In general, there are two main types of fertilizer: inorganic and organic. In practice, both types may be used; for example a chemical inorganic fertilizer may be supplemented with the application of a bulky organic fertilizer such as manure.

Inorganic Fertilizers

Inorganic fertilizers include naturally-occurring examples such as limestone, or chemically-synthesized inorganic versions such as ammonium nitrate, potassium sulphate and superphosphate.

If the fertilizer contains one of the three primary ingredients of nitrogen (N), phosphorous (P) and potassium/potash (K), they are named according to that particular element. They may thus be grouped as nitrogenous fertilizers, phosphate fertilizers or potassic fertilizers.

Some inorganic fertilizers contain more than one of the three principal elements, and are called compound fertilizers. They are named or labelled according to the content of these three elements, thus a 5-10-5 fertilizer would have 5% nitrogen and potassium and 10% phosphate in its ingredients.

Soluble inorganic fertilizers are easy to transport and apply, and are generally the cheapest source per unit of nutrient. They can act as a quick boost to plants deficient in nutrients, and allows the gardener strict control over the timing of nutrient release. However, do bear in mind that it is easy to waste a large proportion of soluble fertilizer due to leaching - especially on sandy soils. Soluble inorganic fertilizers should be applied little and often, rather than in one large dose.

The leaching effect may be reduced by using a slow-release fertilizer; whilst they are much more expensive than soluble fertilizers, they make feeding container-grown plants much less time-consuming, as they remove the need to add fertilizer each time the plants are watered. Slow-release fertilizers release nutrients gradually, either by degrading slowly in the soil or by absorbing water until they swell and burst open. However, this may make the nutrient timing release difficult to predict as breakdown may depend on the moisture content, pH and temperature of the soil.

Organic Fertilizers

Examples of naturally occurring organic fertilizers include manure and slurry, peat, and seaweed, whilst synthesised organic fertilizers include compost, dried blood, bone meal and seaweed extracts. Green manure crops are also grown to add nutrients to the soil.

Although the density of nutrients in naturally occurring organic material is comparatively modest compared to inorganic fertilizers, they do have some advantages. Their bulky composition improves the soil structure and helps to retain soil moisture. This encourages root growth and increases the uptake of nutrients by plants. Nutrients tend to be released at a slower, more consistent rate and many organic fertilizers, such as manures, also contain high levels of micronutrients.

Organic fertilizers also avoid some of the long-term problems associated with the regular heavy use of artificial inorganic fertilizers, such as the possibility of 'burning' plants with concentrated chemicals and the need to re-apply regularly to maintain fertility.

Synthesised concentrated organic fertilizers such as proprietary mixtures of blood, fish and bone are easy to handle and contain fairly consistent proportions of nutrients. However, they remain a relatively expensive option for each unit of nutrient. Their breakdown and release of nutrients is partially dependent on soil organisms, which means that they may not be effective in cold weather when these organisms are inactive.

Green manures are plants that are grown simply to be dug back into the soil to add to the organic content and improve fertility. They are particularly useful on land that would be left fallow as they also prevent nutrients from being washed away. Green manures may include plants such as peas, which have nitrogen-fixing bacteria in root nodules, enabling them to obtain nitrogen from air in the soil. Borage (Borago officinalis), rye-grass (Lolium perenne), and comfrey (Symphytum officinale) are also excellent perennial green manures.