Deciding the amount and which type of fertilizer to use can be complex. Understanding how each type of fertilizer works will make gardening more productive and even perhaps more environmentally friendly.
Bulky organic fertilizers
Weight for weight, bulky organic fertilizers supply fewer nutrients than inorganic fertilisers. For example, 1 ton of manure typically contains 6kg of nitrogen, 1 kg of phosphorus, and 4 kg of potassium. The equivalent amount of nutrients in a chemical form is provided by only 30 kg of inorganic fertilizer. However manures are integral to organic growing because the organic waste matter that makes them so bulky provides greater benefits than a simple nutrient analysis would suggest.
Manures normally contain high levels of micronutrients, and are a long term source of nitrogen, although they may vary in quality. Additions of manure improve the structure and water content of most soils and since this encourages root growth it increases the uptake of nutrients by plants.
Proprietary mixtures of Blood, Fish and Bone are good examples of concentrated organic fertilizers. They are easy to handle and contain fairly consistent proportions of nutrients. Their characteristic slow release of nutrients is partly dependent on breakdown by soil organisms, so may not be effective in cold weather when these organisms are inactive.
Soluble inorganic fertilizers contain high percentages of a given nutrient, weight for weight, and most are easy to transport, handle and apply. However, a few are unpleasant to handle. They are usually the cheapest source per unit of nutrient. They give a quick-acting boost to plants deficient in nutrients. They permit precise control over the timing of nutrient release. A large proportion of soluble fertilizers however may be wasted on sandy soils because of leaching.
When using soluble inorganic fertilizers be selective about which mineral ions are applied to the soil, some may be damaging to particular plants. Redcurrants for example are sensitive to chloride salts, such as potassium chloride, so sulphates should be used instead.
These are complex fertilizer formulations that are designed to release nutrients gradually. Some slowly degrade in the soil, while others absorb water until they swell and burst open. Many have membranes that gradually release nutrients from an internal store. By varying the thicknesses of these membranes, a mix is produced that will supply nutrients to plants over many months, even years in some cases.
These are plants are grown purely to be dug back into the soil to improve fertility and add to the organic content. They are used on land that would otherwise be left fallow and can help to prevent nutrients from being washed away, since fallow land is more prone to leaching.