Q. Plants can make most of their food from Carbon dioxide, Water and Sunlight - so why do we still need to feed them?

A. Well Plants can make all of their Food except for Minerals and these they have to take in through their Roots along with their water.

With this constant taking up of Minerals, Soil or Compost can run low in Minerals and Plants soon become unhealthy.
Fertilizers replace the minerals used up by the Plants in the Soil or Compost.

Straight Fertilizers:
These are are simple inorganic chemicals usually with only one active ingredient, two at the most.
They are usually used for boosting the concentration in the soil of the one or two active ingredients that they contain.
e.g. Sulphate of Potash has Potassium as its active ingredient and would be used if the soil was low in Potassium or if we were going to grow plants that needed a lot of Potassium such as Tomatoes.

Compound Fertilizers:
These are manufactured, complex mixtures of Chemicals that provide a wider range of active ingredients than the "Straights".
As such they are better all round Fertilizers.
They can be split into Soluble and Granular/Powder types.

Soluble:
Soluble Fertilizers are easily applied.
Simply dissolve in water and apply with a Watering Can or Hose end attachment.
Because they are already dissolved they can be quickly taken up by the plant's Roots.
Their disadvantage is that because they are used up so quickly they have to be applied more frequently - but then they are easy to apply.
e.g. "Miracle Grow" and "Phostrogen".

Granular / Powder:
Powdered fertilzers have to be scattered by hand - this isn't as difficult as it sounds.
They are slower acting than Soluble ones as they have to be broken down in the soil first.They tend to last longer so need to be applied less often.
Granular fertilizers are simply powdered ones, compacted into granular form, for easier handling and scattering.
e.g. " Rose Plus", "Growmore" and "Vitax Q4".

Slow Release:
This type of fertizer has become very popular in recent years due to the fact that one application can last all season.
These granules of fertilizer are designed to release regular amounts of fertilizer evenly over a period of weeks.

Manures:

Manures are Organic in nature and as a result are considered more environmentally friendly.
They tend to contain less feeding than fertilizers but they have the advantage of also being a good source of Humus.
In raw form Manure contains bacteria that can generate a lot of heat and raise the temperature of the soil thus stimulating root growth even if the air temperature is still cold.
Manure used to be available direct from farms but nowadays farms tend keep their Manure for their own use as it is now considered economic and environmentally desirable to recycle animal waste .

There are four main types:

Farmyard Manure:
As it says this is Manure from farmyard animals mainly cattle.
Provides an excellent source of Humus.
Usually dehydrated and in bags in Garden Centres.

Hen Pen Manures:
Again as it says Manure formed from Hen droppings. Tends to be very acidic.
Usually in Bags in raw form or dehydrated in pellet form.

Horse Manure:
Harder to get nowadays in sealed bags.
An excellent source of humus.

Spent Mushroom Compost:
Consists, usually, of partially decomposed Peat and is an excellent form of Humus.

Farm Manure:
A versatile 100% Organic soil conditioner designed to add humus and plant nutrients to the soil to naturally increase the yield and improve the vitality of roses, flowers, fruit and vegetables.