WHAT'S ALL THIS HERE 'NPK' STUFF ON MY BOX OF PLANT FOOD?
As well as air and water, plants need a variety of chemicals to build healthy growth. Some chemicals, like boron or magnesium, they only need tiny amounts of, and these are called 'trace elements'. The bulk of what plants need is made up of nitrogen - usually in the form of a nitrogen compound called "nitrates", phosphorus, in the form of "phosphates", and potassium, usually called 'potash' by gardeners. Think of them as like the protein, fat and carbohydrates in your food; three vital components, which are present in different proportions in different foods.
So, different plant foods have these elements in different proportions. In order to set out the proportions on the packaging which is the same from one product to another, to make comparison simpler, the proportions are set out using the chemical symbols for those three elements : N for nitrogen, P for phosphorus and K for potassium. (Don't ask why it's K for potassium, it's too boring to trouble you with). The numbers give the respective proportions of the different nutrients and are always given in the same order; nitrogen, then phosphorus, then potassium, usually set out something like 20:4:8. This would be a 'high nitrate fertiliser'; a tomato food might have numbers more like 7:7:14, indicating higher levels of potash
As well as air and water, plants need a variety of chemicals to build healthy growth. Some chemicals, like boron or magnesium, they only need tiny amounts of, and these are called 'trace elements'. The bulk of what plants need is made up of nitrogen - usually in the form of a nitrogen compound called "nitrates", phosphorus, in the form of "phosphates", and potassium, usually called 'potash' by gardeners. Think of them as like the protein, fat and carbohydrates in your food; three vital components, which are present in different proportions in different foods.
So, different plant foods have these elements in different proportions. In order to set out the proportions on the packaging which is the same from one product to another, to make comparison simpler, the proportions are set out using the chemical symbols for those three elements : N for nitrogen, P for phosphorus and K for potassium. (Don't ask why it's K for potassium, it's too boring to trouble you with). The numbers give the respective proportions of the different nutrients and are always given in the same order; nitrogen, then phosphorus, then potassium, usually set out something like 20:4:8. This would be a 'high nitrate fertiliser'; a tomato food might have numbers more like 7:7:14, indicating higher levels of potash
The Royal Horticultural Society is full of good advice about fertilisers. Click the green bars to go to their website with a list of articles which you can read, or a guide to fertiliser labels.
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Lush, leafy MARROWS and SQUASH are among the plants that need a lot of NITRATES to grow well
To keep it simple, you can remember the following; NITRATES for leaves, POTASH for flowers and fruit, and PHOSPHATES for roots.
When you are feeding your plants, or preparing the soil for a new crop, you can adjust the fertiliser you choose according to the results you're looking for. For example, tomato feeds are always high in potash, to encourage flowers and fruit. But we use farmyard manure, high in nitrates, for leafy cabbages and lettuce. |