How To Understand What NPK Means
Basic plant nutrients are essential for a healthy happy garden
N nitrogen P phosphorus K potassium
Nitrogen is needed to grow leaves and the green foliage.
Phosphorus is needed to produce strong roots and shoots.
Potassium is needed to produce fruit and flowers; it also regulates water and nutrient movement in the cells.
Plants require different nutrients depending on what type they are and also at different stages of their life. As a general rule if you don’t know what your plant requires then feed with a general fertiliser such as fish, blood and bone or mulch with a well-rotted compost. There are also specific fertilisers available for specific plants; such as roses, conifers, tomatoes, houseplants and orchids. Don’t feed with a fertiliser with a specific purpose if it is not designed for that plant.
If you need to know what to feed your plants then ask advice at the garden centre, as it is quite a complex subject; for example, bedding plants need a high nitrogen fertiliser at the start of the season to grow a nice healthy plant, then switch to a high potassium feed about April as this will encourage the production of flowers. Some vegetables such as tomatoes and chillies need a high potassium feed once the first fruits appear, green vegetables such as cabbages and salad leaves need a high nitrogen feed and root crops such as carrots and turnips need a high phosphorus fertiliser.
Only apply fertiliser at the recommended dose as too much could turn the soil toxic and kill the plants. Too strong a dose can also reverse the osmotic pressure, with the soil taking water and nutrients from the plant instead of the other way around.
Animal manures are generally low in nutrients but good for adding humus and structure to the soil. Make sure that they have been rotting down for at least 2 years as they need to have the ammonia leached out. Too much ammonia in the manure will result in the roots or any tender growth being burned.
Cow and horse manure has an NPK value of: 01-00-00 which means that it is only good for adding structure and humus.
Poultry manure has an NPK value of: 03-02-02 which is a low value all round feed.
Bone meal has an NPK value of: 04-12-00 which makes it invaluable for developing the basics of roots and shoots.
Fish, blood and bone is a good balanced fertiliser with an NPK value of: 05-05-06.