Have you ever found yourself staring at the array of fertilizers offered by gardening and horticultural shops, unsure of what to buy? All of them seem similar, but have slightly different combinations of numbers on the front. Even soil these days often come with their own NPK ratios due to the soil conditioners used in the mix. Believe me when I say there’s a rhyme and reason for using these numbers on these products.
They represent something called the N-P-K ratio, which is actually very important when choosing what to feed your plants to support their growth. This is the ratio of nitrogen (N) - to phosphorous (P) - to potassium (K). These three nutrients form the three musketeers of plant nutrition. They provide the material for the vast majority of what the plant produces. Learn how to decipher NPK ratios for yourself so you can make the best (and fastest) decision when your faced with the array of plant fertilizers available online and at your local gardening center.
Breaking Apart The Ratio
Let’s break apart our very own Cre soil as an example.
Our Blue All Mix soil is used as an all around soil to cover the needs of about 90% of common garden plants. It contains an added fertilizer with an NPK ratio of 14-16-18.
This means that is contains about 14% nitrogen, 16% phosphorus, and 18% potassium.
The remaining 52% contains other trace minerals like calcium, magnesium, manganese, iron and zinc. It also contains chelating agents that help bind the nutrients to make them dissolve better in water so the plants can absorb them efficiently through their roots.
Some formulas will contain only one of these macronutrients. These fertilisers will show something like 12-0-0, meaning that the fertiliser contains 12% nitrogen, and 0% phosphorus and potassium. These are referred to as an incomplete fertilizer and are usually reserved for specific purposes and only for short amounts of time.
Nitrogen: Upward Growth
Nitrogen is the main driver for growth of the plant. It’s used to build leaves, roots, and flowers.
This is the nutrient that provides the biggest change in growth speeds. I’ve seen gardens absolutely explode with growth after the addition of a nitrogen source like blood meal.
Most all purpose garden soils or fertilizers will have a similar amount of nitrogen to the other elements, or even slightly higher. Some examples are 12-12-12 or 12-8-12
Some fertilizers will have a very high nitrogen content, such as a 12-6-4 ratio. These products are only used once or twice in early spring to boost vegetative growth.
These fertilizers shouldn’t be used all year.
Phosphorus: Downward Growth
Phosphorus has many roles in plants, but the most abundant use of it is in the development of both the roots, and flowers. Therefore, phosphorus is important at all stages of growth, but is most important during the early stages of seedling development, and final stages during flower production.
Some species of flowers will actually gorge themselves to death on phosphorus.
They love it to such a degree that they won’t stop consuming it until it builds up to toxic levels and begins to decay the plant from the inside out.
It’s pretty nasty stuff.
For this reason, many soil and fertilizer manufacturers will limit the phosphorus content. Fertilizer and soils containing high phosphorus are usually preferred for the flower stages of growth, but can be of benefit throughout the plant's life cycle due to its role in root production.
Some specific ratios for promoting flower and fruit development is 3-20-20, or 4, 12-12. These fertilizers contain little nitrogen and high phosphorus and potassium to really push the plant towards flower development rather than vegetative growth.
Potassium: All Around Growth
Potassium is used throughout the plant, and is an essential component of their defense systems. Without enough potassium, plants become lethargic and lackluster. They start to lose their defenses and can become infected by bacteria, fungi, protozoa, or insects much more easily.
This is why many fertilizers will have a slightly higher potassium rating than the other components. It’s used throughout the lifecycle of the plant, and doesn’t have the same risk of overconsumption as phosphorus.
The “Other” Macronutrients
Although nitrogen, phosphorus, and potassium are the most abundant macronutrients a plant uses, they aren’t the only ones your plants need.
Calcium, sulfur, magnesium, carbon, oxygen, and hydrogen are all considered macronutrients as well, but in lower concentrations.
These other macronutrients are also usually contained in common fertilisers, and usually aren’t necessary to supplement because the soil will have a lot of these nutrients in it already. The oxygen, hydrogen, and carbon are also given to the plant in the form of water, which the plant breaks apart using the sun's energy to release its components.
Our Cre soil comes with a rich supply of organic fertiliser naturally high in all of these elements.
This helps you focus on the three main macros when it comes to plant nutrition while knowing that your plants are getting the other elements they need to thrive.