Fertilisers and their uses

Understanding fertilisers to get more from your allotment.


All fertilisers contain at least one of the major plant foods-that is:

  • Nitrogen (N)-for stem and leaf growth;
  • Phosphate (P)-for root growth
  • Potash (K)-for flower and fruit production.
Fertilisers can be either organic (derived from animal or plant remains) or inorganic (mined from the earth or man-made). Inorganic fertilisers are usually quicker acting. 
All proprietary brands of fertiliser, whether in bags or bottles, should display the analysis, or the N:P:K ratio. For example, if the N:P:K ratio is 7:7:7, as in Growmore, you know that the fertiliser contains equal amounts of the three major plant foods, nitrogen, phosphate and potash and is, therefore, suitable for general use with all plants.
Every plant requires all three major plant foods, but at different stages of growth they are needed in different proportions. For example a flowering or fruiting plant, which includes many vegetable crops, requires some phosphate in the early stages, to promote root growth, followed by a fertiliser high in nitrogen for the growth stage and then one which is higher in potash to ripen the plant and assist in flowering and fruiting.
Some examples of inorganic are:
Sulphate of Ammonia 23% N
Sulphate of Potash 48% K
Superphosphate of Lime 18% P
  • Growmore. Equal parts N,P,K. Mainly for incorporating in the soil before planting.
  • Rose fertilisers. Higher in potash than Growmore. Suitable for all flowering shrubs as well as roses.
  • Spring and summer lawn fertilisers. High in nitrogen to promote quick growth.
  • Autumn lawn fertilisers. Low in nitrogen, higher in phosphate (to strengthen roots) and potash.
  • John Innes base. Mainly for making compost to the John Innes formula. Add 4 oz per bushel (8 gal.) for J.I. No. 1.
  • Fish, blood and bone. For raking into ground before planting. High in potash by the standards of organic fertilisers, so, good for flower beds.
  • Calcified Seaweed. Similar to fish, blood and bone.
Wilted comfrey leaves are an great source of organic potash? Use them around soft fruit or when planting potatoes. Or add them to the compost heap at any time. Cultivated strains are available but don't despise the wild plant which grows beside ditches on many allotment sites.  I hope to start using mine next year.