We check a soil pH against a plant's range only when it was measured in water, the way the ranges are written
We check a soil pH against a plant's range only when it was measured in water, the way the ranges are written. A reading in calcium chloride is kept and shown, and never converted.
As recorded: Milpa reads every soil pH range and threshold it checks plants against as water pH. A pH a lab measured in calcium chloride, or in another salt solution, is recorded and shown with its method. It is never compared with those numbers, and never converted to water pH by a fixed amount.
Promising A person has checked it against its sources. Read against a range written in water pH, a pH measured another way can flag a plant that’s fine or miss one that isn’t. The planner keeps that reading and checks no plant against it.
How it works
A pH read in a calcium chloride solution is commonly 0.5 to 1.0 unit lower than the same soil read in water, by an amount that changes with the soil's type and salt content, so a range written in water pH cannot be compared with it or reached by adding a fixed number.
This reading wasn't taken in water, or the report doesn't say how it was, and the ranges here are in water pH. Readings in calcium chloride usually run half a unit to a unit lower than in water, and the gap moves with your soil's salt, so we keep the reading without checking plants against it.
What to do about it
Ask your lab for the pH in water. Many labs measure both, and some print a water figure they worked out themselves; that one counts, because it is the lab's. Enter the water pH and the plants are checked against it.
The evidence
Evidence: checked against 11 sources in the research-based literature.
The 11 sources
As with most measurements on soil, pH values vary with the procedure used.
This method, the most widely used in Australia, is based on a soil/water ratio of 1:5 at 25°C.
Results are commonly from 0.5 to 1.0 pH unit lower than those obtained by Method 4A1 (e.g. Conyers and Davey 1988), although smaller differences are possible when soils contain high levels of soluble salts (naturally and/or from fertiliser additions) and/or when soil colloids exhibit variable surface charge characteristics.
This method provides a convenient laboratory procedure when there is a requirement to report soil pH values from both water and 0.01 M CaCl2.
The pH measured in 1 M KCl or 0.01 M CaCl 2 solution is typically lower than the pH of the same soil measured in water due to the higher concentration of H + in the resultant suspension from cation exchange phenomenon.
We now measure soil pH in a 0.01 molar calcium chloride solution, rather than in deionized water.
Soil pH drops nearly one pH unit from the very low salt condition (following heavy winter rains) to a salt content that would occur in late spring following fertilizer application
Note: Prior to February 10, 2026, soil pH was measured using a 1:1 soil/water slurry (i.e., water pH).
Soil pH is determined on a 1:1 (V/V) soil/water mixture composed of a 10 gram NCR-13 volumetric soil scoop and 10 mL deionized water.
The target pH values and the low pH limits suitable for vegetable crop production are listed in Table B-1.
Read next
How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- A person read the sources and confirmed it
- Wording made clearer
- Added to the rules
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Milpa Gardens. “We check a soil pH against a plant's range only when it was measured in water, the way the ranges are written. A reading in calcium chloride is kept and shown, and never converted.” Evidence grade: Promising. https://milpa.garden/rules/milpa-reads-soil-ph-range-threshold-checks/
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.