All fertilizer products sold in South Africa must be registered in terms of the Fertilizers, Farm Feeds, Agricultural Remedies and Stock Remedies Act, 1947 (Act 36 of 1947) and are sold with a specific registered plant nutrient concentration which is printed on the fertilizer packaging.
The South African fertilizer industry manufactures a broad range of fertilizer products as compounds or mixtures which contain two or more of the Primary Macro plant nutrients Nitrogen (N), Phosphorus (P) and Potassium (K) which are broadly referred to as NPK products.
In South Africa NPK products are named using a numerical system e.g. 4:3:4(33) which is called the “grade” of the product. The grade of a product is also a guarantee of analysis for that specific product; the tolerance on nutrient concentration variances in a product are regulated by Act 36 of 1947.
The grade 4:3:4(33) contains the primary macro nutrients in a ratio of 4 parts N : 3 parts P : 4 parts K, the sum of which totals 11 parts.The total plant food concentration is displayed in brackets, in this example 33%.
The NPK concentrations can be calculated from the grade accordingly:
- The concentration of N: 4⁄11 X 33 = 12%
- The concentration of P: 3/11 X 33 = 9%
- The concentration of K: 4⁄11 X 33 = 12%
The total nutrient concentration of NPK adds up to 33% as displayed in brackets.
The other 67% of the product consists of the elements Ca, H, O, S, Cl and a few other elements which are constituents of the raw material compounds and filler material.
Other Secondary Macro Nutrients such as Sulphur (S) and micro nutrients such as Zinc (Zn) may also be registered as plant food ingredients in NPK products; their concentrations are expressed as a percentage and included in the grade notation of the product, e.g.:
4:3:4(33)+0.5%Zn or 4:3:4(33) +2% S +0.5%Zn.
In some SADC countries, Europe and America fertilizer grades are expressed differently, where all the numbers in the grade refer to percentages of plant nutrient however the P and K content are expressed as oxides:
Total N is as % N
Total P is as % P2O5
Total K is as % K20
To convert P2O5 to % P multiply by 0.4364 (conversely divide by 2.2915)
To convert K2O to % K multiply by 0.8302 (conversely divide by 1.2046)
For example the Zambian product with the grading 8: 24: 16 would contain elemental nutrient concentrations equating to:
P: 24 X 0.4364 = 10.47%
K: 16 X 0.8302 = 13.28%
Please note: Consult a qualified person (Act 36 of 1947) for specific applications / recommendations.