Fertilizers are any solid, liquid or gaseous substances containing one or more plant nutrients in known amount, that is applied to the soil, directly on the plant (foliage) or added to aqueous solutions (as in fertigation) to maintain soil fertility, improve crop development, yield and/or crop quality.

The purpose of fertilizer use, especially for higher yields, is identical in temperate and tropical climates:

• to supplement the natural soil nutrient supply and build up soil fertility in order to satisfy the demand of crops with a high yield potential;
• to compensate for the nutrients exported by the harvested products or lost by unavoidable leakages to the environment in order to maintain good soil conditions for cropping.
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to supplement the natural soil nutrient supply and build up soil fertility in order to satisfy the demand of crops with a high yield potential;
• 
to compensate for the nutrients exported by the harvested products or lost by unavoidable leakages to the environment in order to maintain good soil conditions for cropping.

Fertilizers are classified into two major forms:

• organic,
• mineral/manufactured.
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organic,
• 
mineral/manufactured.

Manufactured fertilizers are classified according to different criteria as follows:

• Number of nutrients
  • single-nutrient or straight fertilizers (whether for macro or micronutrients)
    examples: urea (46-0-0), triple superphosphate (0-46-0),muriate of potash (0-0-60), zinc/iron chelates, boric acid, etc.
  • multi-nutrient/compound (multiple nutrients) fertilizers, with 2, 3 or more nutrients
    examples: compound fertilizers (15-15-15), diammonium phosphate (18-46-0), monopotassium phosphate (0-47-31), etc.
• Type of combination
  • mixed fertilizers or ‘bulk-blends’ are physical mixtures of two or more single-nutrient or multi-nutrient fertilizers;
  • complex fertilizers are products in which two or more of the nutrients are chemically combined (e.g. nitrophosphates, ammonium phosphates).
• Physical condition
  • solid (crystalline, powdered, prilled or granular) of various size ranges;
  • liquid (solutions and suspensions);
  • gaseous (liquid under pressure, e.g. ammonia).
• Nutrient release
  • quick-acting (water-soluble and immediately available);
  • slow-acting (transformation into soluble form required, e.g. direct application of phosphate rock);
  • controlled-release by coating;
  • stabilized by inhibitors.