Since their main benefit is in improving the soil structure, the plant food they supply should really be considered a bonus rather than a source of food. Most manures contain animal waste and should not be applied within six weeks of using lime, otherwise some of their food value will be wasted, and it is possible that ammonia gas can be produced in the soil with harmful effects on your plants. These materials are of organic origin, and usually contain a high proportion of one or more chemical plant foods. Examples include Hoof and Horn, Fishmeal, Bonemeal etc. The chemicals they provide are generally more consistent than the manures, and it is therefore usually easier to select one to provide the food that your plants are short of.
Organic fertilisers tend to break down slowly in the soil, so they are usually quite long lasting.
They are also easy to apply and, being organic, there is little danger of scorching your plants. They are not usually as concentrated as the neat chemical fertilisers, so you generally need to use more of them for a given amount of plant food. This means that they generally cost slightly more than the neat chemical types for the same amount of plant food. Many of these are natural in origin, often occurring as natural mineral deposits in other parts of the world. These are mined, ground and purified before sale.