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Information about fertilisers and how to use them




Jump to:-  Basic Elements (N-P-K)  | Trace Elements  |  Types of fertilisers  |  How to Use Fertilisers


All plants and trees need food in order to remain healthy and produce reasonable crops.  Although a good soil which has been enriched by the addition of manure or compost is essential and the best place to start, as plants grow, they will be constantly feeding on the nutrients in the soil, so the addition of fertilisers is a good way to ensure a continuous supply of all the necessary nutriments  throughout the growing season.


Fertilisers needed for specific crops are given on the individual growing pages, however below is a guide to some the various fertilisers available and what they are used for. If it seems too complicated take heart,  as there are various brands of general balanced fertilisers available to buy. Try not to get too bogged down with the figures below and certainly, if you're just starting out, don't allow it to rule your life. Remember - in most cases a balanced fertiliser is better than the wrong fertiliser or no fertiliser at all.


Having said that, familiarising yourself with the different elements could help you make a more informed choice when buying and could save you money.


Basic Fertiliser Elements (N-P-K)


Whenever you read about feeding plants the letters N P and K will inevitably used. These letters stand for the basic fertiliser elements which are Nitrogen (N) Phosphorus (P) and Potassium (K). Just in case you are wondering, the letters assigned to the elements are the same as those in the periodic table.


Each of these elements supply specific benefits to growing plants and are essential for various growing requirements and whilst most plants will need all 3 elements to grow successfully. In general the benefits of these 3 elements are as follow:-


  • Nitrogen (N)  - encourages healthy foliage growth - beneficial to leafy vegetables such as cabbages, lettuce and spinach

  • Phosphorus (P) - encourages root growth  - essential for all plants

  • Potassium (K) - encourages bigger, healthier blooms and fruit - beneficial for fruit, flowers and vegetables such as tomatoes, aubergines and peppers.  Potash  is the common name given to a water soluble form of potassium so has the same attributes.




Although you can buy all three individually, there are specialist fertiliser mixes which have been designed to provide the elements in varying percentages depending on the crop being grown.  A balanced (compound) fertiliser will contain all three elements in the same proportions,  however a fertiliser which is produced specifically for fruiting plants such as tomatoes, will contain a higher percentage of potassium (potash) than then other two elements. So when reading the label, the three elements will  be written as a %  e.g.  7-7-7.


Why not 33.3 -33.3 -33.3  (which equals 100%) ?  Because there are always other substances in the mix,  the figures equate to the percentage of elements in the whole mixture by weight - much like the ingredients on food packaging. A chocolate cake may only contain  5% of chocolate when compared to the other ingredients used.


So, if you are growing fruit bearing plants or flowers you could choose a fertiliser which is high in potassium (potash)  e.g.  6-5-9.  However if you are growing leafy vegetables such as cabbages you could choose a fertiliser which is high in nitrogen  e.g.  10-6-4


Try not to get too bogged down with the figures and certainly, if you're just starting out, don't allow it to rule your life. Remember - in most cases a balanced fertiliser is better than the wrong fertiliser or no fertiliser at all however ALWAYS read the instructions on the packet as to how, when and how much fertiliser to use and stick to what the manufacturers advise..


Trace Elements in Fertilisers


Many fertilisers also contain small amounts of other elements such as Calcium, Magnesium, Boron,  Sulphur and  Manganese all of which help stimulate growth in crops and prevent deficiencies.


Types of Fertilisers


There are many types of fertilisers which provide the different elements as mentioned above. Broadly speaking, these fall into two categories - Chemical and Organic both of which can be further divided into straight (single) element fertilisers and compound (NPK) fertilisers. The type you choose will depend on your personal point of view, with both having their up and downsides however your plants won't be able to tell the difference between them.


Chemical Fertilisers

Chemical fertilisers are synthetically produced from inorganic materials, often made using high-energy industrial processes. Many contain acids which, when over-used,  can be harmful to microorganisms in the soil, which may be of concern so they are not generally approved of  for use in organic gardening.  They can be purchased as single element fertilisers or compound mixes,  many of which have trace elements added and are a good choice when a quick release of nutrients is called for.

Sample List of Straight Chemical Fertilisers
  Nitrogen % Phosphorus % Potassium
(Potash) %
Sulphate of Ammonia 20
Sulphate of Potash 50
Superphosphate 18.5
Sample List of Compound Chemical Fertilisers
  Nitrogen % Phosphorus % Potassium
(Potash) %
  Growmore 7 7 7
  Vitax Q4 5.3 7.5 10
  John Innes Base 5.2 7.7 10

 Organic Fertilisers

Most organic fertilisers are by-products of the meat processing industry and are therefore not used by vegan or vegetarian gardeners on ethical grounds. Furthermore concerns regarding BSE have also placed restrictions on the use of  bonemeal for organic growers.  Most are single element fertilisers although there are some compound fertilisers. They are generally slow-release fertilisers which means they have to be applied less often during the season.


Sample List of Straight Organic Fertilisers
  Nitrogen % Phosphorus % Potassium
(Potash) %
Dried Blood/Blood Meal 12
Bonemeal 18  
Potassium Sulphate   50
Sample List of Compound Organic Fertilisers
  Nitrogen % Phosphorus % Potassium
(Potash) %
Blood, Fish & Bone 6 6 6
Chicken Manure Pellets 4 2.5 2.3

How to use Fertilisers


Because different manufacturers make their fertilisers in different strengths, it is impossible to give definitive advice about rates of use. Always read the labels on fertilisers, even if you think you have bought the same one before as the rates of application may have changed.  Below are some general tips.


Liquid Fertiliser

Liquid fertiliser is usually sold in concentrated forms and will have to be mixed with water before they are applied. Once diluted ideal for use as a foliar feed using a fine nozzled spray  as well as being watered into the soil using a watering can. Note that they are better absorbed when used on soil which is already damp. These are probably the easiest to use when making several applications during the growing season.


Granular or Powder Fertiliser

These are usually applied in their dry state and should should be watered in immediately to avoid 'burning' the plants. Rates of application vary so, once again, be sure to read the instructions carefully.



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