NPK & the Fertiliser Code

What does NPK mean

What does NPK mean?

“Any old food will do! Right?”

No, it’s not quite as easy but it’s simpler than you think.

You probably know that many plants need feeding during the growing season, especially those grown in pots, toughs and hanging baskets. However, if you know NPK & the Fertiliser Code you can match the correct fertiliser with the correct plants and their stage of growing.


NPK represents the element present in the feed & stands for the Nitrogen (N), Phosphates (P) & Potash/Potassium (P) and are followed by a series of 3 numbers, such as 7:7:7. These numbers show you the relative proportion of each, an indication of how much there is of each element and are in the order for NPK.

Nitrogen, Phosphates & Potash are called major elements. many fertilisers also include ‘micro-elements’ which are little like vitamins from a plant diet point of view. More later.

What do Nitrogen, Phosphates and Potassium do for plants?


Nitrogen is the food that mainly develops leaves. They are essential for the production of amino acids, that produce proteins for cell growth. That means bigger and more leaves. Ideal for leafy vegetable plants such as cabbage & for growing young plants.

Higher Nitrogen proportions have a ratio such as NPK 10:3:3.


Phosphates are essential for cell membranes and DNA production. THe proportion for P (Phosphates) is the middle number of the 3.

Potassium (Potash)

Potassium is essential for the production of enzymes involved in respiration and photosynthesis. For the gardener the big benefit is that Potash (Potassium) is the increased initiation of flower buds & more fruit where it’s needed. It also toughens foliage up that may help them resist diseases.

High Potash feeds is a phrase you may hear. High Potash feeds have the proportions such as 3:3:12

Types of fertiliser


Compound fertilizers are fertilizers containing more than one of the three primary nutrients–N:P:K. They may also contain one or more micronutrients.

Typically a compound fertiliser that you might use will include Growmore, Blood, Fish & Bone, Rose food, and tomato food.


Straight fertiliser are fertiliser that include single nutrient. A few examples of straight fertilizer are urea, ammonium sulphate and potassium sulphate.

Straight fertilizers can be used for feeding but This can lead to oversupply of one nutrient and/or deficiency of other nutrients. The advantage of a straight fertilizer is they can be used as quick fix to overcome a deficiency.

Inorganic Fertilisers

These are fertilisers that are produced from man made constituents. These fertilisers have been produced for many years and produce standard mixes every time.

Organic Fertilisers

These are plant feeds derived from natural materials and are often approved by the Soil Association, GrowOrganic or Organic Farm and Growers.

Do not confuse the term ‘Organic Chemicals’ that we learned at school, which is a term for chemicals that have a carbon, hydrogen & oxygen that we are now aiming reducing to reduce carbon dioxide release.

Controlled Release Fertilisers

Controlled Release Fertilisers, sometime called ‘Slow Release’ fertilisers, are plant foods that have a resin coating that controls the rate that the plant food is available to the plant roots via soil or compost. The resin coating allows soil/compost water inside the small coated balls of compound fertiliser. The water dissolves the fertiliser and it can only leave the ‘ball’ when the temperature rise enough for the concentrated liquid to leave. The higher the temperature the more food is released. It reacts to seasonal and daily temperature change. The coatings vary depending upon the length of feeding time required. Gardeners controlled release usually feed for upto 6 months, equivalent to the UK growing season.

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)

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