What are those NPK numbers on plant food labels?

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Understanding NPK and giving the right nutrients to your plants

When buying fertiliser from the garden centre it often has a NPK ratio on the packet which stands for:

(N) Nitrogen
(P) Phosphorus
(K) Potassium

NPK numbers refer to the percentage of each nutrient in the packet. Below is a brief overview on these elements and how they benefit your plants. These three are referred to as macro nutrients there are also micro nutrients like copper, magnesium, sulfur etc which the plants also need so it’s a good idea to buy a fertiliser that includes these.

(N) Nitrogen Image1

Nitrogen is associated with vigorous leafy growth and is a key component in chlorophyll which is responsible for Photosynthesis which is the process plants use to convert the suns energy into food.

  • Nitrogen is mobile in the plant so the plant can move it to where it is needed which is often the new growth and why deficiencies are often seen in the older growth.
  • Nitrogen helps the plants to grow quickly and increase the production of seeds and fruit.
  • Nitrogen improves the quality of leafy vegetables.
  • Nitrogen deficiency symptoms include reduced plant growth and a pale green to yellow colouring of the foliage often on older leaves first.
  • Too much nitrogen results in leggy or stretched growth and weak stems.
  • Nitrogen in the soil can also come from organic material that is mineralised by microbes into a form that can be taken up by plants.

(P) Phosphorus Image2

  • Phosphorus is associated with plant reproduction improving flowering, Seed and Fruit production.
  • Helps develop a strong root system.
  • Like nitrogen, phosphorus (P) is an essential part of the process of photosynthesis. And is involved in the formation of all oils, sugars, starches, etc.
  • Phosphorus is mobile in the plant so the plant can move it to where it is needed which is often the new growth and why deficiencies are often seen in the older growth.
  • Deficiency symptoms are purplish leaves, retarded growth, fruit and flowers are poor. Premature drop of fruits and flowers may occur
  • Can be toxic to some Australian native plants like Banksia.
  • Helps improve general plant strength and resistance to stress.
  • Can also be found in Blood and Bone fertiliser and Super Phosphate.

(K) Potassium Image3

  • Plays a key role in a wide range of physiological processes.
  • Helps maintain the plant / water balance
  • Potassium deficiency is characterized by reduced plant growth and a yellowing and/or burning of leaf edges.
  • Potassium is mobile in the plant like Nitrogen and Phosphorus
  • Potassium helps in the building of protein, photosynthesis, fruit quality and disease resistance.
  • Can increase strength and survival in Winter.
  • Potassium is necessary for formation of sugars, starches, carbohydrates and cell division in roots and other plant parts.
  • Improves stem rigidity.

About Warren Downes

Warren has been commercially growing plants for over 26 years in his wholesale nursery, Downes Wholesale Nursery and in that time has developed a wealth of knowledge of plant growth and the horticulture industry.

Warren is also a keen creator and educator on YouTube check out his Woodworking channel.

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