THE FERTILISER INDUSTRY

Contents for this page	Related topics
Introduction Nitrogen Phosphorus Potassium NPK values Magnesium Additional questions	The petroleum industry The chloralkali and Solvay processes The polymer industry (1) The polymer industry (2) Ammonia - Nitric acid - Sulphuric acid Electrochemical cells	Data Glossary
Learning Outcomes
After studying this section, you will be familiar with the fertilisers that supply the plants macro nutrients.

Introduction:

Modern agriculture, an all-important component of any developed nation's economic activity, could not function without the fertiliser industry. Plants require various elements for healthy growth, and frequently, soils are either naturally deficient in certain elements, or certain elements become depleted as a result of cultivation over a period of time. The purpose of fertilisers is to make good any such deficiencies. Apart from carbon, hydrogen and oxygen, which are either derived from the air or water, plants require elements that are conveniently divided into MACRO NUTRIENTS (that represent more than 0.1% of dry plant tissue), and MICRO NUTRIENTS (sometimes called TRACE ELEMENTS, that are present in much smaller amounts).

Macro nutrients	Micro nutrients
Nitrogen Phosphorus Potassium Calcium Magnesium Sulphur	Boron Copper Iron Manganese Molybdenum Zinc

All of the above are normally available from the soil. The fertiliser industry is primarily concerned with the production of fertilisers aimed at supplying nitrogen, phosphorus and potassium in a suitable form for application to agricultural lands.

Nitrogen:

Nitrogen is essential to plants for the production of proteins and other nitrogenous compounds. It is normally available to plants from nitrate ions in the soil, which in turn may be derived from ammonia and ammonium ions. Therefore, ammonium nitrate, NH₄NO₃, is a widely used fertiliser. It is produced by the reaction between ammonia and nitric acid:

Thus, the production of ammonium nitrate is dependent of a supply of ammonia and nitric acid, which rest on two processes, namely the Haber-Bosch process, which converts nitrogen and hydrogen into ammonia, and the Ostwald process, which converts ammonia into nitric acid. Both of these processes are discussed in another topic.

Phosphorus:

Phosphorus promotes flowering and seed production in plants. It is made available to plants in the form of the phosphate ions (H₂PO₄^-, HPO₄^2- and PO₄^3-). Commonly, these are supplied as fertilisers made up of SUPERPHOSPHATE (a mixture of calcium phosphate, Ca(H₂PO₄)₂ and calcium sulphate, CaSO₄), or TRIPLE SUPERPHOSPHATE ( a mixture of calcium phosphate, Ca(H₂PO₄)₂, and calcium fluoride, CaF₂). Note that superphosphate also contains the macro nutrients calcium and sulphur.)

Superphosphate is produced industrially by the action of sulphuric acid on phosphate rock, Ca₃(PO₄) ₂. The mining of phosphate rock and its conversion to superphosphate is carried out in South Africa at Phalaborwa.

Triple superphosphate is obtained by treating phosphate rock with phosphoric acid (obtained from phosphate rock and sulphuric acid):

It is clear from the above that two substances are essential for the manufacture of both types of superphosphates, namely, phosphate rock and sulphuric acid. The industrial preparation of the latter will be discussed in another topic.

When phosphoric acid and ammonia are reacted together, another valuable fertiliser, ammonium phosphate, (NH₄)₂HPO₄ is produced. Note that this supplies both phosphorus and nitrogen.

Potassium:

Potassium in fertilisers is normally provided as potassium sulphate, K₂SO₄. This is obtained by the reaction of various potassium-containing minerals with sulphuric acid.

NPK values:

Fertilizers are labelled with numbers such as 2:3:2 (26). What does this mean?

The first number, (2), refers to the proportion of elementary nitrogen, N, in the fertilizer.
The second number, (3), refers to the proportion of "available phosphorus" (measured as P₂O₅), which is simply labelled as "phosphorus", P.(Please do not spell it "phosphorous"!)
The third number, (2), refers to the proportion of "soluble potash" (measured as K₂O), simply designated as "potassium", K.
The fourth number, in parentheses (26), is the combined percentages of "pure" nitrogen, "phosphorus" and "potassium". The difference between this number and 100 is the "filler" or "ballast", which are largely inert, but which could still contain appreciable percentages of macro nutrients such as sulphur, calcium and magnesium.

A fertilizer can be chosen on the basis of the NPK values. A relatively high N value would indicate a suitability for lawns, while a high P value would suggest that the fertilizer is suitable for fruit-bearing crops. The actual percentage of the nutrients is calculated as follows:

% N = 2/7 x 26 = 7.4%;
% P (actually P₂O₅) = 3/7 x 26 = 11.1%;
% K (actually K₂O) = 2/7 x 26 = 7.4%.

Magnesium:

Magnesium is a constituent of the green plant pigment CHLOROPHYLL. It is applied to the ground in the form of DOLOMITIC LIME, which is a mixture of calcium and magnesium carbonates.

What is meant by "eutrophication", and why is it an ecological problem?

(Click here for a discussion)