If soil pH is too low for a plant, we say so and which way to fix it
As recorded: A measured soil pH below a plant's sourced floor (the lowest pH a published source gives for it) is worth saying out loud, with the shortfall and the direction of the fix. Above the ceiling too, but only for a plant that is documented to require acid soil. For every other plant the upper limit is Milpa's own figure, not a source's, so going over it raises no warning.
A good hunch A person has checked it against its sources. If you skip this, it still grows, just less well. The planner suggests it.
How it works
Nutrient bioavailability is pH-dependent, so below the crop's optimum band phosphorus and molybdenum become less available; below about 5.5 the effect turns acute as aluminum and manganese dissolve fast enough to damage roots directly. Above an acid-requiring plant's range the same curve runs the other way. Iron becomes unavailable and the plant goes chlorotic.
When this applies to a garden, Milpa says: This bed's measured pH sits below what this plant wants. Near the floor that is a question of yield. Survival isn't at stake. The floor is the pH at which this plant's own source says to lime (for most vegetables, University of Maryland Extension's table). The plant doesn't die there, and the figure is an estimate. Below about 5.5 it becomes a harder problem, because aluminum and manganese dissolve and damage roots. The soil card shows how to reach a soil lab.
What to do about it
Lime raises pH. Use the amount your soil lab gives: only its lime requirement test of your own soil measures how much your ground needs, so we do not give a rate. For a plant that needs acid soil and sits above its range, sulfur lowers pH; for how much, ask your lab for its sulfur recommendation, and we give no rate for that either.
The evidence
Evidence: checked against 2 sources in the research-based literature.
The 2 sources
At pH 5.5 or lower, the solubility of Al increases 1000-fold for every pH unit decrease. For example, at pH 5.0, Al solubility is only 0.05 ppm, but at pH 4.0, Al solubility increases to a toxic level of 51 ppm.
Plus one working note on how the sources were read. Every note is in the data download.
Read next
How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- Details updated
- Advice on what to do added
- Sources updated
- Details updated
- Advice on what to do added
- Advice on what to do added
- Wording made clearer
- Wording made clearer
- Wording made clearer
- Grade changed to A good hunch
- Details updated
- Added to the rules
More ways Milpa can help
When do I plant? · What grows well together? · Plan my garden · What can I plant now? · Something wrong in the garden? · Keep a garden diary · Heard a tip?
Cite this page
Milpa Gardens. “If soil pH is too low for a plant, we say so and which way to fix it.” Evidence grade: A good hunch. https://milpa.garden/rules/measured-soil-ph-below-plant-s-sourced/
A garden planner built on graded evidence. 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.
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How we grade evidence
- Well established
- The rule names its mechanism and is settled, by sources a person has read and checked or by plain arithmetic and geometry.
- Promising
- The mechanism is named and the literature backs it. A few questions are still open.
- A good hunch
- Plausible by its mechanism, but field studies are thin, so treat it as a lean.
- Contested
- The evidence points both ways, or no study has tested it, so the planner does not enforce it.
- Refuted
- The claim did not survive its sources. It is kept so the folklore can be answered.