Garden fertilisers

Plants need three main nutrients to help them grow and remain healthy:

  • Nitrogen (N) is used by plants for leaf growth. Adding fertiliser containing nitrogen in the spring promotes new leaf and shoot growth for trees and shrubs. It is also a large constituent in lawn dressing, to feed and 'green up' a lawn.
  • Phosphorus (P) is used by plants to grow strong, healthy root systems, especially in spring. Adding phosphorus in the form of bone meal, provides a great start for newly planted shrubs, vegetables, trees and bulbs. Bone meal can be spread on the boarders without diluting.
  • Potassium (K) It's used before and during the flowering season, to encourage the plant to set flowers and develop fruit. It also controls the water content and transportation of other plant foods from roots to shoots. It is required by most plants and in the greatest amount. Ideal on fruit bushes/trees, roses, tomatoes and hanging baskets. Liquid tomato food is potassium rich. You will often see references to potash, in gardening books.
    • Potash (K20 - Potassium oxide) is a common term for minerals containing water-soluble potassium. It is used throughout the growing season to regularly feed tomatoes after the first fruit (truss) have set, on legumes including runner and broad beans, on exotic like cannas and dahlias and hanging baskets to encourage flowering.
      If you purchase a potash liquid feed, it must always be diluted with water, else it will burn the plant.
      Canada is the biggest mining country of potash, and over 90% of all that is mined is used in fertiliser.
    • Bonfire ash contains high levels of potash, the ash can be spread, without diluting with water, around the base of flowering plants in the garden, rain and watering will wash the potassium into the soil. Alternatively mix with water and use on the base of tomatoes and flowers. Make sure you collect and spread the ashes before the rain causes the potash to leech into the ground where you had the bonfire and be wasted.
      Note: Plastics, chemicals, paints, tar and coal shouldn't be burnt on the bonfire as this could transfer carcinogens onto the fruit or vegetable plot.
    • Potassium/potash increases the effectiveness of other nutrients on the plant and helps reduce the loss of nitrogen from the soil.

Plants also require trace elements: Boron (B), Calcium (Ca), Chlorine (Cl), Copper (Cu), Iron (Fe), Magnesium (Mg), Molybdenum (Mo), Manganese (Mn), Nickel (Ni), Sulphur (S) and Zinc but in small quantities. Bacteria in the soil break these chemicals down into a form that can be taken up by the plant.

Natural vs artificial fertilisers

In the wild, plants take up nutrients from the soil. When the plant dies, insects and bacteria break the plant down and these nutrients are returned to the soil and the cycle repeats itself. However, in a garden, we harvest the fruit and vegetables and cut the flowers for arranging and the prunings are removed, which depletes the soil of these nutrients*. A shortage of these will reduce the chances of growing strong, healthy plants. We therefore need to add nutrients back into the soil, either by adding compost, mulch or fertilisers

As an example, potatoes and beans contain large amounts of potassium (approximately 0.6% by weight, for potatoes and 1.2% for beans), so when they are dug up and collected, the amount of potassium remaining for next year's crop is significantly reduced.

* Watering and rain also washes away these elements from the soil.

Natural fertilisers are made from animal or plant material such as chicken and horse manure, bonemeal, blood fish and bone, seaweed and compost. They release nutrients slowly and add 'body' to the soil, improving soil structure, however they can be awkward to apply and may not provide all of the nutrients that a plant may need.

Natural fertilisers such as chicken or horse manure are high in nitrates and may scorch plants if used fresh. Dry out chicken manure and use it sparingly around the plants, compost horse manure for at least a season (12 weeks/3 months), alternatively add it to the soil when double digging.

Artificial fertilisers such as Growmore, are manufactured by a chemical process, they are easy to apply and give a carefully controlled mixture of the 3 main nutrients required by plants. Overuse of artificial fertilisers can create an imbalance in the environment and excess nutrients can leech into the water table or nearby streams and rivers.

How are fertilisers labeled?

Let's take lawn fertiliser as an example, you'll see something like this on the label:

NPK fertiliser 10-2-4

This means that, by weight, there is 10% Nitrogen, 2% Phosphorus and 4% Potassium (in most cases potassium oxide/potash). There are also trace elements included, these should be labeled separately. The remaining 84% is made up of bulking agents that make it easier to apply, or delay the release of the fertiliser.
Bone meal is 4-12-0

Notes:

  • Adding too much potassium and phosphorus can prevent other nutrients being released by micro-organisms. Always apply fertiliser at the recommended dosage, it's not only a waste of money to overfeed, but the excess can leach into nearby ponds, lakes, water courses or drainage, causing algae blooms or affecting aquatic life.
  • Yellow leaves normally suggests a lack of nitrogen.
  • Beetroot love sodium.
  • Don't give nitrogen to chilli plants - they prefer potassium.
  • Blackcurrants need a lot of nitrogen feed and can tolerate it neat.
  • Feed Mountain Ash (Sorbus) towards the end of summer.
  • Lime hating plants include:
    Azaleas, Camellias, Heathers and Rhododendrons.

Other gardening basics pages: