Introduction

Fertiliser is generally defined as "any material, organic or inorganic, natural or synthetic, which supplies one or more of the chemical elements required for the plant growth". Sixteen elements listed in Table 1.1 are identified as essential elements for plant growth, of which nine are required in macro quantities and seven in micro quantities.

Of the elements listed in Table 1.1, carbon, oxygen and hydrogen are supplies by air and water and are, therefore, not treated as nutrients by the fertiliser industry. The main aim of the industry is to provide the primary and secondary nutrients which are required in macro quantities.

Table 1.1 - Essential elements for plant growth

Note: As per the Fertiliser Control Order (FCO) 'fertiliser' means any substance used or intended to be used as a fertilisers of the soil and/ or crop and specified in part A of Schedule I and includes a mixture of fertilisers and special mixture of fertilisers.

Primary nutrients are normally supplied through chemical fertilisers. They are chemical compounds containing one or more of the primary nutrients and are generally produced by chemical reactions. Whatever may be the chemical compounds, its most important ingredient for plant growth is the nutrient content.
The primary nutrients are nitrogen, phosphorus and potassium; however, their concentration in a chemical fertiliser is expressed as a percentage of total nitrogen (N), available phosphate (P₂O₅) and soluble (K₂O). Thus, ammonium sulphate contains 20.6 per cent N; single superphosphate 16 per cent P₂O₅ and muriate of potash 60 per cent K₂O.
The grade of a fertiliser is expressed as a set of three numbers in the order of per cent N, P₂O₅ and K₂O. If a nutrient is missing in a fertiliser, it is represented by a zero. Thus ammonium sulphate is represented as 20.6-0-0 (since it does not contain phosphorus and potassium), single superphosphate as 0-16-0 (as it does not contain nitrogen and potash), muriate of potash as 0-0-60 ( as it does not contain nitrogen or phosphorus). When a fertiliser contains more than one nutrient, for example diammonium phosphate, it is shown as 18-46-0, indicating that it contains 18 per cent of nitrogen, 46 per cent of P₂O₅ and no potash. Similarly, "Suphala", a nitrophosphate fertiliser produced by RCF, Trombay, is shown as 15-15-15 indicating that the product contains 15 per cent N, 15 per cent P₂O₅ and 15 per cent K₂O.