All fertiliser labels have to display the same basic information. This allows one garden-fertiliser packet to be compared with another and should show clearly what is being bought.
The composition, packaging and labelling of fertilisers in Great Britain is controlled by legislation. This may be either British or European; both differ slightly in their content, with EC regulations becoming more prevalent, but both are satisfactory.
Fertiliser packaging is required by law to include a number of details, including the following:
- The prescribed name of the product content, such as ‘NPK Fertiliser'
- The major nutrient contents, nitrogen (N), phosphorus (P) and potassium (K)
- Forms in which the nitrogen content is present
- Solubility of the phosphorus content: P and K are usually expressed as their oxide content, followed in brackets by the content of the element
- Levels of secondary nutrients present such as magnesium (Mg) and other trace elements
- Any pesticide content (lawn weed-and-feed products, for example) as well as a statutory declaration to comply with pesticide legislation
- The name and address of the manufacturer
- The guaranteed weight of the product
- An EC product declaration, if the product is EC approved
Elements, such as nitrogen, phosphorus and potassium are the most useful part of fertilisers, rather than compounds such as phosphorus pentoxide and potassium oxide. So you will usually see on a label two ratings, one for the compound (oxide) rating, and then in brackets, the actual rating for the element. It will often appear as so:
Potassium oxide soluble in water: 7% (5.8% K)
- Growmore has a ratio of 7:7:7 for these materials, but is actually 7 percent nitrogen, 3.1 percent phosphorus and 5.8 percent potassium, or 7:3.1:5.8. Therefore it is a high-nitrogen feed, rather than the ‘balanced’ fertiliser commonly assumed
- As garden soils are more likely to lack nitrogen than other elements, Growmore is deservedly popular, especially as a spring fertiliser
Manufacturers of garden fertilisers usually add comprehensive information to help gardeners get the best from their products. Major manufacturers will conduct tests and trials, but generic fertilisers such as superphosphate and Growmore have recommendations based on experience as well as field trials. Guidelines cannot cover every possibility and gardeners may have to use ‘trial and error’, ideally in conjunction with occassional soil analysis, to achieve the best results.