Plants need three main nutrients to help them grow and remain healthy:
- Nitrogen (N) (i.e. lawn feed) used to green up a lawn and also promotes leaves and shoots in shrubs and trees.
- Phosphorus (P) (i.e. bonemeal) provide a great start for newly planted shrubs, vegetables, trees and bulbs, it's also good to use in spring to promote strong, healthy root development.
- Potassium (K) (i.e. tomato feed) is required by most plants
in the greatest amount. 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. Ideal on fruit bushes/trees, roses, tomatoes and hanging baskets.
- Potash (K20 - Potassium oxide) is regularly used in gardening, it's a common term for minerals containing water-soluble potassium.
- Bonfire ashes typically contain high levels of potash, which can be spread on the garden. Make sure you collect and distribute the ashes before the rain causes the potash to leech into the ground where you had the bonfire and be wasted.
- 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.
- Canada is the biggest mining country of potash, and over 90% of all that is mined is used in fertiliser.
Bacteria in the soil break these chemicals down into a form that can be taken up by the plant.
Trace elements such as Boron (B), Calcium (Ca), Chlorine (Cl), Copper (Cu), Iron (Fe), Magnesium (Mg), Molybdenum (Mo), Manganese (Mn), Nickel (Ni), Sulphur (S) and Zinc are also required by plants, but in small quantities.
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 apply fertilisers back into the soil.
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.
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.
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 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 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.
Yellow leaves normally suggests a lack of nitrogen.
Ash from a bonfire contains water-soluble potassium, mix with water and use on tomatoes and flowers.
Beetroot love sodium.
Don't give nitrogen to Chili 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:
Azalias, Camellias, Heathers and Rhododendrons.