Fertilisers are given an NPK rating, the proportions of the major plant nutrients, Nitrogen, Phosphorus and Potassium. Each chemical element has a particular effect on plant growth. So, when we see a fertiliser bag with the numbers 20:10:10, we know it contains 20% Nitrogen, 20% Phosphorus and 10% Potassium. This would be a concentrated inorganic general garden fertiliser that would take effect in a short time. Organic fertilisers, from natural sources, are generally bulky and take effect much more slowly. Organic growers do not normally have the luxury of receiving them prepackaged with such information printed on the sack. As gardeners, it’s a good idea to know the fertility rating of the fertilisers we use.
NPK rating of commonly used organic fertilisers
Contains weed seeds, should be composted
Fast acting, breaks down quickly, use carefully, may burn
Medium breakdown time
Most concentrated manure in fresh form
Releases nutrients slowly
High nitrogen value, fast acting
Fast acting and highly alkaline.
Valuable as a source of trace elements
Similar to cow manure
Organic fertilisers contain the same elements as inorganic fertilisers and can be divided into two groups, bulk fertilisers and concentrated fertilisers. Bulk fertilisers are generally plant derived and are slow acting. Concentrated organic fertilisers are often by-products of the meat and fish processing industry. Vegetarians may have issues with animal derived fertilisers and prefer chemical inorganic fertilisers. Chemical fertilisers are often the result of high-energy industrial processes. Using bulk fertiliser requires some forethought and may need to be applied the season before, allowing the matter to break down and release nutrients. Organic gardeners generally prefer low impact methods involving the use of recycled natural products, giving then greater control over their food source. It may not matter which you choose. Produce from an organic home grower will always appear to have more appeal than high input, industrially produced vegetables.