Garden questions, answered
Short, sourced answers to the questions gardeners ask most
Each one comes with the rate, the timing and the product that fixes it.
We start with soil pH, because it decides what every feed can do. More topics follow.
Free UK delivery over £40 · Dispatched in 1 to 2 working days · Packed and posted from Stockport
Test and understand your soil
Start here. Every fix below depends on a reading.
How do I test soil pH?
Test it at home in ten minutes
- Five or six samples, 10 to 15 cm down, mixed
- Colour-chart kit, distilled water, daylight
- The vinegar fizz test for chalk
Which soil pH tester should I buy?
Skip the two-prong probe
- Cheap metal-probe meters failed a 2017 university test
- A colour-chart kit is enough for most gardens
- Glass-electrode meters for frequent testing
Is my soil acidic?
Acidic soil: the signs, and what to do
Granulated Lime · from £10.49 for 1.5 kg
- Moss with sorrel, thriving rhododendrons, clubroot
- Over 40% of UK arable soil is below target
- Lime for veg and lawns; leave it for acid-lovers
How much lime or sulphur do I need?
The soil pH calculator
- Your reading, soil type and area in
- The dose, the split and the timing out
- The bags to buy, at live prices
Make soil more acidic
For blueberries, rhododendrons, camellias and blue hydrangeas.
How do I make soil more acidic?
Mix in sulphur and give it a season
Sulphur Soil Acidifier · from £12.00 for 1.5 kg
- 55, 110 or 175 g/m² per pH unit, sand to clay
- Spring to early autumn, retest after three to six months
- Chalky soil cannot realistically be acidified
Can I make soil acidic naturally?
Most home remedies barely move pH
Sulphur Soil Acidifier · from £12.00 for 1.5 kg
- Used coffee grounds sit at about pH 6.5 to 6.8
- Vinegar is buffered back within days
- Soil bacteria turn sulphur into acid for years
How do I make soil acidic for blueberries?
pH 4.5 to 5.5, started a year ahead
Sulphur Soil Acidifier · from £12.00 for 1.5 kg
- Sulphur into the bed six to twelve months before planting
- Up to 85 g a bush a year once planted
- Rainwater only
Iron sulphate or sulphur?
Iron is quick; sulphur goes eight times further
Sulphur Soil Acidifier · from £12.00 for 1.5 kg
- Iron sulphate regreens leaves in weeks
- Sulphur changes the soil for years
- Both: iron for now, sulphur for next year
What is ericaceous compost?
Lime-free compost for pots
Sulphur Soil Acidifier · from £12.00 for 1.5 kg
- Pots and planting pits: ericaceous compost
- Borders: sulphur changes the soil itself
- No sulphur in pots (RHS)
Raise pH and lime
For veg beds, brassicas, lawns and pink hydrangeas.
How do I raise soil pH?
Add lime; a test tells you how much
Granulated Lime · from £10.49 for 1.5 kg
- 0.4 to 0.6 kg/m² lifts a bed from 6.0 to 6.5 (RHS)
- Autumn or winter on empty beds
- Never before potatoes
Should I put lime on my lawn?
Only after a test, below pH 5.5
Granulated Lime · from £10.49 for 1.5 kg
- 225 to 300 g/m² per half pH unit
- September to November is best
- A month away from lawn feed
Does lime kill moss?
No. It helps grass on acid soil
Granulated Lime · from £10.49 for 1.5 kg
- Moss is mainly shade, wet and compaction
- Lime only below pH 5.5
- Rake the moss, then fix the cause
Hydrangea colour
Mophead and lacecap hydrangeas only. White ones stay white.
How do I make hydrangeas blue?
Acid soil, about pH 5.5 or below
Sulphur Soil Acidifier · from £12.00 for 1.5 kg
- Up to 25 g/m² of sulphur around established plants
- Rainwater, not tap water
- One or two seasons to change
How do I make hydrangeas pink?
Soil near neutral, about pH 6.5
Granulated Lime · from £10.49 for 1.5 kg
- Granulated Lime over the root area
- No ericaceous compost
- Expect a mauve stage
Does aluminium sulphate work on hydrangeas?
Fast, but it builds up in borders
Sulphur Soil Acidifier · from £12.00 for 1.5 kg
- Quick colour in a pot, at the pack rate
- Aluminium can build up in the ground (RHS)
- Sulphur is safer for borders
Yellow leaves on acid-lovers
Yellow between green veins usually means the pH is too high.
Why are my rhododendron leaves turning yellow?
The soil is not acid enough
Sulphur Soil Acidifier · from £12.00 for 1.5 kg
- Young leaves yellow, veins green: pH too high
- Old inner leaves dropping: normal
- Up to 25 g/m² of sulphur, spring and late autumn
Why are my camellia leaves turning yellow?
Look at the veins first
Sulphur Soil Acidifier · from £12.00 for 1.5 kg
- Hard tap water is the usual cause in pots
- Sulphur in the ground; ericaceous compost in pots
- Camellias want pH 5.0 to 6.5
Why are my azalea leaves turning yellow?
Check which azalea you have
Sulphur Soil Acidifier · from £12.00 for 1.5 kg
- Deciduous azaleas colour up every autumn
- Evergreen with green veins: the pH
- Rainwater, always
Why are my blueberry leaves turning yellow?
Above about pH 5.3, iron locks up
Sulphur Soil Acidifier · from £12.00 for 1.5 kg
- Up to 85 g of sulphur a bush a year
- Autumn red and yellow is normal
- Tap water is the usual cause in pots
Common questions
What soil pH do most plants need?
Most garden plants are happy between pH 6.0 and 7.0, and the RHS gives 6.5 as the best all-round figure. Blueberries want 4.5 to 5.5, and rhododendrons and azaleas 4.5 to 6.0.
Lime or sulphur?
Lime raises the pH; sulphur lowers it. Test first, then use lime where the reading is below what you grow needs, and sulphur only where acid-lovers grow on soil that reads too high.
How long does it take to change soil pH?
Fine lime shifts the pH within weeks. Sulphur takes a season, because soil bacteria turn it into acid slowly, and needs warm, moist soil.
Can I change the pH of chalky soil?
You can raise it, but you cannot realistically lower it. If a dry spoonful fizzes with vinegar, grow acid-lovers in pots or raised beds of ericaceous compost (RHS).
Work it out in a minute
Your soil test reading, what you grow, your soil type and the area. The calculator gives the dose, the timing and the bags.
Free UK delivery over £40 · Dispatched in 1 to 2 working days
Written by Joe Barrington, Dr Forest, Stockport. Rates and sources are on each answer page.