or what the numbers on the side of your box of fertiliser mean.
The premise that Nitrogen, Phosphorus and Potassium are fundamental to healthy plant growth has been around since the early 1800's. Modern science tells us that plants need more than these basic three elements to thrive. Trace elements such as magnesium and calcium (to name two of many), good soil aeration, and of course, water and sunlight are also essential. However, every packet of fertiliser has an NPK rating somewhere on the box, so it's useful to know what it means. The information grid (below) gives brief explanation of NPK. The rating you'll find on fertiliser packets tells you how much, as a percentage of each element is in that packet.
All round feeds like Growmore will have a balanced NPK (7-7-7). Specialist feeds like Tomorite will have a bias towards a particular element (4-3-8). Make sure you get the right feed for your plants and remember - don't over do it! Read the instructions and don't overfeed your plants, it won't benefit them