Aquarium sciences: Plant biochemistry – Part 2 Nitrates, Phosphates and Potassium (NPK)

In this three part series we will look at the key processes that power plant growth. By understanding these processes we can better control growth of plants in our aquarium and avoid unwanted algae. For the first in this series will looked at Photosynthesis, this second installment will look at Nitrogen, Phosphorus and Potassium and the third will look at Micro nutrients such as Iron, Magnesium and Calcium. How all of these things work together is the key behind a successful high tech planted tank.

By now most of us are aware that plants use nitrates to grow and you’ll probably have an appreciation that phosphates and potassium also play a role, what we’ll look at here is exactly what these are used for and why it is essential to make these available for your plants especially in a high energy aquarium. Finally we will take a look at the symptoms of deficiencies in these nutrients.

To start let’s take a look at nitrates, what do they look like?


Image 1. Nitrate molecule

So this is a nitrogen atom with 3 oxygen atoms attached, as you will know the majority of nitrates in the aquarium are made by bacteria. The bacteria necessary to convert these nitrates into nitrogen gas or other gases like nitrous either find it difficult to survive, or the conditions required for them to perform these processes do not exist in most home aquariums. Fortunately plants ‘soak’ up these nitrates and use them to grow but how?

To understand what nitrogen is used for we need to go small, really small; DNA contains a lot of nitrogen and as plants grow they need to make more DNA and so they will use some of the nitrates to make this. DNA is the set of instructions that all living things use for everything that they do. For example let’s take humans; inside our DNA there are a set of instructions for how to make muscles, skin, bones and which order they are put together. In plants the DNA will instruct them how to make the stem, leaves, flowers and even things like how to make DNA or how to make the special pumps that they use to take up nutrients. The process of making DNA is very complicated and beyond the scope of this article but it’s useful to know that this is one thing nitrates are used for.

Another use of nitrates is making proteins, proteins like enzymes help plants build new parts or break down old bits to reuse. If you look back to part 1 of this series you will see a number of proteins that were key in photosynthesis highlighted below.

Image 2. Protein molecules involved in photosynthesis

Proteins are made up of something called amino acids, you probably will have heard of these. They are the building blocks that make the enzymes that we spoke about earlier and also make the proteins that help make plants bendy. So what we now know is that nitrates are used for making DNA and amino acids, these amino acids are like lego and you put them together to make proteins, some of these proteins are in a group called enzymes and enzymes are very important they help break things apart or put things together.

What are phosphates used for? Well we already know this, let’s look at part 1 again

Image 3. Adenosine triphosphate

If you look at these two diagrams from part 1 you can see I’ve highlighted ATP.  Let’s take a closer look at this molecule.

Now what we have here is ATP at the top and ADP at the bottom (take a look back at part 1 to familiarise yourself with them) all the times you see a P in this image that is a phosphate. If you remember ADP is bound with a phosphate to lock in energy converting the ADP to ATP, this ATP then goes to where energy is needed and breaks apart releasing the energy it had trapped inside it. So ATP is a major use for phosphates but phosphates are also present in the proteins and enzymes we mentioned earlier.

The last of the macro nutrients we will look at in this instalment is potassium, if you are familiar with macro and micro dosing then you will know potassium forms part of the macro dosing and is represented by the K in NPK. If you are not familiar with micro and macro dosing it is basically a method of adding nutrients to your aquarium, the macro dosing covers the nutrients that plants use more of; nitrogen, phosphorous and potassium or NPK and micro dosing covers the trace elements that plants use in smaller amounts, things like iron, magnesium, calcium and other earth and transition metals.

So what do plants use potassium for? If you imagine a leaf is made up of lots of little bags of water and in these bags of water there are lots of other things dissolved in them. Now if one of these bags has too much other stuff dissolved in it then certain processes can’t take place. Think of it like a saltwater aquarium you dissolve a certain amount of salt in the water to make it the right concentration for your livestock, too much and they might die, too little and they might die; well it’s the same with plants and potassium helps regulate how , much other stuff is in the plants. Potassium also plays a role in photosynthesis and production of carotenoids (the compounds that help make red plants red). Potassium plays many other roles in the plant that are beyond the scope of this article and that are difficult to explain without first explaining some of the fundamentals of microbiology and biochemistry.