Soil Test Results, Explained

A standard garden soil test tells you five things: pH, phosphorus, potassium, organic matter, and an estimate of texture. Start with pH, because it controls whether the other nutrients are usable. Then check phosphorus, because in long-composted gardens it is the number most often too high. Ignore the urge to fix everything at once. Most reports need one or two changes, and many need none beyond "stop adding phosphorus."

What's on a standard report

The University of Minnesota's regular lawn and garden test is typical: phosphorus, potassium, pH (with a lime requirement if needed), organic matter percentage, and estimated texture, for $22 a sample. Soluble salts, lead, micronutrients, and nitrate are extra-cost add-ons. Other land-grant labs offer similar packages, though the extraction methods and interpretation ranges differ by region, which is why two labs can put the same number in different categories.

The recommendations section at the bottom of the report is calibrated to that lab's methods and local soils. If a number seems to disagree with something you read online, trust the report.

pH

pH is the one to read first. It measures the acidity of the soil water, and it governs how available nutrients are to roots. Cornell's guide to vegetable garden soil test reports puts the optimum for most vegetables at 6.0 to 7.0. Iowa State gives a slightly wider 6.0 to 7.5 for most horticultural crops, and the University of Maryland says 5.5 to 7.0. The overlap, roughly 6.0 to 7.0, is a safe target. Blueberries are the main exception and want much more acidic soil.

If pH is low, the report will usually include a lime recommendation. If it's high, elemental sulfur lowers it. For heavily composted or manured beds that have drifted alkaline, UMN's guide to correcting too much compost and manure gives rates of 0.8 lb of sulfur per 100 square feet on sandy soils and 2.4 lb on loam or silt loam.

Phosphorus: Bray, Olsen, and others

Phosphorus is reported by an extraction method, and the method matters. The Bray-P1 test is designed for acidic and neutral soils, and the Olsen test for alkaline ones. The UMN lab runs Bray on every sample and adds Olsen when pH is above 7.4, basing its recommendation on the Olsen number when one is reported. Oregon State uses Bray west of the Cascades and Olsen east of them. Other labs use other extractions again. The numbers are not interchangeable.

Here's how two extension services rate the same tests. Note how much the cutoffs differ:

All values are in ppm. The gap reflects different soils, crops, and calibration histories, not a mistake on either side. UMN's working rule for gardens is simple: above 25 ppm Bray or 18 ppm Olsen, don't add phosphorus for a few years, and that includes compost. If your number is high, read can you use too much compost? before your next delivery.

Potassium

Potassium (K) is usually less of a problem. Iowa State says no potassium fertilizer is needed above 125 ppm and that deficiency is likely below 50 ppm. Oregon State rates under 150 ppm as low and over 800 ppm as excessive. Again, use your lab's ranges.

If you need potassium but your phosphorus is already high, use a source without phosphorus. UMN suggests potassium sulfate or langbeinite.

Organic matter

Organic matter is reported as a percentage by weight, and more is not always better. Cornell suggests about 2 to 2½ percent for sandy soils and 3 to 5 percent for clay soils, and warns that twice those levels could lead to nutrient imbalances. Iowa State calls 3 to 6 percent ideal. The University of Maryland treats 2 percent or more as healthy.

Organic matter also changes slowly. Oregon State notes that a 2 percentage-point increase, say from 4 to 6 percent, is a large change and about the most you can expect in most situations. A reading well above the ranges above in a mineral garden soil usually points to years of heavy compost, not exceptionally good soil.

Labs also use organic matter to estimate nitrogen needs, since it releases nitrogen as it decomposes. Cornell's rule of thumb: for every 1 percent organic matter, you can cut applied nitrogen by 0.5 ounce per 100 square feet for the year.

CEC

Cation exchange capacity is a measure of how well the soil holds positively charged nutrients such as potassium, calcium, and magnesium. It's mostly determined by clay and organic matter content, so sandy, low-organic soils have low CEC. You can't change it quickly. The University of Maryland suggests adding organic matter if CEC is below 10. Not every lab reports it.

Soluble salts

If you ordered a salts test, it will be reported as electrical conductivity in mmhos/cm, which is the same as dS/m. Oregon State's guide rates soil EC under 1.0 as suitable for crops, 1.0 to 2.5 as marginal, and over 2.5 as poor. Salts accumulate from fertilizer, manure-based compost, and road salt, and they're worth testing if you've applied a lot of manure or plants are struggling for no obvious reason.

What's missing: nitrogen and lead

Nitrogen. Most home tests don't report it. Iowa State explains that nitrate isn't held by soil and dissolves easily in water, so it leaches and a reading taken today may not reflect what's there in a few weeks. Labs estimate nitrogen needs from organic matter and the crop instead.

Lead. Lead isn't part of a fertility test and has to be ordered separately. It matters most near older houses and busy roads. UMass Amherst's soil lead fact sheet explains how to interpret results and which practices, such as raised beds and favoring fruiting crops over leafy and root crops, reduce exposure.

What to do next

1. Fix pH first if it's outside about 6.0 to 7.0.
2. If phosphorus is high or very high, stop adding it, including compost and manure.
3. Add potassium only if it's low, from a source that doesn't also contain phosphorus.
4. Supply nitrogen based on the report's recommendation and your crop.
5. Retest every three to five years, or when you make a big change, such as turning lawn into a garden bed.

The rest of the approach, from mulch to cover crops, is in building garden soil organically.