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N P K, what is it anyway?

Published on September 15th 2020
A close up of a flower
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.

What do the numbers on fertiliser mean?

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.
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What are the functions of each nutrient?

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. Excessive nitrogen, however, may delay the flowering and fruiting of plants.
Signs of nitrogen deficiency:
  • Yellow and stunted foliage on plants.
  • Early falling of tree leaves.
  • Premature flowering and seeding of growing plants.
*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, fishbone meal, worm castings and various liquid organic fertiliser.
Signs of phosphorous deficiency:
  • Plants remain stunted during early growth.
  • Leaf pigments change colour. For example, tomato leaves turn purple.
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.
Signs of potassium deficiency:
  • Plants become mottled or yellow between the veins and are scorched at the margins.
*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.

Fertilising in the garden

Although compost is the best source of food for your plants, they can often do with a little extra boost.
Types of fertilisers
  • Organic fertiliser: Mainly comprised of compost, manure and other animal and plant products, organic fertilisers help to build the soil structure which supports improved water and nutrient retention. These fertilisers also encourage beneficial microbial activity and do not build up toxins in the soil. There is no measurable amount of any specific nutrients in organic fertilisers however some bags will print estimates.
  • Chemical fertiliser: Also known as inorganic fertilisers, chemical fertilisers are mostly comprised of man-made/synthetic materials. These fertilisers provide rapid nutrition for successful, short-term growth, however, it is not beneficial for building soil structure and can often build up excess salts.
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Fertiliser forms
  • Granular: these fertilisers are easily spread on top of the soil and, as you continue to water your garden over time, the nutrient granules soak into the soil.
  • Powder: powdered fertilisers can be spread by hand and watered thereafter to allow for absorption.
  • Liquid: these fertilisers are diluted with water and can be spread whilst watering the garden.
A close up of a bottle
Before making amendments, 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).

When do I need to fertilise my garden?

The best time to fertilise your garden is during spring as new growth sprouts and most plants enter the growing season. Plants can be fertilised until mid-summer. Plant growth slows down in the colder months as most plants enter into dormancy in winter, so applying fertiliser in this time would be wasteful.

What fertiliser do different plants require?

Grass and lawns mainly consist of roots and foliage, therefore the ideal fertiliser would be high in nitrogen and phosphorus content for healthy, green leaf growth and strong root development, and low in potassium as flowering is not encouraged in lawns. Fertilisers ideal for lawns includes 7:1:3 or 3:2:1.
For more information on lawn care, dig into the article below.
A close up of a flower

Spring lawn care


Plants grown for their flowers and fruits need a fertiliser high in potassium for encouraging the formation of flowers and fruit, high nitrogen content to maintain healthy green growth, and low phosphate content. A fertiliser like 6:1:5 is great for fruit plants and shrubs, and 8:1:5 are perfect for boosting flowering in roses.
A close up of fruit on a branch
All vegetables need nitrogen, especially leaf and stem crops. However, too much nitrogen in root and fruit crops results in too much leafy growth at the expense of flower and fruit production. Phosphorous is important for root development and potassium is essential for flower formation and immunity against disease. A good general-purpose fertiliser for vegetables is 2:3:2.
A hand holding a plant in a garden
If you're planting new trees, shrubs, seedlings or lawn it is beneficial to add general fertiliser like 2:3:2 and Super Phosphate to the mixture. For newly planted veggies use a fertiliser like 2:3:4.
A person standing in a garden
The best fertilisers to use for indoor plants are slow-release and liquid fertilisers. It's important to know that every plant has its own specific requirements so do some research before you start feeding. Liquid fertiliser allows you to control the timing of feeding as you can stop during the dormant winter months and continue feeding again when spring comes or new growth sprouts.
Liquid fertilisers can be given bi-weekly or monthly. Granular products can be given once every month or two. Slow-release houseplant fertilisers slowly break down to release nutrients in small amounts, over an extended period of time. These fertilisers only need to be applied once every three to four months.
A cup of coffee and a vase of flowers on a table

Dig into the article below to learn how to make your own compost!

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