Plants Eat Food, pt 1 – The Big 3

This is part 1 of a 4 part series on plant nutrients and how to use them. Stay tuned in the following weeks to learn about the other 3 main nutrients (part 2 of this series) and trace elements (part 3). Then, in part 4, we’ll do a broad look over the common forms that fertilizer or plant nutrients can take.

A Slight Change

The blog style has been getting a bit of an overhaul, and the website is soon to follow. If you’d looked at my account in the past, you’ve probably read that I’m planning major changes.

The time is now.

Recently, I’ve decided it’s time to get my butt in gear and start diving deep into the nature like I never have before. I’m planning to study all the important topics that I don’t know enough about, and apply those lessons to my garden or explore them in the nature around me.

A handy quick-release liquid fertilizer

Plant growth is limited by the nutrient in shortest supply.

The big 3.

I’ve only recently started applying fertilizer to my garden in any meaningful way. Over the course of the winter, I reasoned with myself that I didn’t want to cause root burn or over-nutrify my little balcony garden.
I was wrong.

Generally, nature will put the nutrients it has available into the soil (or the air), and the plant will make use of what it can. Most nutrients will get taken up by a plant’s roots, which means the nutrients have to make it into the soil for the plant to be able to get it out. The roots aren’t heat seekers. They actually only make use of a small area right around the roots, and can’t pull nutrients in from much farther. On top of this, water, in the form of heavy rain or irrigation, can leach nutrients from the soil.
All the meanwhile, plants are gobbling up everything they can of 3 key macronutrients – Nitrogen, Phosphorus, and Potassium – alongside more than a dozen other nutrients in smaller quantities.

And oxygen and carbon and all that stuff. But that’s in the air, so we don’t have to worry about adding that.


Nitrogen is the big one. Probably the one you’ve heard of the most – nitrogen fertilizer. Nitrogen is in all the plant cells, the proteins, everything else. If you have a lot of nitrogen, you’ll probably have a lot of leaves. If you want a lot of leaves, you’ll probably need a lot of nitrogen. This is the green nutrient, if nothing else. Adding too much, though, will cause the plant to forgo healthy flower and fruit production in favor of leaf growth.

Nitrogen has to make its way into the soil to be useful for the plants.
Atmospheric nitrogen is one good source. It’s 78% or so of the air, so it makes sense that plants would want to use it. Some plants can fix the atmospheric nitrogen into their roots (legumes!), but most don’t. Instead, they make use of the mineral form, nitrate. Atmospheric nitrogen is very commonly found processed into ammonium sulfate, ammonium nitrate, and urea. After application to the soil, the nitrogen is converted through different (often fungal or bacterial) processes into nitrate.

Nitrogen can be leached out of the soil pretty easily by heavy rain. It then acidifies the soil, which many plants don’t like.
To prevent this, apply the fertilizer in small amounts over time, or use an organic source that can break down slowly over time, such as composted manure.

N : Quick Facts

  • Makes leaves.
  • Makes pretty much everything, too.
  • Plants need it, and a lot of it.


Phosphorus is a key ingredient for how the plant actually functions.

It helps transfer energy from sunlight to plants, stimulates early root growth, and helps the plant mature. I can also say it’s a key one for fruit production for at least a few of my balcony plants. It also helps in flower production.
In a roundabout way, it helps the plant convert other nutrients into useful building blocks.

The easiest way to see if you’re low on phosphorus is to check : are your flower-producing plants not producing many flowers? Are your fruit-bearing plants not making much fruit? Sounds like you need some phosphorus.

Manures, especially from grain-fed animals, are a rich source. Bone meal might be a less scented option. Compost can help the plants take up nutrients regardless, so it’s a good idea to throw some of that in too, just in case.

Some states or counties watch phosphorus runoff carefully due to the potential environmental harm. If you’re doing something large scale, you probably aren’t going to use this blog… but also watch your phosphorus runoff.

The other big thing, the even bigger thing for me:

It’s very difficult for a plant to get too much phosphorus due to the fact that it’s difficult for plants to absorb phosphorus in the first place.


P : Quick Facts

  • Helps convert other nutrients into useful plant building-blocks
  • Phosphorus = Flowers & Fruit


Potassium is the other really important plant nutrient, perhaps even more important than phosphorus. (Nitrogen is still the champion)

This is a big one for disease resistance, helps move the various foods and fluids throughout the plant (oils, starches, sugars), assists in overall balance and top growth, and can help with fruit quality. The long and short of it is, this nutrient helps overall plant functionality.

K plays essential roles in enzyme activation, protein synthesis, photosynthesis, osmoregulation, stomatal movement, energy transfer, phloem transport, cation-anion balance and stress resistance

Int J Mol Sci. 2013 Apr; 14(4): 7370–7390.

This stuff usually comes from something called potash. Pot-ash used to come from plant ashes soaked in a pot, but now comes from salts that have water-soluble potassium in them.

Out of all the mineral nutrients, potassium (K) plays a particularly critical role in plant growth and metabolism, and it contributes greatly to the survival of plants that are under various biotic and abiotic stresses. 


If you’re running low on potassium, plants will show some wilting and some necrosis, usually in the older leaves. You can’t quite over-feed the plant on potassium, but too much can cause issues with the regulation of other nutrients.

K : Quick Facts

  • Helps the plant stay healthy.
  • Complex, but very needed.

How can I use the numbers?

The usual style of measurement you’ll see is something like #-#-#. This refers to the N-P-K value of the fertilizer. 2-1-1 would have 2 parts nitrogen for each 1 part of phosphorus and potassium.

Here’s where things get weird… or useful.
The numbers are there to help you figure out the amounts of these macro-nutrients by weight. Let’s take a 10-0-0 fertilizer. That means there’s only nitrogen in it. Now, divide 100 by 10 to get 10. This means it’ll take 10 lbs of fertilizer to add 1 lb of nitrogen to the soil.
5-10-20 would need (100/5=20) 20 lbs of fertilizer to add 1 lb of nitrogen to the soil, or (100/10=10) 10 lbs of fertilizer to add 1 lb of phosphorus,
or (100/20=5) 5 lbs of fertilizer to add 1 lb of potassium to the soil.
That’s a funky fertilizer.

You can look at these three numbers as a simple proportion – but if you’re willing to weigh or measure your fertilizer, this method may help you figure out exactly how much you’re adding and what changing fertilizers might add to your soil. For liquid fertilizers in my balcony garden, I typically measure in teaspoons. I’ll be switching to a dry, slow-release fertilizer soon. I’ve figured out how often and how much fertilizer my plants need, so with a little math I should be able to switch over to dry without a huge upset.


NSW – Department of Primary Industries
A refreshing overview of the main plant nutrients, their sources, their uses, and some potential harm they can cause if used improperly.

Gardening KnowHow
A straightforward overview of the main macronutrients that plants need. (And here’s Phosphorus in particular)

A look at NPK, the over- and under-usage of these macronutrients, and a different way of doing the math on the 3 key numbers.

An in depth look at potassium and how it works in the plant.

Thanks for stopping by! I hope you had a pleasant time checking out the plants. If you’re in the mood for more nature, please stay in touch!


2 thoughts on “Plants Eat Food, pt 1 – The Big 3

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