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The short answer: five numbers matter. pH should sit at 6.0 to 7.0 for most plants, because it controls whether other nutrients are even available. Phosphorus under about 15 ppm is low, over 50 ppm is high — stop adding. Potassium is optimum around 175 to 250 ppm. Organic matter should be 3 to 5 percent. CEC tells you how much the soil can hold and how hard it will be to change.
A soil test is the cheapest decision-making tool in gardening, and the report is intimidating for about ten minutes. Once you know which numbers act on which, most of the page turns out to be supporting detail around a small number of real decisions.
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Take the Sample Properly or the Report Is Fiction
The lab analyzes what you send. A careless sample produces precise numbers describing soil that does not exist anywhere in your yard.
- Sample each area separately. Vegetable beds, lawn and ornamental borders have different histories and different targets. Do not blend them.
- Take 10 to 15 cores scattered across the area in a zigzag, not one scoop from one spot.
- Go to the right depth: 6 to 8 inches for garden beds and new plantings, 3 to 4 inches for established lawns, since that is where lawn roots and amendments actually sit.
- Mix in a clean plastic bucket. Never galvanized metal, which contaminates the zinc reading, and never a bucket that has held fertilizer.
- Remove roots, stones and thatch, then air-dry the sample on newspaper. Do not oven-dry it.
- Avoid recently amended ground. Wait at least six to eight weeks after applying lime, fertilizer or fresh manure, or you will measure the amendment rather than the soil.
A state extension lab test typically costs $15 to $30 and returns pH, buffer pH, phosphorus, potassium, calcium, magnesium, organic matter, CEC, base saturation and often micronutrients, with amendment recommendations by crop. A home kit gives you a usable pH reading and rough NPK for less money and no waiting. For anyone planning a new vegetable garden or chasing a persistent problem, the lab test is decisively the better spend — the comparison is in best soil test kit.
pH: The Number That Controls the Others
pH is a logarithmic measure of acidity, so 5.5 is ten times more acidic than 6.5. It matters because it governs nutrient solubility. Below about 6.0, phosphorus locks up with iron and aluminum, and calcium and magnesium become scarce. Above about 7.5, iron, manganese, zinc and again phosphorus become unavailable — which is why plants in alkaline soil show yellow leaves with green veins while the soil test reports plenty of iron. A soil that tests low in everything is very often a soil at the wrong pH.
| Plants | Target pH |
|---|---|
| Most vegetables and herbs | 6.0 to 7.0 |
| Lawns, cool and warm season | 6.0 to 7.0 |
| Potatoes | 5.0 to 6.0, which also suppresses scab |
| Blueberries, azaleas, rhododendrons, camellias | 4.5 to 5.5 |
| Brassicas, asparagus, most stone fruit | 6.5 to 7.5 |
| Lavender and Mediterranean herbs | 6.5 to 7.5 |
To raise pH, use ground agricultural limestone. As a rough guide, raising pH by one full point over 100 square feet takes about 3 lb of lime on sandy soil, 5 lb on loam and 8 lb on clay — the heavier the soil, the more it resists. Use dolomitic lime if your magnesium is low and calcitic lime if magnesium is already adequate, since dolomitic lime supplies both. Lime is slow: apply in fall, water it in, and retest the following season. Never apply more than about 5 lb per 100 square feet in one season.
To lower pH, use elemental sulfur. Roughly 1 lb per 100 square feet lowers sandy soil by a point and about 2 lb does the same on loam and clay. Sulfur works through soil bacteria oxidizing it, so it needs warm, moist soil and several months. Aluminum sulfate acts faster but adds aluminum, which is toxic to plants at concentration, so most extension services advise against it outside specific blueberry situations.
If the report shows a buffer pH as well, that is the lab measuring the soil’s resistance to change, and it is what the lime recommendation is actually calculated from. Trust the recommendation over any generic rate table, including the one above.
Phosphorus and Potassium
Nitrogen is usually absent from a standard soil test or reported with a caveat, because it is mobile and changes week to week — the reading would be obsolete before the report arrived. Recommendations for nitrogen come from the crop, not from the soil. Phosphorus and potassium are stable enough to measure meaningfully.
| Rating | Phosphorus, ppm | Potassium, ppm | What to do |
|---|---|---|---|
| Very low | Under 8 | Under 60 | Correct directly this season |
| Low | 8 to 15 | 60 to 100 | Amend and retest in two years |
| Medium | 15 to 30 | 100 to 175 | Maintenance only |
| Optimum | 30 to 50 | 175 to 250 | Add nothing beyond compost |
| High or excessive | Over 50 | Over 250 | Stop adding entirely |
Those bands vary with extraction method — Mehlich-3, Bray-1 and Olsen give different absolute numbers — so read your level against the rating your lab printed beside it rather than against a figure from another source.
Low phosphorus: bone meal, rock phosphate for a slow long-term supply, or a balanced organic fertilizer. Phosphorus barely moves in soil, so mix it into the root zone rather than surface-spreading it. Note that cold soil suppresses phosphorus uptake, so purple-tinged seedlings in a cold spring often indicate temperature rather than deficiency.
High phosphorus is genuinely common in established gardens, because compost and manure both carry a lot of it and gardeners add both for years. Excess phosphorus interferes with zinc and iron uptake and runs off into waterways, where it drives algal blooms. If your test reads high, switch to a low-phosphorus or phosphorus-free fertilizer and keep composting — the compost is still worth it for the organic matter.
Low potassium: potassium sulfate, sul-po-mag if magnesium is also low, greensand for slow release, or wood ash used sparingly since it raises pH sharply. Potassium leaches readily on sandy soils, so split applications there.
Organic Matter
Organic matter is measured by burning off the carbon in a sample and weighing the loss, and it is the single most informative number on the report because it drives three things at once.
| Organic matter | Interpretation |
|---|---|
| Under 2 percent | Low — typical of new-build lots and long-tilled ground |
| 2 to 3 percent | Below par for a garden; improving is the priority |
| 3 to 5 percent | Healthy garden soil |
| Over 5 percent | Excellent; typical of long-established beds with an annual compost habit |
Organic matter holds water, feeds soil biology, and mineralizes slowly into plant-available nitrogen — a commonly cited extension figure is roughly 20 to 30 lb of nitrogen released per acre per year for each percent of organic matter. It also builds the crumb structure that makes clay drain and sand hold together, which is why it is the answer to both problems. See how to improve clay soil.
Raising it is slow and there is no shortcut: an inch or two of compost annually, permanent mulch, cover crops, and minimal tillage. Expect to gain a fraction of a percentage point a year. If your supply is the limiting factor, the systems are compared in tumbler vs compost bin, with the method in how to make compost and equipment in best compost tumbler.
CEC and Base Saturation
Cation exchange capacity, reported in milliequivalents per 100 grams, is the soil’s capacity to hold positively charged nutrients — calcium, magnesium, potassium, ammonium — against leaching. It is set mainly by clay content and organic matter, which is why it is the number you cannot quickly change.
| CEC | Soil type | Management |
|---|---|---|
| 1 to 5 | Sandy | Nutrients leach fast — feed little and often; pH shifts easily, so lime lightly |
| 5 to 15 | Loam, sandy loam | Straightforward; standard rates apply |
| 15 to 30 | Clay, clay loam | Holds nutrients well; needs more lime or sulfur to shift pH |
| Over 30 | Heavy clay or high organic matter | Very high reserve; changes take multiple seasons |
Base saturation breaks that capacity down by which nutrient is occupying it. Broadly healthy ranges are calcium 60 to 70 percent, magnesium 10 to 20 percent and potassium 2 to 5 percent. The ratios between them are less important than most soil-balancing marketing suggests — the research consensus is that as long as each nutrient is individually sufficient and pH is right, the ratios themselves rarely limit yield.
One base-saturation figure is worth real attention: sodium. Above roughly 15 percent exchangeable sodium, the soil is sodic, clay structure disperses, and gypsum is genuinely the correct remedy. That is the only common situation in which gypsum improves soil structure, and it is why gypsum sold as a general clay-breaker is overselling.
Turning the Report Into Three Decisions
Work through it in this order and most reports resolve quickly.
- Fix pH first. Nothing else works properly until it is right, and everything else is cheaper once it is. Follow the lab’s lime or sulfur recommendation, apply in fall, and retest the next season.
- Correct anything rated low or very low, using the specific amendment for that nutrient rather than a balanced fertilizer that adds things you already have too much of.
- Raise organic matter, always. This is the only recommendation that is right on essentially every report, and it improves water retention, nutrient supply, structure and biology simultaneously.
Then do nothing else. The most expensive habit in home gardening is applying amendments nobody measured a need for — lime on soil that was already at 6.8, phosphorus on soil that was already high, and micronutrient blends for deficiencies that were really a pH problem. Retest in three years and let the numbers tell you what changed.
If the test was prompted by a struggling lawn rather than a bed, pair the results with the practical fixes in dethatching vs aerating and the feeding programs in best lawn fertilizer — soil pH below 6.0 is a frequent and rarely suspected cause of a lawn that will not thicken no matter how much fertilizer it gets.