How to understand fertilizer: a guide
Have you ever found yourself in front of a box of fertilizer at the garden centre, doing your best to decipher numbers that don’t mean anything to you?
Like 20-20-20, or 15-30-15, or 3-3-2?
What do these numbers mean? What do I need? Which one is the right one for my plant that’s all droopy?
There’s a lot of meaning behind these numbers. But, not to worry — we’re going to explain the basics. By the end of this post, you’ll have a general understanding of fertilizer, and a little more confidence for your next trip to the garden centre.
What is plant fertilizer?
You may have heard people refer to fertilizer as “plant food” or “nutrients”. Fertilizers are actually sort of like multivitamins for plants, and provide essential plant nutrients to soil to help them thrive and grow optimally.
Soil itself is an important part of plant growth, in that healthy soil is full of organic matter to begin with that aids in healthy growth. Healthy soil also both holds water and allows it to drain well.
Without the help of soil and fertilizers, we likely wouldn’t get the results we seek from our plants — they don’t grow as well as we’d like, and they don’t bloom as often, either. Particularly with plants grown in containers, plants are limited to the soil in their pots, which can become depleted of essential elements over time. Plants in pots can’t reach their roots out to find nutrients in adjacent soil, either, which is another limitation on their ability to get the nutrients they might lack.
Plant fertilizers can be a chemical/synthetic or natural ingredient that you add to your plant’s soil to amend its nutrient value.
Natural or chemical fertilizer
When you use a natural or organic fertilizer (like worm castings or compost) plants are able to get a more balanced diet. From a practical standpoint, you may find it most useful to use compost and worm castings on outdoor plants and gardens.
For indoor plants, chemical fertilizers (liquids, water-soluble granular fertilizers and slow-release fertilizers) are most often the route that indoor gardeners and professionals take. When you go the chemical route, it’s important to study the labels on fertilizers, so you know what it is you’re about to feed your plants.
So let’s dig in to that.
What are the main components of fertilizer?
The three numbers you’ll see on a bag or liquid container fertilizer label stand for three chemical elements, marked as N-P-K. They are always in that order and will never switch around. Here’s what those letters mean:
N = Nitrogen
P = Phosphorus
K = Potassium
Nitrogen, phosphorus and potassium are the three macronutrients that help support plant growth.
NPK = Shoots-Roots-Fruits
Now that you know what each of the letters stands for, let’s talk briefly what each of each of these elements does for the plant. Each plays a separate role in aiding plant health. This is where the mnemonic shoots-roots-fruits comes in.
- Shoots = Nitrogen
- Roots = Phosphorus
- Fruits = Potassium
To interpret that further:
- Shoots = Nitrogen = Assists with foliage and vegetative growth (leaves and anything that grows above ground)
- Roots = Phosphorus = Aids with healthy root growth (underground and what we don’t see)
- Fruits = Potassium = Helps promote flowers and fruit (vegetable) formation and size
Is there such a thing as too much fertilizer?
Yes, there absolutely is such a thing as too much fertilizer. Excessive use of fertilizers can damage plants and decrease soil’s fertility. For example, if your tomato plant is tall and green but lacking in fruit production, there may be too much nitrogen in the soil.
When you’re choosing fertilizer, larger numbers don’t necessarily make for a better fertilizer. It’s essential to monitor your plants, assess their growth and health, and pick a fertilizer to meet their needs.
How do I choose a fertilizer?
- What does your plant need? Does it need help with foliage (shoots)? Choose a fertilizer that has a higher N value. Do you want to encourage blooms (fruits)? Perhaps choose a fertilizer with a higher P value.
- If you want to get a little more scientific – do a soil test. You can find soil testing kits to test for NPK levels, as well as pH levels, on Amazon.
- If you’re uncertain, you can always choose an all-purpose fertilizer (5-5-5), which contains an equal ratio of Nitrogen, Phosphorus and Potassium (Shoots, Roots and Fruits).
The next time you go into a store to buy some fertilizer, just remember ‘Shoots, Roots and Fruits’, in that order, and consider what it is you seek to improve for your plant. And when in doubt, go for an equal ratio of fertilizer numbers: 5-5-5 or 10-10-10, and so on.