How much fertiliser do I actually need?

Fertiliser rates are given in grams per square metre, gardens are measured in paces, and almost nobody owns a set of scales in the shed. The gap between the two is where over-application happens — and over-application is far more damaging than under-application, because a hungry plant merely grows slowly while a scorched one dies.

Safety

What you'll need

• A tape measure
• Kitchen scales, once
• A marked scoop, cup or trowel
• A measuring cap for liquids

Step by step

1. Measure the area properly.Length times width in metres. Split an irregular border into rectangles and add them up. A circle is radius squared times 3.14. Pacing is fine at this scale as long as you know the length of your pace.
2. Read the rate off the pack, not off memory.Typically somewhere between 30 and 100 grams per square metre for a granular general fertiliser, and it varies a great deal between products. Chicken manure pellets and Growmore are applied at quite different rates.
3. Multiply, and get a total.A 6m by 1.5m border is 9 square metres. At 70g per square metre it needs 630g — a little over half a kilogram for the whole border. Written down like that it is usually far less than people were about to apply.
4. Calibrate a scoop, once, and never weigh again.Weigh out the correct amount for one square metre and see what it looks like in a cup, a yoghurt pot or your hand. For most granular fertilisers a rounded adult handful is around 30 to 40g. Do this once and you can apply by eye accurately forever.
5. Spread it evenly, in two passes.Half the total walking in one direction and half walking across it. Even spreading matters more than precise quantity — a concentrated patch scorches while the rest of the bed gets nothing.
6. Rake it in and water it.Granular fertiliser on the surface does nothing until it dissolves, and left on top it is available to pets, birds and rain run-off. Rake into the top 50mm and water unless rain is imminent.
7. Get liquid dilutions right by measuring, not by capfuls.Most liquid feeds state millilitres per litre. A standard watering can is 9 or 10 litres — check yours. A bottle cap is not a standard unit and varies between products; use the cap the manufacturer supplied and read what it says.
8. Apply half and repeat, when you are unsure.Half the rate now and the other half in three or four weeks is safer than the full rate at once, gives a smoother supply, and reduces leaching. On sandy soil it is simply better practice regardless.
9. Reduce for containers, do not scale up.Container rates are given separately on the pack and are usually much lower per unit of volume than open-ground rates, because there is nowhere for excess to go and salts accumulate. Never work out a container dose from a per-square-metre figure.

Tips

• A kitchen scale left in the shed in a sealed bag is one of the more useful things you can put there, and it costs almost nothing.
• Organic fertilisers have a much wider safety margin than mineral ones. Getting a chicken manure pellet dose 50 per cent wrong rarely matters; getting a soluble mineral dose 50 per cent wrong on a dry bed can kill plants.
• Applying to dry soil is worse than applying too much to moist soil. Water the ground first if it has not rained.
• The one time to be precise rather than approximate is with anything containing a herbicide, such as lawn weed and feed. Over-application there does damage you cannot undo.
• Keep the pack. The rate is on it, it varies between products, and a decanted fertiliser with no label is a guess waiting to happen.

Common mistakes

• Applying by the handful without ever having weighed a handful — Handfuls vary by a factor of three between people and between products. One calibration with a kitchen scale removes the guesswork permanently.
• Doubling the dose because the plant looks unwell — An unwell plant is usually short of water, waterlogged, root-damaged or diseased. Doubling fertiliser on any of those makes it substantially worse.
• Using a per-square-metre rate on a container — A pot has no drainage into surrounding soil and no buffer. Salts accumulate and roots scorch. Container rates are on the pack separately for a reason.
• Applying more organic fertiliser because 'you cannot overdo organic' — You can. Chicken manure pellets at several times the rate will scorch, and repeated heavy applications build up phosphorus and nitrogen that run off into watercourses.

Questions people ask