Posted: 18/03/2015 at 11:07
The general idea is to get your soil in good heart first by digging in well rotted compost to improve soil structure, this will encourage worms and so the cycle goes on.
After this, a top dressing of choice may be added at planting time...this may be blood fish and bone meal, Vitax 4. There other fertilizers available for specific purposes such as bone meal which is a slow release fertilizer and used mainly when planting out trees & shrubs.
Truth is you need to understand the NPK table, which explains the three main plant foods ie Nitrogen, Phosphates and Potash. Then familiarise yourself with the requirements of the crop you are growing and choose accordingly.
Bit of a pain, but it takes much of the guesswork out of it.