FERTILISING AGENTS
This category includes all those agents that are beneficial for the soil in order to grow crops. Three categories of agents can be distinguished:
  1. Fertilisers: substances whose main function is to provide nutrients for plants.
  2. Soil amendments: substances whose main function is to improve the structure of the soil.
  3. Soil corrective agents: substances whose main function is to change the pH of the soil.
FERTILISERS
Based on their origin, fertilisers are divided into:
  • Organic fertilisers (of plant, animal or mixed origin);
  • Mineral fertilisers (obtained by extraction or by industrial physical and/or chemical transformation).
They are classified based on the type of nutrients contained:
  • Main nutrients: nitrogen, phosphorus, potassium (essential for the growth of plants which use them in large quantities);
  • Secondary nutrients: calcium, magnesium, sodium and sulphur (important for the life of plants which use them in medium quantities);
  • Microelements: boron, cobalt, copper, iron, manganese, molybdenum, zinc (essential for the life of plants which use them in small amounts).
They are classified based on the number of nutrients contained:
  • Simple fertilisers: they contain only one primary nutrient (nitrogen, phosphate or potassium fertilisers);
  • Compound fertilisers: they contain at least two primary nutrients obtained chemically or by blending.
The titre value of a fertiliser corresponds to its nutrient content, expressed in % over mass.

Nitrogen is the essential constituent of Nitrogen fertilisers.Nitrogen is a fundamental element for the production of amino acids and hence of proteins, molecules which are essential for the vegetative growth of the plant. In fertilisers, the titre of nitrogen is expressed as the % of total N and of N in its different forms.
Nitrogen fertilisers are classified based on the type of chemical molecule that binds to nitrogen; the main ones are:
  • Nitric fertilisers (NO3-), characterised by rapid absorption by the plant but with a high risk of run-off. They are indicated as a starter and as anti-stress agents (for cold stress and for suffering crops). Among them, the most important one is calcium nitrate (titre 15% of N).
  • Ammonium fertilisers (NH4+), which are absorbed by the plant more gradually over time and for a longer duration, if promptly buried. Among them, the most relevant are:
    - ammonium sulphate (titre 20%), which contains approximately 24% of sulphur;
    - anhydrous ammonia (titre 80%); ammonia is gaseous so it is marketed under pressure, in liquid form. It is used as a base molecule to obtain fertilisers containing more nutrients.
  • Nitro-ammonia fertilisers, that combine the features of the two previous ones:
    - calcium ammonium nitrate (titres 26% N or 27% N); a base product as it is diluted with limestone rocks;
    - ammonium nitrate (titre 33-34%); officially considered as an explosive substance.
  • Urea fertilisers (titre 46%); organic synthesised fertilisers, in which the nitrogen does not carry out its action immediately as it requires first the transformation into ammoniacal nitrogen and then into nitric nitrogen. It is the most widely used nitrogen fertiliser in the world, thanks to its high nitrogen titre; it is highly soluble.
Phosphate fertilisers have Phosphorus as their essential constituent, an important micronutrient for the life of plants as it plays a role in their metabolism and physiological processes; in particular, it favours: flowering, fruit set and ripening, the strengthening of tissues, the development of roots and the production of seeds.
The raw material of phosphate fertilisers consists of phosphorites, sedimentary rocks of organic origin containing tricalcium phosphate. Tricalcium phosphate Ca3(PO4)2 is not water soluble and is not absorbed by plants. For it to be assimilated it must be attacked with sulphuric and phosphoric acid with an industrial process to obtain dicalcium CaHPO4 (scarcely assimilated by plants) and monocalcium phosphate Ca(H2PO4)2 (completely soluble and assimilated by plants). In fertilisers, phosphorus is expressed as phosphorus pentoxide (P2O5).
The most commonly used phosphate fertilisers are:
  • simple superphosphate (titre 16-20%); obtained by treating phosphorites with sulphuric acid. 93% of the titre must be of monocalcium phosphate (phosphorus in a very soluble form); physiologically it is an acid fertiliser, with the advantage of containing chalk (12% sulphur);
  • concentrated superphosphate (titre 25-36%); obtained by treating phosphorites with sulphuric and phosphoric acid. 93% of the titre must be of monocalcium phosphate (phosphorus in a very soluble form);
  • triple superphosphate (titre 38-48%); obtained by treating phosphorites with phosphoric acid. 85% of the titre must be of monocalcium phosphate (phosphorus in a very soluble form). It is therefore more concentrated, and hence more economical, but free of sulphur.
Phosphate rocks can be directly marketed also as soft natural phosphate or as ground phosphorite (titre 25%, but with phosphorus soluble in mineral acids). Phosphate rocks are used in organic farming.

Potassium fertilisers have Potassium as their essential constituent, which plays a key role in the formation of plant carbohydrates and helps to increase the dry matter in the plant. Potassium improves the glucose and fructose content in fruits and roots; it regulates the plant’s water balance through the stomata of the leaves; it induces greater plant resistance to thermal stresses. The major sources of potassium consist of natural deposits. Potassium fertilisers are essentially made up of soluble potassium salts, which are obtained from the processing of minerals or of organic residues. The titre of potassium fertilisers is expressed as potassium oxide (K2O), which is soluble in water.
  • Potassium chloride (titre 60-62%); physiologically acid, not used on crops that fear chlorine (e.g. tobacco);
  • Potassium sulphate (titre 48-52%); physiologically acid;
  • Potassium salt (titre 34-45%); a by-product of sugar factories, obtained from sugar beet processing residues or as a by-product in rapeseed oil extraction plants.