Soil pH for vegetables: test it at home, adjust with dolomite, don't overcorrect

Soil pH tells you how acid or alkaline the soil is, and it directly affects whether vegetable roots can take up the nutrients already in the soil. Most vegetables like slightly acid to near-neutral soil, around pH 6.0-6.8. If the soil is much more acid than that, lime fixes it, but measure first, add a little at a time and give the lime time to work, because once you overshoot it is hard to bring back.

This follows the potting-mix lesson. It is most useful if you grow in the ground, or have potted vegetables that are struggling for no obvious reason.

What pH is, and why vegetables care

pH is a scale from about 0 to 14. 7 is neutral; below 7 is acid (“sour soil”, as Thai growers say); above 7 is alkaline. Each step is tenfold, so soil at pH 5 is about ten times more acid than soil at pH 6, not slightly more.

In strongly acid soil, roots grow poorly, the plants cope with drought less well, and they cannot make full use of nutrients. A plant may show several deficiency symptoms at once even though you fertilised. In strongly alkaline soil, some minor nutrients end up in forms the roots struggle to take up.

The pH range vegetables like

Sources give slightly different ranges, depending on the local soils they work with.

Source	Range where vegetables do well
Mississippi State University Extension	6.0-6.5
University of New Hampshire Extension	6.5-6.8
UF/IFAS (Florida sandy soils)	5.8-6.3

A practical summary: 6.0-6.8 is good. Mississippi State states plainly that vegetables do not grow well in acid soils at pH 4.5-5.5 or in alkaline soils above 7.5. If you read 6.0-7.0, change nothing and keep planting.

How to measure: kits, meters and labs

Method	What you get	Limits
Colour-match home kit	Rough pH	Not precise; shows which range you are in
Push-in soil meter	An instant number	Accuracy varies a lot; the probe needs care
Lab soil analysis	pH, nutrients and advice	You must collect and send a sample, then wait

University of New Hampshire Extension sums up home kits as “not entirely precise”, but good enough to show whether pH is in a workable range for your crops and whether it needs adjusting. Before spending on lime for a large area, get a lab test.

In Thailand, the Land Development Department has volunteer soil doctors, a land development station in each province and soil-analysis laboratories that serve farmers. Ask your nearest land development station what kind of sample they accept.

Collect a sample that means something:

1. Dig soil from several spots in the bed, from the top layer where most vegetable roots are, about 10-15 cm.
2. Pick out leaves and roots, and mix all the spots together in a clean bucket.
3. Take a portion and test it following the kit’s or meter’s instructions, every step, without skipping.

Acid soil: fix it with lime

Agricultural lime raises pH. The types differ:

Lime	Supplies	Speed
Dolomite lime	Calcium + magnesium	Slow; apply 2-3 months ahead
Ground limestone, marl	Mainly calcium	Slow
Hydrated lime	Calcium	Faster, use less, irritates skin

Mississippi State advises dolomite when a soil test shows acid soil that is low in magnesium; if magnesium is already high, either type will do. Mix the lime evenly through the top 10-12 cm, because patchy spreading leaves some spots overlimed.

Worked example: a 1 x 2 m bed at pH 5.2

UF/IFAS gives a starting rate for soil below pH 5.5: 2-3 pounds of dolomite per 100 square feet. In metric:

1. 2-3 pounds ≈ 0.9-1.4 kg, and 100 square feet ≈ 9.3 square metres
2. Per square metre ≈ 900 ÷ 9.3 to 1,400 ÷ 9.3 ≈ 100-150 g
3. A 1 x 2 m bed = 2 square metres, so about 200-300 g
4. Start at the low end, 200 g. Spread it evenly, work it in about 10-12 cm deep, and water so the soil is moist
5. Wait 2-3 months and test again. If it is still below 6.0, add another round

This is a starting point, not a fixed recipe; the amount you actually need depends on the soil type and the bed’s history. If you have a lab result, use the lab’s rate instead.

Lime is slow. What you add today will not show tomorrow. Add a little, wait and re-test, rather than adding a lot at once.

Do not overcorrect

UF/IFAS warns that too much lime can be just as bad as too little. Strongly alkaline soils need extra attention to minor nutrients, and bringing pH down is harder than raising it. Where the native soil pH is already above 7.0, adding sulfur to lower it does little good.

What to do instead of chasing a number:

• If you read 6.0-7.0, do not add more lime, even if the number is not exactly in the range you read about.
• If the soil is slightly alkaline, around 7.0-7.5, keep growing, add finished compost as usual to feed the soil, and test again next season.
• Organic matter such as compost improves soil overall, but do not expect it to shift pH quickly the way lime does.
• Limed soil slowly turns acid again, sandy soil faster than clay. Mississippi State recommends re-testing every three years.

Common mistakes

• Liming every time you prepare the bed without testing. Soil that was fine slowly turns alkaline until the plants run short of minor nutrients.
• Testing one spot and trusting it. Soil where construction rubble or old lime was dumped can differ a lot from the rest of the bed.
• Liming on planting day and expecting results. Dolomite takes weeks to months.
• Sprinkling lime on the surface without mixing it in. It stays on top, and the roots below get nothing.
• Blaming pH every time plants look poor. Yellow leaves often come from overwatering, which is the next lesson.

Sources

• Test Soil to Find Its pH Value — Mississippi State University Extension
• Soil Preparation and Liming for Vegetable Gardens (VH024) — UF/IFAS Extension
• Are garden soil test kits a good alternative to lab testing? — UNH Extension
• Use of carbonized rice hulls as a horticultural amendment for an organic soil — Pesquisa Agropecuária Brasileira (Embrapa)
• Soil analysis services of the Land Development Department (Thai) — Thairath