The South African fertilizer industry manufactures a broad range of NPK fertilizer products which are typically mixtures of two or more chemical compounds (raw materials) such as Limestone Ammonium Nitrate (LAN), Ammonium Sulphate, Mono Ammonium Phosphate (MAP), Potassium Chloride and Urea.
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 fertiliser packaging.
Not all of the constituents of the fertiliser raw materials used to manufacture NPK mixtures are plant nutrients (i.e. nitrogen (N), phosphorus (P) and potassium (K)). For example:
- LAN has a N content of 28%, the remaining 72% consists of hydrogen (H), oxygen (O) and approximately 21% limestone which acts as a carrier of the plant food.
- Ammonium Sulphate consists of 21% N, 24% sulphur and the remaining 55% consists of H and O.
- Potassium Chloride typically has a K content of 51% while the remaining 49% consists of chlorine.
- MAP consists of 11% N, and 22% P; the remaining 67% consists of H, O and carrier material.
- Urea consists of 46% N, the remaining 54% consists of C, H and O.
All fertiliser products / raw materials are produced on large scale through industrial processes and are manufactured to the specification of their registrations in terms of Act 36 of 1947. Due to the nature of these processes it is normal that slight variances occur in the production process bringing about a slight variance in the concentration of plant nutrients in the different batches of product that are manufactured. The tolerance on concentration variance both upwards and downwards of a product is regulated by Act 36 of 1947.
When raw materials are subsequently used to manufacture NPK mixtures, the variances in the plant nutrient content of the raw materials will have an impact on the final plant nutrient content of the NPK mixture. In order to ensure that NPK mixtures consistently meet specification, NPK mixtures are typically formulated to carry in the region of 1-2% filler material. All raw materials used in the production of NPK mixtures are analyzed on a continuous basis and where necessary the bill of materials for a specific product is adjusted to ensure that the final NPK product meets specification. In the event that one or more of the raw materials to be used has a plant nutrient concentration towards the lower end of the specification allowed for the raw material, the addition of filler will be reduced. Similarly, in the event that if the raw materials to be used have a plant nutrient concentration towards the upper end of the specification allowed for the raw material, the filler content may be increased.
The filler material that is used in the production of mixtures is typically a course sand which is screened to ensure that 90% of the particle sizes range between 2.00 – 4.00 mm; it has no nutrient value and shouldn’t react with the other compounds used in the production of the fertiliser mixture and therefore have no detrimental effect on the shelf life of the product. The sole purpose of the filler material is to allow the manufacturer to meet the NPK specification of the final product due to the normal variances in the plant nutrient concentration of the raw materials.
Most NPK mixtures are formulated to carry the highest possible concentration (grade) of plant nutrients, keeping the filler content as low as possible. This ensures the highest possible plant nutrient concentration of the product and allows the end user to save on transport costs. Notable exceptions are traditional products such as 2:3:2(22)+0.5% Zn which have been used by farmers historically and have typically been applied by hand or with equipment which cannot be calibrated to apply the lower rates of fertilizer that can be attained when using the higher grade mixtures. Products such as 2:3:2(22)+0.5%Zn will carry as much as 37% filler material and may therefore appear to contain a lot of sand.
Should you have any queries on filler material and fertiliser concentration please do not hesitate to discuss this with your fertiliser advisor.