Limed ground is the wrong ground for the next potato crop: common scab worsens as soil pH rises, and lime raises it.
Promising · suboptimal if ignored → advise
How it works
Common scab (Streptomyces scabies) infects young tubers through the lenticels and is favoured as soil goes less acid; at about pH 5.2 and below it is largely suppressed. Lime raises pH and its effect lasts years, so ground limed for a crop that wants sweet soil carries a scab-prone pH into the seasons that follow.
What to do about it
This ground has been limed, and potatoes want it sour. Scab is skin-deep - the tubers still eat - but where it has been a problem, put the potatoes on ground you have not limed, choose a scab-resistant variety, keep the soil evenly moist for the four to nine weeks after planting while the tubers set, and never lime or add wood ash where potatoes are going next. A later pH reading at or below about 5.2 on this ground would quiet this. We will not give a sulfur rate to get there: the amount needs your lab's buffer pH.
The evidence
Evidence: confirmed against 5 sources in the research-based literature.
the 5 sources
UMass Extension, 'Potato, Scab', umass.edu/agriculture-food-environment/vegetable/fact-sheets/potato-scab (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Common scab is controlled or greatly suppressed at soil pH levels of 5.2 or lower'; 'Avoid application of soil amendments that increase soil pH'; 'Increase rotation with nonhost crops to 3-4 years'; 'Maintain adequate soil moisture during and after tuber set.'
Wisconsin Horticulture Extension, 'Potato Scab', hort.extension.wisc.edu/articles/potato-scab (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Scab tends not to develop when the soil pH is less than 5.2'; 'For home gardens, keeping the soil pH at approximately 6.5 is a good compromise'; 'Avoid using fertilizers (e.g., lime, calcium and potassium nitrate, or fresh manure) that may increase soil pH'; the disease 'can also affect other root vegetables such as beets, carrots, parsnip, radish, rutabaga, salsify and turnip.'
Clemson HGIC, 'Irish Potato & Sweetpotato Diseases', hgic.clemson.edu/factsheet/irish-sweet-potato-diseases (fetched in-container 2026-09-02; read by the maintainer 2026-09-02), common scab: 'Soil pH should be between 5.0 and 5.2'; 'avoid alkaline materials, especially lime and wood ashes, but also avoid mushroom compost and leaf compost. All of these will raise the soil pH'; 'Scab is favored under low soil moisture conditions, so the garden soil must be kept moist during the active growing period of the tubers (particularly 4 to 9 weeks after planting)'; 'Do not plant Irish potatoes after beets, carrots, radishes, parsnips, rutabagas, or turnips, which are also susceptible to scab.'
Univ. of Maryland Extension, 'Common Scab of Vegetables', extension.umd.edu/resource/common-scab-vegetables, and 'Growing Potatoes in a Home Garden', extension.umd.edu/resource/growing-potatoes-home-garden (both fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'The disease is encouraged by soil pH above 5.5'; 'Where potato scab disease is a persistent problem, a soil pH of 4.8 to 5.8 is recommended'; 'It can also be a problem of beets, carrots, parsnips, turnips, and radish.'
Univ. of Maine Cooperative Extension, Bulletin 2077, 'Potato Facts: Growing Potatoes in the Home Garden', extension.umaine.edu/publications/2077e (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'a soil pH of 5.3 to 6.0 is typical for potato production'; 'higher soil pH levels are more conducive to scab, a potato disease caused by a soil-borne pathogen'; 'Lime will also raise the pH of soil. A soil test will tell you how much lime to add.'
Plants this fires on
Change history
Every change to this rule is one commit. This is what the project's version history records — the date and the reason — not a summary of it.
- rules: another rule here - potato scab vs. the liming sequence (verified), the first rule fired by a recorded action (O133 step 5)
Cite this page
Milpa Gardens. "Limed ground is the wrong ground for the next potato crop: common scab worsens as soil pH rises, and lime raises it." Evidence grade: Promising. https://milpa.garden/rules/limed-ground-wrong-ground-next-potato-crop/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.