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What is in a plant's diet? (1/4): Understanding some types of plant foods

Plants eat. That is a universally known fact amongst all gardeners from all periods of human history. But what dothey eat? When scientists investigated this, beginning by the Dutch scientist Jan Ingenhousz in 1779, they found out a quite unexpected fact: plants actually do not eat, in the sense that they don’t feed on any other living organism. They make their own food through the process of photosynthesis, which allows them to take the carbon dioxide from the air and combine it with the energy they gain from sunlight, to produce sugars. That is what they consume as food.

But then… what do fertilisers actually do? Well, even though plants don’t eat soil, they do need to use some components that are in the soil to build new leaves, flowers, branches, fruits and sustain their growth. If the sugars they produce is what gives them the calories they need, these components in the soil are like our food supplements. You don’t survive by eating calcium, iron, vitamins, and so on: you need to eat actual food, and take these in addition. Same thing happens with plants!

So, we call these plant foods because it’s comfortable and because plants do consume them, but they are not exactly that. And it is key to keep in mind this fact when fertilising, because when you know that you are not actually feeding them but giving them the minerals that they need as supplements, you can ask: what is the mineral (or the minerals) that they need right now?

That’s precisely why several fertilisers exist, that provide either a perfect balance or a specific mixture of the three main minerals that all plants need to grow, and which also happen to be some of the most deficient in our planet’s soil: phosphorus (P), nitrogen (N) and potassium (K). Together they form an acronym that you probably have seen before: NPK. A fertiliser that is rich in all of these three is called ‘all-purpose plant food’, or ‘general-purpose fertiliser’. But there can be other three types of fertilisers as well:

  • Nitrogen plant food or high-nitrogen plant food is, of course, called like this because it contains a particularly pronounced amount of nitrogen in comparison with the rest of the elements it contains. Nitrogen is especially used by plants at the beginning of their growth, because it is essential for them to produce the proteins that they use to build tissues: leaves, stems, roots and everything else. It is also a major component in chlorophyl, without which they can’t do photosynthesis! Nitrogen plant foods or high-nitrogen plant foods are used to amend especial deficiencies of this element in the soil. If they are inorganic fertilisers they are also often the cause of a lot of ecological damage, when the excess nitrogen that they inject in the soil seeps into the nearby bodies of water and upsets their balance.

    Most inorganic high-nitrogen fertilizers are just plain old urea, the cheapest form of nitrogen fertilization there is. It is also one of the most dangerous when over-applied, as it contaminates easily aquatic ecosystems.
  • Phosphorus plant food or high-phosphorus plant food serve the same deficiency-correcting functions. Plants use phosphorus with especial force at the middle of their growth, when they are preparing to bloom and beginning to produce flowers, but by no means it’s useless in the rest of the periods of their growth: it’s essential in energy transfer, which means that without it cells cannot even interact with each other to pass food!

    This is how phosphorus (phosphates, actually, a derivative of pure phosphorus) looks in its raw, purely mineral state. Yummy!
  • Potash plant food or high-potash plant food is the last type of ‘focused’ fertiliser, which presents potassium in the form of a water-soluble salt, that can readily be absorbed by plants. Potassium is, in a similar manner to nitrogen and phosphorus in their respective periods, used by plants with much more intensity as their growth draws to an end, and they begin to produce fruits. Potassium, to put it simply, helps them produce, transfer and store nutrients in the fruits that we eat in the end.


    Potash, also in its raw, mineral state. It looks like salt, doesn't it? Applied to the soil in this state, it can dangerously elevate pH levels.

This is, shortly, what NPK fertilisers and their focused variations contain. And it is precisely what an organic fertiliser, done entirely through natural processes, will contain as well, with the main difference that it is far less likely to kill your plants (or the surrounding environment) through overfertilization. It’s not quite that easy, of course, but definitely makes it worth checking our list of products, which also contain a lot of other minerals and microorganisms that will give your plants an incredible boost. They don’t live on NPK alone, you know!

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