The contents of fertilisers, as described on the packet can seem quite daunting. However, labels explain the composition and nutrient content of the product, helping you to decide what you need to buy and how best to use it.
Why use fertilisers?
Although not always essential if plenty of well-rotted organic matter is available, fertilisers are extremely useful in maintaining soil fertility so that gardens thrive. Fertilisers are especially useful for plants in pots, fruit and vegetables. Fortunately fertiliser labels contain essential information to get the best from them.
Information on labels
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.
Materials not covered by fertiliser regulations may be offered as plant foods, without stating their nutrient content. This may be variable due to the parent material (as with pelleted poultry manure) or there may be other purported benefits. Gardeners should be cautious about using material with no stated nutrient contents – they may contain very little plant nutrient.
What is not listed?
Fertiliser quality - freedom from lumps, solubility and quality of ingredients - is not covered by the label information. Agricultural fertilisers for example may contain potassium chloride. This is cheaper than potassium sulphate, but some garden plants are sensitive to chloride content. However, the fertilisers sold to gardeners are almost always satisfactory in these respects.