Just like us, plants need a balanced diet.
They obtain this in part from soil that has been enriched with bulky manures, but such manures do not contain large quantities of food - their value lies more in their soil-conditioning properties.
For this reason, fertilizers are used to provide plants with nutrients in concentrated form.
General fertilizers Plants have a need for three main foods - nitrogen (N), phosphates (P) and potassium (K).
I always think of these as the starter, the main course and the pudding.
Phosphates are the starter - they promote root growth.
Nitrogen is the main course - it fuels the development of leaves and shoots, making the plant grow well and stay green.
Potassium, or potash, is the pudding - it encourages flower and fruit development when the other two foods have done their bit.
Fertilizers described as general or compound have all three foods or nutrients in balanced quantity.
As well as these three main foods, there are others described as trace elements, which are the plant's equivalent of vitamins.
Things such as magnesium, boron and molybdenum are all present, to a variable degree, in well-fed soil and in general fertilizers.
Organic or Inorganic? Organic fertilizers are derived from carbon, so at some time have been living material.
Inorganic fertilizers are chemically derived.
Both can provide plants with the nutrients they need to grow.
The difference is that organic fertilizers must be broken down by soil bacteria before they are in a form that can be utilized by plants; inorganic fertilizers can be absorbed as soon as they are dissolved in water.
Inorganic fertilizers are thus usually quicker-acting, but organic fertilizers improve bacterial activity in the soil and keep it healthy.
Use organic fertilizers to keep soil fertile and inorganic fertilizers to give plants a quick boost.
Growmore is an inorganic general fertilizer; pelleted poultry manure is an organic general fertilizer.
Application rates for all proprietary fertilizers will be printed clearly on the container.
Sequestered Iron Plants that dislike alkaline or chalky soils - rhododendrons and camellias, for instance - usually do so because they find it difficult to extract iron.
This results in a yellowing of the foliage.
Sequestered iron is a plant tonic which contains iron and trace elements in a form that they can assimilate rapidly, so it is a useful tonic for lime-hating pot plants being grown in hard-water areas.
If you have yellowing lime haters growing in the garden, dig them up and plant them in containers of lime-free (ericaceous) compost.
Liquid Fertilizer Feeds Proprietary liquid feeds, which are diluted in water before being applied to plants, go into action faster than feeds in powder or granular form but they are not so long-lasting.
Apply them to container-grown plants once a fortnight in spring and summer.
Foliar Feeds The fastest-acting plant foods are foliar feeds that are diluted in water and sprayed on to the leaves, preferably on a dull day so that they do not evaporate too quickly.
When to Feed As a general rule, garden plants are fed once in spring and once in summer, while they are growing.
In winter they do not need feeding, but sulphate of ammonia (high in nitrogen) can be sprinkled around fruit trees to give them an early boost in early spring.
Nitrogen is the food most readily leached out of soil by water and this will replace losses due to winter rains.
All plants will enjoy a sprinkling of general fertilizer in early spring to give them a good start.
Do this if you do nothing else.