phosphorus, potassium) is said to be complete. If only one or two of these nutrients are present, it is said to be incomplete. Of course, an incomplete fertilizer can still be a good product. For example, if soluble forms of phosphorus and potassium are already present in sufficient quantities in the soil, which is more often the case than many gardeners think, why add more? It may be helpful to have a soil test done to find out if these two nutrients are missing or abundant in your garden soil.

As for nitrogen, pretty much every garden plant will need nitrogen to grow well: it just isn’t very persistent in soil.

Non-fertilizer Elements

Three other elements are of vital importance to plants and even form the basis of their cell development: carbon, hydrogen and oxygen. However, plants absorb them from water or air. You don’t need to add them in the form of fertilizer.

Secondary Nutrients

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A periodic table of plant nutrients, showing all 17 nutrients essential for plant health. If just one nutrient is lacking in the soil, crop yields will suffer. Ill.:

In addition to macronutrients (NPK), plants need secondary nutrients in fairly substantial quantities. Again, they are often already present in garden soils, but when they aren’t, and if you don’t add them in some form or another, their absence can cause nutrient deficiencies, often indicated by a slow growth and yellowing or reddening of the foliage.

The three secondary nutrients are calcium, magnesium and sulfur. Because of their importance, their presence is often highlighted on the label.


Scientists are still arguing about which micronutrients, also called minor nutrients or trace elements, are actually needed for plant growth, but most accept the following: boron, chlorine, manganese, iron, nickel, copper, zinc and molybdenum. Some specialists add selenium and iodine to that list. And it would appear that some plants need silicon, sodium, cobalt or vanadium.

Micronutrients are only needed in very, very small amounts, yet that doesn’t mean they aren’t important. Without them, plants will suffer serious deficiencies leading to poor, misshapen growth and considerably lower yields.

Synthetic or Organic?

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Organic fertilizers feed the soil, synthetic fertilizers feed only the plants. Ill.:

In general, synthetic fertilizers (also known as chemical fertilizers) are very rich in macronutrients, with rates exceeding 15%. 15-30-15, for example, is a synthetic fertilizer. On the other hand, they rarely contain the full range of secondary nutrients and micronutrients. As their name suggests, they are derived from chemical compounds and are assembled artificially. They’re designed to be absorbed directly by plant roots.

Organic fertilizers are derived from natural sources, such as crushed rock, vegetable waste and manures. They are less concentrated than most synthetic fertilizers and rarely do their nutrients exceed a concentration of 14%. A 5-7-8 or 2-3-1 fertilizer would probably be organic. Most organic fertilizers contain a good deal of secondary nutrients and micronutrients. Some, such as seaweed fertilizer and fish emulsion, contain essentially all the nutrients (macro-, secondary and micro-) that plants need for growth and are therefore useful for curing or preventing nutrient deficiencies.

Organic fertilizers are not directly assimilable by plant roots. Instead, they have to be broken down first by soil micro-organisms. They’re often said to “feed the soil,” then the soil “feeds” the plant.

Many gardeners prefer organic fertilizers because they are generally less polluting than synthetic fertilizers.

Slow Release Fertilizer

These fertilizers are composed in such a way as to release their minerals slowly, over a whole season, rather than all at the same time. Thus, a single annual spring application will suffice. There are both organic and synthetic slow-release fertilizers.

All-Purpose Fertilizer

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Example of an all-purpose slow-release organic fertilizer. Photo:

An all-purpose fertilizer (also called multipurpose fertilizer) is, as the name suggests, designed for use on all plants: vegetables, fruits, trees, lawns, flowers, houseplants, etc. It usually contains either an equal amount of the three major nutrients, such as 12-12-12, in which case it is said to be a balanced fertilizer, or a higher percentage of nitrogen, such as 12-8-6. A good all-purpose fertilizer would contain the full range of other nutrients too, as is the case with many all-purpose organic fertilizers.

So, make your life easier! In most cases, the only fertilizer you’ll need for all your plants is a single all-purpose slow-release organic fertilizer.



Container Plants Are Greedy


Feed me!

Container plants are especially greedy and need more fertilizer than other plants. That’s because they live under unique conditions that don’t quite have their equivalent in nature.

First, they are usually grown in a potting mix that contains few natural minerals (most potting mixes are based on peat, coir, and/or bark, all of which are very poor in minerals).

Plus they are more exposed to moving air than plants in the ground and thus dry out more rapidly, forcing the gardener to water them much more often. And frequent waterings leach the soil of any minerals it originally did contain.

Do note that, when it comes to leaching, the situation is very different for plants growing in the ground. When it rains or you water them, true enough, soluble minerals in the soil tend to descend lower in the soil, carried by the downward flow of water, but then they start to move right back up into the root zone through capillary action as the soil starts to dry out. Thus, minerals tend to remain more or less within easy reach of most in-ground plants.

In a pot, though, dissolved minerals go straight out the drainage hole when water flows out and the plant no longer has access to them.

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We tend to jam our containers, like this hanging basket, full of plants… increasing the competition for minerals. Photo: Proven Winners

Add to this the fact that we usually tend to pack plants grown in containers or baskets much more densely than we would in the ground to give the container garden a fuller look or to increase yields. As a result, there are lots of hungry plants sharing the same growing space, all looking for their share of minerals.

Once final point: when plants growing in the ground lack minerals, they can send their roots out to look for more, often quite a distance from the mother plant. The roots of the potted plants remain imprisoned inside the container and can’t go anywhere searching for a snack.

A Different Environment

What you’ve seen from the information above is that container plants live in an environment far removed from their natural environment, one where minerals, if any there were any in the soil, are quickly exhausted. That’s why you need to fertilize container plants more often.

Two-Step Fertilization

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Add a slow release fertilizer to the container at the beginning of the season… and “top up” with soluble fertilizer throughout the summer! Photo: Nursery Live Wikipedia

Most experienced container gardeners will agree that the best way to fertilize potted plants is to apply, at the beginning of the season, a slow-release organic fertilizer, as it will help to provide minerals gradually as the season progresses. Apply it according to the manufacturer’s recommendations. Then complement that by fertilizing regularly with a soluble fertilizer all through the summer. This is especially true if you notice a decrease in growth or in bloom or if the foliage shows signs of yellowing..

Personally, at the beginning of the season, I give each container a handful of homemade compost and apply whatever organic slow-release fertilizer I have on hand. I never even consider whether it’s a flower fertilizer, a vegetable fertilizer or even a lawn fertilizer. As if plants could really tell the difference! (Read Just Use Any Fertilizer to better understand that point.) Then, during the summer, I apply a soluble fertilizer (again, any kind) about every two weeks. And I’m very satisfied with the results!20170730A

Plants Can’t Read Fertilizer Labels


20150709EngThere is a wealth of fertilizers on the market, each more extraordinary than the next, specifically designed to suit any plant group from lawns to orchids, vegetables, cedars, roses, tomatoes and so much more. You therefore feel compelled to buy at least one box of each kind to satisfy your vast plant family. And you have therefore just been taken for a ride by the powerful fertilizer industry.

You see, plants can’t read fertilizer labels… and really don’t care which fertilizer you give them! So, if you feed lawn fertilizer to your roses or orchid fertilizer to your cedars, they’ll grow just fine!

Indeed, the importance of specific ratios of NPK (nitrogen-phosphorus-potassium) has been greatly exaggerated and often entirely invented. Sure enough, all plants* require the 3 major elements for healthy growth (and all the minor elements as well), but the exact proportion is of very little importance… so little, indeed, that you don’t even really need to consider it.

The most honest fertilizer on the market is all-purpose or multi-use fertilizer. It is aptly named, as it is suited to all plants*. But all those plant-specific fertilizers – the “dishonest” fertilizers as I like to call them – can also go on all plants.

My advice? When it’s time to fertilize, just grab the first fertilizer from your reserve and use it on all your plants, edible or ornamental, outdoor or indoor. When you run out, grab another one and use it up as well. When you just have no specific fertilizers left, buy an all-purpose fertilizer (because I feel one should reward honesty in packaging) or a fertilizer which does not refer explicitly to a type of plant: seaweed fertilizer, fish emulsion, blood meal, etc. The latter fertilizers, by the very fact that they make no claim to a specific use, are also all-purpose and therefore “honest”.

Personally I always opt for organic fertilizer, but that’s a story in itself: one to cover in a future blog!

* Carnivorous plants are exceptions to the “all-purpose fertilizer suit all plants” rule: most don’t need any fertilizer at all!