Soil pH by Plant: Preferred Range for Common Vegetables

In short: Every vegetable in Oklahoma State's table tops out at the same pH; what separates them is how far into acid soil each one copes. Find your crops, then hold your soil test result against each row before you add lime or sulfur.

Reading the table

The table is Oklahoma State University Extension's list of soil pH preferences for 36 vegetables, from its fact sheet on reading soil tests for vegetable crops (the fact sheet says it adapted the list from Knott's handbook for vegetable growers). It sorts crops into three groups, and the groups differ only at the bottom. The 12 least tolerant of acidity, from asparagus and beet to lettuce and spinach, want 6.0 to 6.8. A middle group of 20, including tomato, pepper, beans and squash, is listed at 5.8 to 6.8, with Oklahoma State's note that these crops tolerate the lower end but grow more successfully at 6.0 to 6.8. Only 4 crops (potato, rhubarb, sweet potato and watermelon) are listed down to 5.5 to 6.8.

No crop on the list is given a preference above 6.8. That matches the reason Oklahoma State gives for caring about pH at all: most plant nutrients are most available in mildly acidic soil, 6.0 to 7.0.

What happens outside the range

The University of Florida's Extension publication on soil pH for vegetables, written for commercial growers, explains what goes wrong at the edges. It calls 5.5 to 7.0 suitable for most vegetable crops. At 8.0 or higher, it says, iron and manganese are too unavailable to meet most crops' needs; at 5.0 or lower, aluminum, iron, manganese and zinc dissolve to levels that are toxic to most vegetables.

pH also changes disease pressure. UF/IFAS says clubroot of cabbage, mustard and other crucifers is a major epidemic disease below pH 5.7, is dramatically reduced from 5.7 to 6.2 and is virtually eliminated above 7.3. Potato runs the other way: common scab is favored above pH 5.2, and between that and 8.0 it gets worse as soil turns more alkaline. That is one reason potato sits in the most acid-tolerant group.

Worked example: a reading of 5.6 in a mixed bed

Say your soil test comes back at 5.6 and the bed is planned for tomatoes, cabbage and potatoes. Tomato is in the middle group, 5.8 to 6.8, so the reading sits just below it; UGA Extension's tomato circular gives a target of 6.0 to 6.8, which matches Oklahoma State's best range for that group. Cabbage, in the strictest group at 6.0 to 6.8, is further below, and the reading is under the clubroot threshold UF/IFAS gives. Potato, at 5.5 to 6.8, is inside its range, and raising the pH would favor scab.

The table points to a decision rather than an average. Lime the part of the garden going to tomatoes and cabbage, as your soil test report recommends, and give the potatoes a separate bed that you leave alone. Oklahoma State adds that farmed soils tend to become more acidic over time, so retest the limed bed in a later season.

Before you add anything

The table tells you the target, not the dose. How much lime moves the pH depends on the soil: UF/IFAS notes that clay soils have more buffering capacity and need more lime for the same change than sandy soils, and Oklahoma State's lab reports a buffer index for exactly this when a sample reads below 6.2. Use the amount your state soil-testing lab gives. UF/IFAS also advises against using sulfur to lower a naturally alkaline soil, and suggests talking to your county Extension agent before trying.

For the rest of the bed plan, see crop rotation by plant family, which puts cabbage in the mustard family with broccoli, kale and turnip, and the first-year garden checklist, which starts with the soil test. If tomatoes in the limed bed later show dark, sunken patches on the fruit, the blossom end rot fix covers the calcium side of liming.