There is no denying that supplying sufficient food for the rapidly growing population of the world presents one of the greatest challenges facing mankind at the present time. Because there is so little reserve land suitable for cultivation, it is only possible to increase food production by increasing crop production per unit area. But, it is not only the quantity of food produced that should concern us, its nutritional quality is also important. Supplying the world's food is the business of both farmers and research scientists in developed and developing countries alike.

Fertilizers offer the best means of increasing yield and of maintaining soil fertility at a level sufficiently high to ensure that good yields can be obtained consistently, year after year.

To many, whether they be farmers or laymen or even occasionally scientists, 'fertilizer' means 'NPK', but nitrogen, phosphorus and potassium are only three of the plant nutrients needed. Plants require also large quantities of sulphur, calcium and magnesium and small quantities of a number of minor elements, while some plants require appreciable amounts of sodium. In recent years the fertilizer industry has sought to supply fertilizers more highly concentrated in terms of N, P and K and this has meant that some of the other needs of plants may be overlooked.

The effect of a single nutrient (like K) in a fertilizer may depend upon the way in which it is chemically combined in the fertilizer material and this affects both yield and crop quality. Because potassium fertilizers are obtained from natural products they may contain substances other than K, S and Cl and these substances may affect plant growth. Thus, choosing the right kind of potash fertilizer can be as important as applying the right amount of potash to a crop. This book is concerned with this choice and seeks to answer the questions:

• What, for a particular purpose, is the ' best' form of potash? Here we are concerned essentially with the choice between the chloride and the sulphate.
• How is the value of a potash fertilizer affected by 'accessory' materials e.g. magnesium, sodium, sulphur, contained therein?

The booklet aims to discuss these problems, which are implicit in its title, thoroughly, but it makes no claim to being a complete and exhaustive review of all publications and experimental results. It brings up to date the information contained in an earlier publication:

KAMPFER, M. and ZEHLER, E.:
The importance of the sulphate fertilizers for raising the yield and improving the quality of agricultural, horticultural and sylvicultural crops.
Potash Review, May/June 1967. Int. Potash Inst., Bern (1967).

A more recent publication deals especially with results obtained with sulphate fertilizers in France:

LOUE, A.:
Le sulfate de potasse.
Au Service de l'Agriculture, Dossier K₂O, No 11, SCPA Mulhouse (1978).