Broadfork™

How to

Test and adjust soil pH

The beds are empty and the soil is still workable. Send some of it to a lab before you add lime or anything else.

Grown in Ohio, zone 6a. Your frost dates move the calendar; the signals don't. Broadfork goes by where your garden is, not its zone: frost dates for your ZIP code in the US, or the ones you type in anywhere else.

Four minutes to read. The steps stand on their own.

6.5the usual target
2 to 3 yearsbetween tests
3 to 6 monthsfor lime or sulfur to work

Before you start#

Soil pH decides what the roots can take up. Nutrients come easiest in slightly acid soil, around 6.5. Near 8.0, phosphate, iron and manganese get harder to reach; at 4.5 or below, calcium, magnesium and phosphorus run short and aluminum can turn toxic.

Most vegetables do best from 6.1 to 6.9, and 6.5 is the usual target; the data's bands run from 5.8 to 7.0. If yours is within half a point of the target, changing it probably won't help.

Add nothing without a test. Plenty of gardens are near neutral already, where lime won't help and may hurt.

Changing pH takes months, so test a year ahead of the crop that needs it.

The steps#

  1. Sample in the fall, or any time the soil isn't frozen or wet. Fall gives lime the winter to start working.
  2. Dig 5 to 10 spots, 6 to 8 inches deep, in a zigzag. Mix them in a clean plastic bucket, never metal, and leave out the mulch on top.
  3. Send it to a lab, not a kit. Home kits tend to be unreliable, and only the lab's buffer test says how much lime to use. Extension offices list the labs that take garden samples; call the lab before you sample.
  4. Read the pH and the lime requirement. The report also gives phosphorus, potassium, calcium and magnesium. Within half a point of 6.5, leave it be.
  5. Too low: lime it in the fall, at the rate on the report. Use ground or pelletized limestone, dolomitic if the report says magnesium is short, and mix it into the top 6 to 8 inches. Over 100 lb per 1,000 square feet, split it into doses of 40 to 50 lb. Skip burned and hydrated lime, which are hazardous and hard to spread.
  6. Too high: elemental sulfur, and give it a season. Not aluminum sulfate, which can poison plants with aluminum. Put on no more than 20 lb per 1,000 square feet at once, 8 on ground that's planted. The data splits on when: spring, because the bacteria that turn it to acid work in warm, moist soil, or the fall before, a year ahead.
  7. Test again. The season after a change, 4 to 6 months after sulfur, and every year while you're still correcting it.
acid alkaline 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 most vegetables, 6.1 to 6.9 the target is 6.5 clubroot strikes cabbage below 6.0 potatoes, if scab is a problem nearer 5.2 blueberries, 4.5 to 5.5 at 4.5 or below, aluminum can be toxic near 8.0, phosphate, iron and manganese are harder to take up
  1. 4.5At 4.5 or below, aluminum can be toxic.
  2. 4.5 to 5.5Blueberries.
  3. 5.2Potatoes, nearer this if scab is a problem.
  4. 6.0Clubroot strikes cabbage below this.
  5. 6.5The target. Most vegetables do well from 6.1 to 6.9.
  6. 8.0Near this, phosphate, iron and manganese are harder to take up.
One scale, acid to alkaline. Most vegetables do well from 6.1 to 6.9, and 6.5 is the target. Blueberries want it far lower, and potatoes lower when scab is the trouble.

Give it time#

Lime is slow: three to six months by some of the data, six months to a year by another, and one to three years to reach its peak.

Sulfur is slow too, and the data splits on how long it holds: by one account the pH stays low for more than five years, by another the effect fades and wants repeat doses.

Never lime every year by habit. That advice comes from the acid soils of the Southeast, and without a test it can push a garden too alkaline.

If it goes wrong#

Too much lime
The soil turns alkaline, and near 8.0 plants struggle to take up phosphate and iron. Stop liming, and add sulfur only on a test.
The lime did nothing
Give it time, and mix it in; lime works best tilled into the soil.
The sulfur did nothing
Some soils carry free lime and are impractical to acidify, and hard irrigation water undoes it.
Scabby potatoes
Scab is milder at 5.0 to 5.2. Skip the lime where the potatoes go.
Cabbage with clubroot
It strikes below 6.0. Lime to 7.2 where it's a problem.
Melons yellow and set no fruit
The soil is below 6.0.

The exceptions, by crop#

Every number on this page comes from a published extension source.

Sources for this guide: Ohio State University Extension, Virginia Cooperative Extension, University of Minnesota Extension, Penn State Extension, Clemson Cooperative Extension, Purdue Extension, Cornell Cooperative Extension, Oklahoma State University Extension, University of Connecticut, University of Delaware Cooperative Extension, University of Georgia Extension, University of Maryland Extension, University of Missouri Extension, University of Nebraska–Lincoln Extension, University of Vermont Extension, University of Wisconsin-Madison Division of Extension.

The documents behind this guide (42)

Updated 11/11/2026 · Checked against our sources 11/12/2026

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