Types of fertilisers
There are four main forms of fertilisers:
Organic fertilisers
Concentrated sources of plant nutrients which are derived from living organisms. Bone meal and dried blood, for example, are organic fertilisers.
Inorganic fertilisers
Concentrated sources of plant nutrients which are derived from non living material, such as calcium nitrate or superphosphate.
Bulky organic matter
Substances such as compost, straw and farmyard manure can be used to add nutrients, however their nutrient levels are variable at best and are better used to add humus to the soil to improve its structure.
Green manure
This is the process of growing specific types of plants over an area, which are then dug back into the soil as a form of organic matter, leading to nutrients being re-incorporated into the soil.
Fertilisers can provide single or multiple nutrients:
Straight fertilisers
These supply just one nutrient, generally one of the macronutrients. Dried blood is a straight fertiliser, supplying only nitrogen, as does ammonium nitrate.
Compound fertilisers
These supply two or more nutrients within the same fertiliser, generally combinations of nitrogen, phosphorous and potassium. The proportions are usually expressed as ‘NPK x-x-x’ where the x’s denote the percentage of each nutrient (eg NPK 20-10-10). Bone meal is an example of a compound fertiliser, supplying NPK 3-20-0.
Where the NPK proportions are equal, eg 1-1-1, this is called a ‘balanced’ fertiliser.
The fertilisers can act in two different ways, and their structure often reflects this:
Quick acting
These fertilisers dissolve as soon as they come into contact with water so they deliver nutrients to the plant in a form which it can take up quickly. These are generally available as powders, crystals, granules, prills (where the materials are melted then dropped from a tall tower so they solidify in a spherical form) or ready-prepared liquids. The solid forms are readily soluble. Quick acting fertilisers include ammonium nitrate and potassium chloride.
Slow release
Contrary to quick acting fertilisers, these release the majority of their nutrient content over a longer period of time. The slow release mechanism can be natural (eg bone meal requires microorganisms to break down the fertiliser and release the nutrients) or artificial where they are formulated with coatings which dissolve slowly.
There are various forms of slow release fertiliser:
- Controlled release fertilisers break down to release nutrients at a rate that broadly matches the plant’s uptake.
- Frits (nutrients fused with fine glass powders) are used to release immediately soluble materials, or micronutrients which can quickly become toxic, more slowly.
- Chelates/sequestrenes, compounds which bond with the nutrient required to make it available where the soil conditions would otherwise ‘lock up’ the nutrient and make it unavailable to plants (for example where high levels of zinc makes iron unavailable.
