Hmmmm, very hard as we all have different soil etc....but as a genral rule:
There are three main constituents you need to worry about:
Nitrogen (N), Phosphorus (P) and Potassium (K).
You will see on the back of fertilizer packets the N:P:K ratio. Such as on Growmore which is 7:7:7.
See here for more details:
http://www.rhs.org.uk/advice/profiles0306/fertiliserlabel.aspA equally balanced feed is ideal for a general feed but some plants need more of one and less of another ingredient.
Nitrogen
Promotes vigorous lush green growth ideal for plants such as brassicas, lettuce and leafy crops but is also needed for general plant growth. Too much will promote weak sappy growth and in extremes can scorch the plant. Also too much for fruiting plants will reduce the crop.
Nitrogen deficiency is often displayed as poor leaf colour.
Phosphorus
This element is needed during periods of rapid growth, a lack can be shown up as pale growing points on the plant. It is also a vital ingredient for good root growth
Potassium
Potassium is important to the plant as it is a key element to help the plant photosynthesise. It is also important for flowering plants and you will find it in a high ratio in tomato feeds.
Deficiency's often shown up through bluey tinged foliage and poor growth rate.
So you can tailor feeding to the crop to get the very best.
But you may not need too, I tend to use a mix of general fertilizer such as a general growmore, some high N blasts such as pelleted chicken manure or Sulphate of Ammonia/Ammonia Sulphate, some slow release feed in the form of Fish blood and Bone and finally dedicated tomato feed. This is also coupled with lots of well rotted manure and compost to boost the natural balance of the soil and it's structure.
The approx NPK's are:
Growmore
7:7:7
Chicken Manure
4:2.5:2.5
Sulphate of Ammonia
20:0:0
Fish blood and Bone
6:6:6
Tomato Feed
4:4.5:8
Obviously on top of this there are other trace elements such as calcium, boron, manganese, etc. that are all needed for general health. The riskiest one in my list to use is the neat Sulphate of Ammonia, it can do more harm than good if not careful.
Also different soil types have different needs so that needs to be taken into account as well.
Hope that helps a little.
Jerry
