When you pick up a plant food in the garden centre, you scan the details and there is that code NPK. What does NPK mean? Know the basic facts and change your growing success. Plant food or fertiliser (fertilizer in the U.S.) provide the nutrients for healthy plant growth.
The NPK letters are usually displayed in this way on the side or back of a pack or bottle of plant food
'Compound fertiliser NPK 5 - 3 - 5'
‘Compound’ means it’s a mix of materials.
‘NPK’ stand for Nitrogen, Phosphorous, K for Potassium or Potash.
‘5 – 3 – 5’ are the ratios of NP&K in the pack or bottle. In this case 5 parts Nitrogen, 3 parts Phosphorous, 5 parts Potash.
The ratios vary depending upon the intended use of the plant feed, and the amount of each source that is added to the mix.
What are the the components used for in your plants?
Nitrogen is the fuel for growth. It keeps the plant growing during the right season. It’s naturally found in many soils. High proportions of Nitrogen are used to advance leaves and growth and are the feeds for leafy growth. Ideal for leafy vegetables such as cabbage. Also useful for early season growth in plants such as fuchsias before changing to a different feed to induce flowers. See more later.
A high nitrogen food will have a ratio typically like 11 – 5 – 6.
Phosphorous is the foundations and frame work food for plants. Plants use Phosphorous to build maintain the structures and systems for health growth and fruitfulness. It’s essential for all forms of growth.
Potash, or Potassium is the flower and fruit food. It’s essential in combination with the other nutrients above but it’s the ‘quality food’ improving flower, fruiting, colour, and taste in edible crops.
A high Potash food will have a NPK ratio more like this 7 – 3 –12.
How plants absorb nutrients
Plants absorb most of the nutrients, needed for growth and fruitfulness, dissolved in water and they are drawn in via their small feeding roots.
Plant food is sold as granules/power, liquids and soluble feeds. Granules are taken from the pack and scattered over soil surface or mixed into the soil or compost. Liquids might be ready diluted to pour onto compost and soil directly, or as a concentrate that you dilute with water into a form that is suitable for the roots to absorb. Soluble feeds are like sugar added to coffee or tea. They dissolve when added to water create a liquid feed to be watered in the same way liquid feeds work.
A special form of granular feed are controlled release/slow release feeds. These are the easiest way to feed many plants especially any you grow in pots. The are applied ONCE a season and release the food to the plants gradually as the plant needs them and in relation to soil temperature. As soil and compost warms up in spring more food is provided when the plants need it.
When to use
Most soils have enough of the NPK elements in them already or may benefit from an initial feed at planting with a general feed such as Fish, Blood and Bone. The NPK ratio 5 – 3 -5 is found here an is a balanced feed.
High nitrogen feeds are very useful in the early months of growth for new young plants and also for brassicas (cabbage family) in spring when over wintered leafy vegetables are grown.
High Potash feeds are probably the ones that have the most obvious impact for growers. Feeding spring flowering shrubs and trees, including fruit trees with High Potash food will initiate and produce more flowers (and more fruit where applicable) when fed at the end of June. Read more here. Rose Food In June is my motto. Rose food is a High Potash plants food. When used with roses in borders new flowers are initiated with in weeks. With Spring flowering shrubs and fruit trees bud initiation takes place in July & August for flowers that open in spring.
Pots and Containers
Many of us grow plants in pots, containers and hanging baskets. Where as garden plants growing in garden soil can draw nutrients from the soil as they need it, plants grown in pots can’t. Compost on pots and containers have no supply to draw on once the food in the compost has been used up. You MUST feed plants in containers and pots. This includes indoor plants.
The easiest way to feed pots and containers is with the controlled/slow release granules. Avoid using using compound or straight* granules feeds in pots unless instructions are included on the pack as they may scorch and damage the small feeding roots, stop them working and will not only starve the plants of food but also essential water that the plants need.
Water and plant foods
Since plants mostly absorb food dissolved in water via their roots, plants short of water plants are also starved of food. This is most obvious in hanging baskets. There may be sufficient food being ‘stored’ in the compost but unless it can travel into the roots as a dilute liquid feed plants don’t flourish, look weak, and have few flowers. Regular watering is essential.
Helping plants absorb water in compost is improved when compost contains ‘water holding agents’. These are often included in bags of composts produced by your favourite supplier. A good example is Miracle-Gro Moisture Control compost.
You can add these water holding agents to your own compost and are readily available.
A basic understanding of plant foods and feeding will change the results you achieve with very little effort. Enjoy the results.
*concentrated forms of the single components of compound feeds)