N P K, what is it anyway?

Published on March 19th 2019
A close up of a rock
Have you ever found yourself staring aimlessly at the three numbers on fertiliser packaging, wondering what they mean and which one to choose to best suit your garden's soil needs? Well, stare no more, because we are about to break it down for you ... much like organic matter breaks down in the compost bin.
N-P-K are abbreviations for the three macronutrients that are essential for plant growth i.e. nitrogen (N), phosphorus (P) and potassium (K). The series of three numbers printed on fertilisers represents the amounts in percentage or ratio of each of these three nutrients e.g. 3:1:5. The higher the number, the more concentrated the nutrient is in the fertiliser.
A close up of a logo
Why is NPK important?

N | Nitrogen

Nitrogen promotes leaf development and growth. It is responsible for the green colour (chlorophyll) in plants and plays a key role in photosynthesis. Nitrogen also feeds new shoots and plant growth. Common amendments with a high nitrogen content include Bloodmeal, fish emulsion and Alfalfa meal.
Tip: Grow nitrogen-fixing cover crops, like legumes, clover and rye, to naturally replenish nitrogen in the soil.

P | Phosphorus

Phosphorus builds healthy roots and promotes healthy flower and fruit development, an essential nutrient during spring. Often, fertiliser for bulbs and blooms are high in this nutrient. In nature, plants usually take up phosphorous through decaying organic matter. You can apply it to your garden as rock phosphate, fish bone meal, worm castings and various liquid organic fertiliser.
A close up of a bunch of pink flowers

K | Potassium

Potassium is important for the overall function and vigour of the plant. It improves disease resistance and general plant health. It can be applied organically as SUL-PO-MAG, greensand, kelp meal, wood ash and liquid fertiliser.
Tip: Wood ash is a great source of potassium and can be mixed into the compost heap.
Not all plants have the same nutrient requirements, therefore, understanding the NPK values will help you select the appropriate fertiliser for the plants that you are growing. For instance, applying a fertiliser that is high in nitrogen will cause some plants to spend all their energy on producing leaves at the expense of flowers. Similarly, if you are growing leafy vegetables, you want to use a fertiliser high in nitrogen to encourage leaf growth for an abundant harvest.
Before making amendments though, it is best to get your soil tested first as this will help in determining what balance of fertiliser will be appropriate for your garden’s soil needs and deficiencies. Also, make sure your plants get the most out of the soil by checking the pH first. Nutrients are most available when the pH is in the sweet spot (6.0 - 6.5).

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