All plants need mineral ions, especially nitrogen.
Intensive food production makes large demands on the soil because mineral ions are continually taken up by the crops being grown on it.
Mineral ions crops have absorbed are being removed. In natural ecosystems the minerals that have been removed from the soil by plants are removed when the plant dies and is broken down by microorganisms.
In agricultural systems the crop is harvested and then transported from its point of origin. The minerals are rarely returned to the same place and therefore the levels of mineral ions in agricultural land will fall.
It is therefore necessary to replenish these mineral ions otherwise they will become a limiting factor to plant growth and thus reducing productivity.
To offset this loss of minerals, fertilisers need to be added to the soil.
These fertilisers are of two types:
Natural fertilisers
Which consist of dead and decaying remains of plants and animals
Artificial Fertilisers
Mined from rocks and deposits which are then converted into different forms to give the appropriate balance of minerals for a particular crop
Inorganic Fertilisers
Since the invention of the Haber process in 1905 it has been possible to use inorganic fertilisers to improve yields, and this is a keystone of intensive farming. The most commonly used fertilisers are the soluble inorganic fertilisers containing nitrate, phosphate and potassium ions.
Inorganic fertilisers are very effective, easy to apply, and can be tailored to each crop’s individual mineral requirements, but they can also have undesirable effects on the environment. Since nitrate and ammonium ions are very soluble, they do not remain in the soil for long and are quickly leached out, ending up in local rivers and lakes and causing eutrophication. They are also expensive and their manufacture is very energy-intensive, requiring fossil fuels, so it contributes to the greenhouse effect.
Organic Fertilisers
An alternative solution, which may do less harm to the environment, is the use of organic fertilisers, such as animal manure, composted vegetable matter, crop residues, and sewage sludge. They contain the main elements found in inorganic fertilisers, but contained in organic compounds such as urea, proteins, lipids and organic acids. Plants cannot make use of these organic materials in the soil: their roots can only take up inorganic mineral ions such as nitrate, phosphate and potassium. But the organic compounds can be digested by the soil decomposers, who then release inorganic ions that the plants can use.
