At least thirteen mineral nutrients are essential for plant growth, including nitrogen, phosphorus and potassium. Other major nutrients are sulphur, calcium, magnesium, which are required in lesser but still substantial quantities; sulphur deficiencies are increasingly prevalent. A number of ‘micronutrients’ are required by plants in very small quantities but each is necessary for healthy plant growth. As with humans and animals, a healthy plant requires a balanced diet. When adequate supplies of plant nutrients do not occur naturally in a plant's immediate vicinity, these must be provided through other sources.

There are several sources of plant food, the two most important being organic manures and mineral fertilizers. When all the other available sources have been utilized, mineral fertilizers supply the outstanding balance needed for good crop yields. In most parts of the world the margin that must be covered by mineral fertilizers is substantial.

It is estimated that about 40% of the world's protein needs are derived from atmospheric nitrogen fixed by the Haber-Bosch process and its successors to produce ammonia. The process was initially developed early in the twentieth century. It was clear to scientists even at the beginning of the twentieth century that the then-available sources of nitrogen would shortly prove insufficient. If it were not for this fixed nitrogen, it would be necessary to increase substantially the area of land cultivated, sometimes expanding cultivation into fragile environments, in order to produce the quantities of agricultural produce needed today. 

2^nd edition, October 2002