‘N-P-K’ …What does this mean? Fertilisers Explained.

This is something that mystified me, until it was explained to me at college a few years ago, when I took my RHS Horticulture Course.  When it’s explained properly, it’s not really that hard to understand at all, though some books make you feel like you need a scientific degree just to open the page, let alone read it.

As spring is around the corner, I thought it would be nice to take a look at ‘N-P-K’, in the hope that my explanation will help to unravel the mystery surrounding it.  I’ll apologise now for those who know this already, but it’s great to refresh your knowledge sometimes.



So what is ‘N-P-K?’

A German scientist called Justus Von Liebig (1803-1873), came up with a theory that Nitrogen, Phosphorous and Potassium are mainly responsible for healthy plant growth.

Of course it isn’t quite this simple, as there are lots of things that promote healthy plant growth, such as soil structure, soil pH, and trace elements (I’ll cover them another time).  But mainly, when choosing a fertiliser, it’s the Nitrogen, Phosphorous and Potassium (N-P-K) that we look for.

Fertilisers are used to improve plant growth.  If your soil is healthy, you don’t necessarily need a fertiliser, but by using them you may produce a higher yield of fruit and vegetables or a better show of flowers.  Though it must be noted, you can over feed plants too, which can also cause problems, so always follow the product instructions.



If you look on your box or tub of fertiliser, it will have three numbers.

The picture above shows: Fish, blood & bone  (4-7-4)

The numbers reflect the percentage of the nutrient, per pound of plant food. The first number represents Nitrogen (N), the second number represents Phosphorus (P) and the third number represents Potassium (K). 

So you can see from the picture above, the feed is slightly higher in Phosphorus (P).

(Please stay with me  and try not to switch off, as these figures really do mean something to your fruit, vegetables and flowers).




Nitrogen (N):

Nitrogen promotes a good, green leafy growth.  This is particularly important for green leafy vegetables like cabbages, spinach etc.  If you have a nitrogen deficiency, you will most likely see spindly yellow plants and leaves, occasionally with a pink tint.  A nitrogen deficiency will cause stunted growth.

High nitrogen feeds include sulphate of ammonia and poultry manure.



Phosphorus (P):

Phosphorus promotes strong healthy root growth and shoot growth.  It is rare to have a phosphorus shortage in the soil, but it can happen if you have a heavy clay soil or suffer with a very high rainfall.  If your plants have a phosphorus deficiency, they will be slow to grow and have a dull, yellow foliage.

High phosphorus feeds include superphosphate or bonemeal.



Potassium (K):

Potassium helps promote better fruit and flowers and the overall hardiness of the plant.  If your plants have a potassium deficiency, the leaves will become yellow, with brown around the edges and you will have poor fruit and flowers.  Shortages of potassium are more likely to be found on light sandy soils, as a clay soil will hold potassium well.

A high potassium feed is sulphate of potash.



So what is the difference between a ‘Compound Fertiliser’ and a ‘Straight Fertiliser’?


A ‘straight fertiliser’ has just one nutrient in it

e.g. sulphate of potash (which is used around fruit bushes in February / March).

A Straight Fertiliser

A Straight Fertiliser


A ‘compound fertiliser’ is a mixture of different nutrients:

e.g. Blood, fish and bone (organic) or Grow more (in-organic).

A Compound Fertiliser

A Compound Fertiliser


How do you apply fertilisers?

There are different ways to apply fertilisers, but these are the three main ones:

  • Top dressing:  Sprinkle the fertiliser around the plant and hoe or lightly fork into the ground.  Be careful not to get any fertiliser onto the leaves of the plant, as this can cause scorching. 
  • Base dressing:  This is where you rake the fertiliser into the soil before you plant into it.  I usually spread Blood, fish and bone two weeks before I plant into the soil.
  • Liquid fertilisers / soluble powders:  These can be watered straight onto the plant roots.  A good example of this is a tomato feed, which is watered onto the plant roots each week or fortnightly.  The nutrients are available to the plant straight away.  Again, if you water directly onto the leaves, it can cause scorching.


My barrel of 'Comfrey Feed'

My barrel of ‘Comfrey Feed’


To finish off with, I just wanted to remind you that fertilisers should not be used as an alternative to soil conditioners.  Leaf mould, compost and manure all help to break up the soil and improve the soil structure.  This in turn, will help the plant to take up the nutrients, which makes them stronger and able to provide us with our wonderful, fruit, vegetables and flowers.

I had some new organic manure delivered on Tuesday, at my allotment:


  It smelt very fresh!  I covered it, to stop the nutrients leaching out when it rains.

I won’t be using this manure until September, so this will give it a good six months to continue rotting down.


 My husband had the day off work to help me with the delivery, ready to clear any manure that dropped onto the road.  The delivery man was great though and managed to drop the whole load exactly where I wanted him too, which made it very easy for us.


Am I the only woman that gets excited over a ton of manure?

Thank you for reading my blog today.

I’ll be back on Monday.