What does NPK mean on a bag of fertiliser?

So what does NPK mean? If you’re a seasoned gardener with a grasp of the science behind growing fruit, veg or flowers then you’ll already be checking this when you buy fertiliser for your garden, and you’ll perhaps even be keeping an eye on nutrient levels with a soil test. But if you’re a veg gardening novice, or have become hooked by the houseplant trend that’s boomed over the past year or two, then it’s worth knowing about NPK and what it means for your plants.

NPK: what does it mean?
These letters are symbols from the periodic table – so conjure up what memories you have of school chemistry lessons. N stands for nitrogen, P stands for phosphorus (so far so simple) and K stands for potassium (well, P was already taken…).

The NPK numbers – that is to say the percentages of nitrogen, Phosphorus and Potassium, essential nutrients for plant health – refer to the make-up of the fertiliser.

Nitrogen for foliage health
Nitrogen is important for growth, and also to help plants fight diseases and withstand difficult growing conditions, because it’s critical to the photosynthesis process – it’s what gives plants their green colour. This is why you’ll see plants in need of nitrogen either yellowing or turning a paler green. However, it’s worth exercising restraint.

If you feed too much nitrogen, then you’ll find plants thriving in one sense – with more stems and foliage – but lacking in flowers, fruit or vegetables (and if you feed extreme amounts, you can even kill the plant). It’s also worth bearing in mind that some plants – such as the legumes: beans, peas and others – take their nitrogen from the air, returning it to the soil when they die.

Nitrogen can be washed away by watering or rainfall, so it’s not uncommon for gardens to need more. It’s most needed during the early stages of growth, so make sure you feed early on in the season, rather than late (as it will leach away over winter and only need re-applying in spring).

Phosphorus for strong roots and stems
Phosphorus is another vital nutrient – it’s key to root growth and stem strength, and boosts flower and seed formation. Again, it’s critical to the process of photosynthesis and important for disease resistance. Feed too little phosphorous and you’ll find plants are stunted and have poor numbers of flowers and fruit/vegetables, too much and you’ll find the plant’s leaves yellowing.

Potassium for bumper crops!
Potassium is central to overall plant health, but what gardeners really find important is its positive impact on edible plants – with the right levels, you’ll get higher crop yields and the quality will be better, too. If your plants are lacking in potassium then you’ll notice growth is stunted, and the lower leaves will start to turn yellow.

You’ll notice that chemical fertilisers have higher levels of NPK, while organic fertilisers (like our Soil Association Organic Approved fertiliser, Gro, made from fruit waste and grains) have lower numbers – but this isn’t actually a bad thing at all. Importantly, these organic options tend to have a slower release of nutrients, which is good news for your garden, and are also less likely to burn any leaves which come into direct contact with it (one good reason why synthetic fertilisers come with warnings about using too much).

So now you know what NPK means, give Gro a go and test out the positive impact of these essential nutrients on your houseplants, vegetable patch, flower garden or lawn.