There are very few gardens that can boast an ideal soil for every requirement, and even if you start off with a good-quality loam not lacking in plant foods, as your plants use up what is already there, it will need to be replaced.
Also there are many compounds that can be added to a soil to correct some physical defect. Many of these, such as substances which provide humus, will also improve soil texture as well as breaking down to supply elements essential for healthy growth.
These additives can usually be divided into 3 categories: manures, fertilizers, and other soil conditioners.
Manures: A manure has come to be regarded as a bulky organic substance which is incorporated into the soil to supply plant foods during its breakdown. Before the introduction of concentrated fertilizers this was the only way in which plant foods could be replenished as they were used up. The drawback of this is that a comparatively large quantity is required to provide enough of every element needed, especially for areas which are heavily cropped. Manures are the humus-forming substances needed for good soil structure; in addition to forming plant foods, they hold water to prevent a soil drying out and the actual digging-in process ensures that air is introduced.
However, during decomposition they give off acids which lower the pH of the soil. As a soil gets more acid, bacterial activity slows down, and foods stop being released, therefore at some point the alkalinity has to be increased to encourage bacteria to continue to break the material down. This is done by adding lime, or calcined seaweed, but lime should not be added at the same time as manure, as it will cause it to break down so quickly that the plants are unable to absorb much of what is produced.
Manures come from many sources and the foods they supply and their relative proportions depend on the type of manure.
Fertilizers: Technically, all substances capable of supplying plant foods are fertilizers (that is, they make a soil fertile). However, fertilizers are now thought of as concentrated compounds added to soil for the purpose of supplying one or more elements necessary for plant growth.
Nitrogen: Plants require nitrogen for quick, healthy, green growth, especially leaves and stems. A shortage of this element causes pale green colouring and stunted growth. Too much makes a plant very lush, prone to diseases, and reluctant to flower and set fruit, so it is essential not to create an imbalance with other elements.