All the Minerals that a plant needs have to be absorbed from the Soil along with the Water it takes in.

When we feed a Plant it is these Minerals that we are supplying.

The Main Minerals are:

Nitrogen: usually abbreviated to "N" - the most important Mineral and essential for growth in general

Phosphorous: usually abbreviated to "P" - essential for actively growing areas in the Plant

Potassium: - usually abbreviated to "K" - necessary for the production of the colours in Plants (other than green).

Always look for the N:P:K ratio on a box of fertilizer to show you what it can do.

e.g. Growmore has an N:P:K: of 7:7:7 and is a good all-round fertilizer.

       Bonemeal has an N:P:K of 0:20:0. Note that it only contains Phosphorous.

       Look for a high level of "K" in Tomato/flower fertilizers, to provide the colour.

As Well as these 3 main Minerals Plants require a whole raft of other Minerals.

Because these other Minerals are only required in small quantities they are known as Trace Elements. e.g. Iron, Calcium, Magnesium, Copper, Zinc. Molybdenum etc.

It is rare for the soil to run out of these Minerals - but it can happen, particularly in older gardens, or in tubs containing old compost that hasn't been fertilized regularly.

Types of Fertilizer:

Soluble: e.g.  Phostrogen, Miracle-Gro, the Chempak series.

These are dissolved in water and applied with a watering can or sprayer.
They are very quick acting - because they are soluble they can go right to the roots -
but because they are soluble they are easily washed out of the soil by rain.

Powder or Granular: e.g. Growmore, VitaxQ4, Bonemeal, Fish Blood & Bone.
Require to be broken down to a greater or lesser extent so take a bit longer to work but don't get washed out of the soil so easily.

Manures:
Horse, Cow, Farmyard or Chicken manures are very popular.
The Fertilizer content in manures is on the low side but they are an excellent source of Humus. "Fresh Manure" contains bacteria that can provide heat to the soil which can stimulate earlier root growth, particularly early in the season when the ground is still cold.
Freah Manure is hard to come by nowadays - there must be many a grateful neighbour!

Specialist Fertilizers:

Ericaceous: specially formulated without using Lime for Lime hating plants. Essential for Plants such as Heathers, Azaleas, Camellias etc.

Tomato: high in Potassium to bring out the colour.

Slow Release: fertilizer is given out over a period of time in a controlled manner. Some brands will feed all season.

Lawn Fertilizer: high in nitrogen for quick growth.

Potato: high in nitrogen for quick growth. Difficult to get nowadays - use double strength Growmore as a substitute.