What Do The Numbers Mean On Fertilizer? NPK Ratio Explained

If you have ever wondered what the three numbers on a fertilizer bag mean, you’re not alone. They refer to the NPK ratio, by weight, of the fertilizer in the bag.


What does NPK Stand For?
The NPK ratio is the amount of Nitrogen (N), Phosphorus (P), and Potassium (K). They always appear in that order and may be equal (e.g. 10: 10: 10) or each may be different (e.g. 18: 24: 10), depending on each compound’s concentration.


Nitrogen, Phosphorus, and Potassium are the three macro-nutrients needed to sustain plant life. Each one plays a different role, and, in combination, these compounds support healthy plant growth.


What the N, P, K, are used for in plants?
Nitrogen (N) primarily promotes upward growth and leaf health. It supports numerous chemical processes with the plant, such as vital photosynthesis and plant metabolism. It is especially good for leafy green plants (think lawns, corn, and lettuce). Nitrogen-deficient plants do not grow fast and will show yellowed or pale green leaves.


Phosphorus (P) is essential for strong structural growth and also supports chemical processes such as photosynthesis. It is a key component in the maturation of plants and the development of flowers and seeds. Plants lacking Phosphorous are slow to mature and prone to physical damage like broken stems.


Potassium (K) promotes root growth, energy conversion, and the development of protein and starches in the plant. It also activates numerous different enzymes that are essential for healthy plant growth.
These macronutrients are naturally present in healthy soils and regenerate through natural environmental processes. The problem here is that very few cultivated plants are in a natural environment where these processes can occur uninterrupted and nutrient levels in the soil are not very rapidly depleted.


Synthetic Fertilizers
As great as a bag of concentrated nutrients sound, there are drawbacks and limitations to using chemical fertilizers.


Chemical fertilizers are easily depleted from the soil and are not always completely available for the plant to take up if they are not in the right conditions. They can negatively impact soil quality and kill off healthy microorganisms. They also kill earthworms and some plant-friendly insects. You can damage or even kill your plants by ‘burning’ them with too high a concentration of fertilizer.


Chemical NPK fertilizers are also limited to the three macronutrients and do not provide all the support and nutrition a plant needs to be healthy and happy.


This is where organic fertilizers, composts, and liquid plant foods come out on top by fulfilling a broad range of needs, in both the plant and the soil. They contain a variety of nutrients and healthy microbes, supporting plant growth and promoting healthy soil conditions.

Natural-based Fertilizers
Worm castings and the liquid ‘worm tea’ made by certain varieties of earthworms take the natural decomposition process, speed it up by eating organic matter, digesting it, excreting it, and a number of healthy microbes from their gut. The solids are called worm castings, and the liquid is called worm tea. While worm castings typically do not have a very high NPK content (which varies, but is commonly around 0.6: 0.7: 0.2), they are preferable to chemical fertilizers for several reasons.


Worm castings are high in a variety of nutrients, such as iron, sulfur, and calcium, and NPK macronutrients. They also contain healthy microbes that develop during the worm’s digestive processes. These slow down the release of the nutrients into the soil, meaning that they are fully available for the plant to take up, over an extended period of time. This provides a steady supply of nutrients to the plant. And all the nutrients can be taken up rather than leaching out into the surrounding soil.

Worm tea is a more immediately available fertilizer. When diluted and watered into the soil or sprayed directly onto the plant as a foliar feed, it is fully absorbed, and all the nutrients are immediately available for the plant to use.


Worm castings are safe to use and won’t pollute the environment or harm humans or domestic animals. They are safe to use directly on your plants and won’t ‘burn’ them. Organic and naturally rich in organic matter, worm castings will actively improve soil quality and continue to improve it over time.

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