Article 2 in Composting Series
It’s worth looking at what the nutritional requirements of our plants are and how we can supply them. Before we do that, it’s important to understand what the components of fertilisers are and what they do – see the accompanying article “What are NPK”.
Different fertilisers will provide different percentages of various nutrients so the quantity to add will depend on the type and content.
Its important then to read the instructions or look up the amounts to use per square yard or metre as you can add too much. You can also add fertiliser at the wrong time of the year or in unfavourable weather conditions and that means you’ll just be wasting your money and the fertiliser.
Straight fertilisers are those that purely supply one element, like sulphate of ammonia or Superphosphate.
Straight Artificial Fertilisers:
NPK Levels in Straight Artificial Fertilisers
| Fertiliser | N Nitrogen % | P Phosphorus % | K Potassium (Potash) % |
| Sulphate of Ammonia | 20 | ||
| Nitro Chalk | 27 | ||
| Nitrate of Soda | 16 | ||
| Sulphate of Potash | 50 | ||
| Superphosphate | 18.5 | ||
| Rock Phosphate | 26 | ||
| Triple Superphosphate | 45 |
Compound Artificial Fertilisers
Compound artificial fertilisers are produced by combining straight fertilisers in various proportions to form balances suitable for general growing or for specific crop requirements.
Trace Elements & Micro-Nutrients
Just as people need vitamins and minerals in their diet for long term health, plants require other elements in their diet to thrive. Many of the compound fertilisers add these trace elements just as vitamins are added to some of our foods. These other elements are covered in Additional Elements of Plant Nutrition and in Trace Elements or Micro-Nutrients of Plant Nutrition.
Having covered the range and types of fertilisers available let’s take a look at some specific plant requirements and how we can meet them. The main element for growth is nitrogen. Nitrogen is vital to the production of the leaves, which in turn power the plant’s growth and the more leafy growth a plant produces the more nitrogen it will require. Because nitrogen has the shortest life in the soil, being easily washed out by heavy rain for example, it is the one element to concentrate on and with crops that are in the ground for a long time worth applying in stages rather than one go
Nitrogen Requirements of Various Crops
| Very High Nitrogen | High Nitrogen | Med Nitrogen | Low Nitrogen | Very Low Nitrogen |
| Brussel Sprouts | Beetroot | Broccoli | Asparagus | Carrots |
| Cabbages | Celery | Calabrese | Runner Beans | Radish |
| Rhubarb | Leeks | Cauliflower | Parsnip | Peas |
| Spinach | Lettuce | Swede | Broad Beans | |
| Onion |
Remember that legumes (Beans) produce much of their own nitrogen due to a symbiotic relationship with bacteria that fix nitrogen from the air for the plant, which is why peas and broad beans generally need little to no nitrogen supplement and runner beans with all their foliage need just low levels to supplement their own produced nitrogen.
How Much To Use – Quantities to Apply
My advice here is to
Remember – You can add too much fertiliser AND at the wrong time of the year or in unfavourable weather conditions and you’ll just be wasting your money and the fertiliser.
Quantities Guide (read the manufacturers instructions)
Organic Fertilisers
Bone meal – 100-125g per sq mtr
FBB – 50-200 g per sq mtr
Seaweed – all you can get or liquid feed (follow instructions)
Poultry manure – higher in nitrogen with some potash and phosphate in small quantities so use sparingly
Garden Lime – Fork into the soil in the autumn in areas where you plan to grow brassicas and other alkaline loving plants such as spinach and onions, but NOT where you plan to grow potatoes. Young plants can be burnt if planted into soil that has recently been limed, so autumn liming in preparation for later sowing is preferable. DO NOT dig in at the same time as manure as they will counteract each other.
Use 05 to 1 kg per sq. metre, depending on the pH of soil.
Inorganic Fertilisers
Growmore – 50-125gms per sq mtr
Sulphate of ammonia – 25 gms per sq mtr
Sulphate of potash – 25gms per sq mtr
Super phosphate – don’t use it kills your worms!
Nitrate of chalk 27-0-0 – 50gms per sq mtrNote: A small 150 g yogurt carton holds about 200g (7oz) fertiliser. This applies to granular fertiliser like Growmore or potato fertiliser NOT POWDER’S
Quantities from – http://www.roehamptonallotments.co.uk/growing-things/fertilisers/