It is important to understand what nutrients are needed for each stage of plant growth.

Essential Soil Nutrients

The three most important elements required in the greatest quantities are:

Nitrogen - for leaf and shoot growth. This can easily be leeched out of the soil but, if there is too much present, the plant will develop too quickly leading to soft growth.

Phosphorous - for root development. Less is required than Nitrogen but any deficiency leads to stunted growth with the leaves turning blue. 

Potassium - for flowers and fruit. Leaves will yellow around the edge if there's not enough in the soil, the plant will become stunted and both fruit and flowers will be disappointing. 

Other important elements that soils must contain are Magnesium, Calcium and Sulfur. 

Further trace elements are also required but in very small quantities, these include: Iron, Zinc, Copper, Manganese, Boron and Molybdenum. All can be found in manure, compost and other organic matter, so deficiencies should be rare. Raspberries are susceptible to lack of iron but treating the soil annually with a seaweed fertilizer should solve any problems.

Make sure that the elements are present in correct proportion as an over supply of one can have an adverse effect on others, magnesium for example will become inactive if there is an overload of potassium.

What Fertilizer Is Best?

If you garden organically, in theory all the goodness that plants require will be obtained from manure and compost. However other fertilizers may be required in certain circumstances.

For example, you may not have access to very much compost or manure, or the soil might be very short of one particular essential nutrient.

General:
Blood, Fish and Bone Meal (apply no more than two weeks in advance of planting or sowing); Liquid Extract of Seaweed (rich in trace elements that might be lacking in fish, blood & bone)

Nitrogen: 
Hoof and Horn (excellent for a boost of nitrogen in the spring); Dried Blood (fast acting, nitrogen rich); Fish Meal (good for nitrogen and phosphorous)

Phospherous and Potassium:
Bone Meal (slow release that encourages root growth); Superphosphate; Wood Ash (contains some potassium and phosphate)

Magnesium:
Seaweed (liquid and meal)

Sulphur:
Calcium Sulphate (Gypsum)

So nutrient rich fertilizers do play a part in organic gardening but they must be compatible with the requirements of plants and the soil. As the late Geoff Hamilton once said,'If you supply nature with the tools of her trade, she will do the rest'.