Choosing the right fertiliser 

Fertilisers provide plant nutrients. But what nutrients do your plants need and how do you know what to use?

Garden centre shelf full of an array of different fertilisers
I should mention right up front that this article has been a long time in the making. There has been much debate at Quest for Veg HQ about what to include and how to make it easy to understand. We have tried to produce a simple, readable guide to a complicated and sciency area. If you have any questions or suggestions, please let us know.


Why do we even need to use fertilisers?

We all need food to keep us alive. It’s especially important that we get good nutrition when we’re growing. And it’s the same for plants.

Nutrients are usually available to plants in the soil. They can be created when organisms decompose – such as when plants shed their leaves or animals or bacteria die. However we gardeners, well, we do like to tidy up, don’t we? And we also take away the plants we are growing as crops. All of this activity means we’re reducing the nutrient content in our soil.

We can replace some of these nutrients by adding well-rotted organic matter, also known as garden compost. But this doesn’t replace all of the nutrients our crops need. Not only that, some plants, such as carrots, prefer soil with very little organic matter so adding compost isn’t always an option. That is why we need to add nutrients by applying fertilisers.

What’s in a fertiliser?

All fertilisers should have an analysis label showing what they contain. Here’s a picture from a tub of New Horizon All-round Fertiliser which we were sent by Westland to try out.

snapseed-25

You can see that the label says 6-2-4. That’s shorthand for what is explained further down on the label

  • Nitrogen (N) – 6%
  • Phosphorus (P) – 2%
  • Potassium (K) – 4%

This analysis is sometimes referred to as the NPK after the chemical symbols for the three elements.

Nitrogen , phosphorus and potassium are the most important elements for plants. Each encourages the plant to grow in certain ways:

  • Nitrogen – leaves and shoots
  • Phosphorus – roots
  • Potassium – flowers and fruits

So you will probably want to use different fertilisers at different points in the growing season depending on what you want the plant to do.

For example, Gro-Sure tomato feed  (another product sent to us by Westland to try out) is 6-3-10 because it is designed to encourage tomato plants to produce fruit.


Any fertiliser containing 10% or more of any element is very strong and should be handled with care. Follow the packet instructions.

What nutrients do plants need?

Plant nutrients are divided into 3 categories: primary macro nutrients, macro nutrients and micro nutrients (or trace elements).

Plants need these nutrients in diminishing quantities in relation to the categories. They need more of the primary, less of the macro and a little of the trace elements. Here is a list of some of the elements in each category.

  • Primary macro (main) nutrients
    • Nitrogen – N
    • Phosphorous – P
    • Potassium – K
  • Macro nutrients
    • Magnesium – Mg
    • Calcium – Ca
    • Sulphur – S
    • Iron – Fe
  • Micro nutrients (trace elements)
    • Boron – Bo
    • Manganese – Mn
    • Copper – Cu
    • Molybdenum – Mb
    • Zinc – Zn

The New Horizon All-round Fertiliser is known as a complete fertiliser because it contains all of the primary macro nutrients.

You can also get incomplete (or compound) fertilisers that contain more than one of the primary macro nutrients. For example, Ammonium phosphate (11-48-0), which you might use, for example, if you were planting trees and wanted them to produce leaves and establish roots.

Then there are straight fertilisers that contain only one of the primary macro nutrients such as sulphate of ammonia (21-0-0) which provides nitrogen.

A garden centre display showing mainly straight fertilisers

A garden centre display showing mainly straight fertilisers

If your plant is looking unhealthy even though you have given it enough soil to grow in, have kept it watered and there are no obvious signs of pests, you should consider whether it is suffering from a nutrient deficiency. This could show as yellowing or blueing of the leaves, for example, depending on what is deficient, although there are all sorts of signs to look out for.

A leaf showing yellowing which could indicate nutrient deficiency

Yellowing of the leaves can indicate a nutrient deficiency

This is a complicated area and you would need to do some research. One obvious place to turn to is the internet but bear in mind that not everything you read online is accurate!

We have produced a simple overview of some signs of nutrient deficiency and excess, which you can download here.

What form to choose?

Fertilisers come in a array of different guises, including:

  • liquid
  • powder
  • soluble powder
  • soluble crystals
  • granules
  • prilled

They can also be:

  • immediately available – usually as a liquid or soluble type (dried blood)
  • slow release – these usually need to be decomposed by soil organisms (fish and bone meal)
  • controlled release – this type has a clever coating that allows water to enter the pellet and wash out a little fertiliser when conditions are right

Plants are only able to take in nutrients in solution. So, if you want to feed the plant immediately, choose a form that is either already a liquid or that you make up into a liquid form.

You might choose a fertiliser that is immediately available because you want to give the plant a boost, because the plant has been damaged or because you want to encourage it to grow in a certain way (such as producing a heavier crop).

Generally, you apply liquid fertiliser to the soil but you can get foliar feeds. These are applied in solution to the above ground parts of the plant on a dull day or late in the evening. Only fertilisers marked as foliar feeds should be used in this way.

Slow release vs controlled release fertilisers

Slow release fertilisers release their nutrients through the action of organisms in the soil.

Soil contains an array of different organisms, the majority of which are microorganisms. One teaspoon of soil contains more microorganisms than there are people on the planet.

Microorganisms help break down organic material and other substances into the elements that are plant nutrients. When the conditions are correct for the soil organisms to be active, the plants will also be actively growing and requiring these nutrients.

So that’s OK in the garden or allotment because we have a diverse community of soil organisms. But within a contained area such as a hanging basket, tub or window box, we often use a sterile growing medium to reduce the chances of contains disease in these small growing areas.

As you may have guessed, slow release fertilisers wouldn’t work in sterile growing mediums because there are no organisms. Therefore we need a different means of releasing the nutrients over a period of time. This is achieved by the use of a controlled release fertiliser.

When shouldn’t you use fertilisers?

You can have too much of a good thing. Applying too much fertiliser can slow down or stop the plant absorbing water and in severe cases cause the water in the roots to move back into the soil. So apply your fertiliser(s) at the recommended rates.

And don’t use fertilisers on seeds. The higher concentration of nutrients in the water surrounding the seed will make it difficult for the water to be absorbed by the seed roots.

Compost designed for seeds (seed compost or general/universal compost) should have very low nutrient content. If you use a universal compost, start feeding plants within the first week of pricking out or potting on.

Other ways of providing plant nutrients

There are a few other notable ways of providing plant nutrients that we should mention but which really need their own blog entries.

Garden compost – a science all of its own. There are right and wrong ways to go about making compost and everyone has their own way of doing it.

Homemade teas or tisanes – infusions made by steeping plant materials in water. Again many gardeners have their own recipes. A popular tisane uses nettles and people often think of it as a way of getting their own back on weeds by turning them into fertiliser. However, care needs to be taken as not all weeds can be used and it could be a means of spreading disease.

Mycorrhiza – fungal organisms that join onto plant roots and help the plant to take up water and nutrients. They can also help plants ward off pests and diseases. You can buy mycorrhiza in packets in the garden centre.

We will expand on these in later blogs.

We have produced an information sheet summarising points from this blog with a little bit more about how soil types can affect nutrient uptake, and a list of some leading fertilisers and the primary macro nutrients that they contain.

Let us know what you think

Do you feed your plants? What do you use? Let us know. And if you have any questions do get in touch.

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5 thoughts on “Choosing the right fertiliser 

  1. Reblogged this on Site Title and commented:
    Taking some of the mystery out of what on earth it all means. I’ll never look at the fertiliser shelf at the garden centre in the same way again. AND this blog has a very handy info sheet for further research. Loads of effort clearly been put into this and much appreciated.

    Like

  2. Interesting post. I pretty much use Comfrey liquid for everything (ornamental and edible). If you use the Bocking 14 variety where the seeds are sterile you won’t have to deal with masses of seedlings. One plant will be all you need and it’s easy to make the liquid. It has NPK, and is particularly high potassium so great for plants producing fruits, including Tomatoes. And, it means you grow your own fertiliser, and that you don’t support the multinational fertilising industry, including companies like Monsanto. And to top it off, bees love it, so you are feeding them as well as your plants 🙂

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    • Great tip! thanks for sharing. We intend looking, in more detail, at other methods of providing nutrients at a later date. So keep an eye out. Regards Andrew

      Like

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