How to read a soil test report
There are three kinds of number on the page — what the lab measured, how it rated it, and what it recommends — and almost every mistake comes from reading one as another.
What the report is actually telling you
A home soil report has three kinds of number on it, and they are not interchangeable:
1. Measurements. pH, buffer pH, organic matter, CEC, and extractable P, K, Ca, Mg. These are what the lab found. They are reported in ppm or lb/acre depending on the lab, and the same soil can read differently at two labs because they use different extractants — Mehlich-3, Bray, Olsen, Morgan. A ppm figure is only comparable against the same lab's own ratings.
2. Ratings. "Low / medium / optimum / high". This is the lab interpreting its own measurement for your stated crop. It is the most useful column on the page and the one people skip.
3. Recommendations. Pounds of N, P₂O₅ and K₂O per 1,000 sq ft or per acre, plus a lime rate. This is the lab telling you what to do. It already accounts for the measurement, the rating and the crop you declared.
Why the recommendation says P₂O₅ and not P
Fertilizer labelling is in oxide form by convention — phosphate (P₂O₅) and potash (K₂O), not elemental phosphorus and potassium. Reports state recommendations the same way so the number can be read straight onto a bag. If you ever need the elemental figure: P = P₂O₅ × 0.44, K = K₂O × 0.83. Mixing the two up over-applies phosphorus by more than double.
Buffer pH is the number worth paying for
Soil pH tells you where you are. Buffer pH tells you how hard the soil will resist being moved — its reserve acidity — and that is what actually sets the lime rate. Two beds reading pH 5.5 can need very different amounts of lime. A cheap probe gives you pH; a lab gives you buffer pH, and that is most of the difference between the two.
CEC and organic matter
Cation exchange capacity is the soil's capacity to hold nutrients — low CEC (sands, roughly under 10) holds little and needs feeding little-and-often; high CEC (clays, over 20) holds more and buffers harder, which is the same property that makes it need more lime. Organic matter drives CEC upward, which is why "add compost" is the one recommendation that appears on almost every report.
What to do with it, in order
1. Fix pH first. Nutrient recommendations assume the pH will be in band, and phosphorus in particular is unavailable outside it — fertilizing an out-of-band soil wastes the fertilizer.
2. Then apply the nutrient recommendation, converted to your bag.
3. Then re-test, on the interval your lab suggests — usually every two to three years for a garden, and always before liming a second time.
Your own lab's recommendation wins. Loam converts what the report already says into what your bed and your bag need — it does not set rates.
FAQ
How do you read a soil test report?
Read it in three passes: the measurements (pH, buffer pH, organic matter, CEC, extractable P/K/Ca/Mg), then the low/medium/optimum ratings the lab assigns them for your crop, then the recommendation in pounds of N, P₂O₅ and K₂O per 1,000 sq ft. Act on the recommendation; use the measurements to understand why it says what it says.
What is buffer pH on a soil test?
A second, lab-only measurement of the soil's reserve acidity — how strongly it will resist a pH change. It, not the soil pH, is what sets an accurate lime rate. Two soils reading pH 5.5 can need very different amounts of lime, and the buffer pH is what separates them.
Why does my report say P2O5 instead of P?
Fertilizer bags are labelled in oxide form, so reports state recommendations the same way to make them directly usable. To convert, P = P₂O₅ × 0.44 and K = K₂O × 0.83 — and mixing the two up over-applies phosphorus by more than double.
What is a good CEC for garden soil?
Roughly 10–20 is a comfortable middle. Below about 10 is sandy and holds little, so feed little and often; above 20 is clay-heavy, holds more, and buffers harder — which is the same property that makes it need more lime to move its pH.
How often should I test my soil?
Every two to three years for an established garden, and always before liming a second time. Testing again too soon after an amendment mostly measures the amendment.
Getting a report worth reading
The difference between a probe and a lab is the buffer pH and the recommendation — the two things this page is about.
- A mail-in lab soil test returns buffer pH, CEC, organic matter and an actual recommendation. This is the one worth paying for.
- A pH probe is fine for tracking a bed between lab tests, but it cannot give you buffer pH and so cannot set a lime rate.
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Sources
Report layouts differ by lab — UMass, NCDA, Minnesota and Missouri each format theirs differently — but all four carry the same three kinds of number described here.