How to read a soil test report

pH and buffer pH

Soil pH (usually a 1:1 water slurry) measures the acidity of the soil solution now. Most field crops do well between about 6.0 and 7.5. Iowa State targets pH 6.5 for corn and soybean where the subsoil is acid, 6.0 where calcareous subsoil sits within 4 ft (1.2 m) of the surface, and 6.9 for alfalfa.

Buffer pH (SMP or Sikora) estimates the reserve acidity held on clay and organic matter, which is what lime actually has to neutralise. The lower the buffer pH, the more lime for the same soil pH. In the Iowa tables for a 6-inch (15 cm) sample and a target of 6.5, a buffer pH of 6.5 calls for about 2,800 lb/ac (3,100 kg/ha) of effective calcium carbonate equivalent and a buffer pH of 6.0 about 6,400 lb/ac (7,200 kg/ha); convert with the ECCE or agricultural index of the lime you can buy.

In no-till, strip-till and pasture, Iowa State recommends a 2 to 3 inch (5 to 7.5 cm) sample for pH and lime, and roughly halving the rate if only a 6-inch sample exists. Where soil magnesium tests 100 ppm or less, OMAFRA recommends dolomitic lime.

Organic matter, CEC and base saturation

Organic matter (OM) drives nutrient cycling, water holding and some soil-applied herbicide rates, but neither Iowa State nor OMAFRA uses it directly to set fertilizer rates. Field Crop News lists texture-based targets and warns that labs measure OM by different methods, so track it over years with the same lab.

Cation exchange capacity (CEC, meq/100 g) describes how many positive ions the soil can hold; it rises with clay and OM. Base saturation is the share of that capacity occupied by calcium, magnesium and potassium. Both are routinely over-interpreted. Neither OMAFRA nor Iowa State uses CEC or base saturation for recommendations; Ontario research found CEC-adjusted potash rates less reliable.

Phosphorus and potassium: the extractant decides what the number means

Each extractant pulls P from a different pool, so the same soil gives different numbers:

• Bray-1 (Bray P1) is an acid extractant for acid and neutral soils. Iowa State does not recommend it above pH 7.3, and Minnesota uses it only to pH 7.4, because free carbonate neutralises the acid and the test reads low.
• Olsen (sodium bicarbonate) was built for calcareous and high-pH soils and is the accredited P test in Ontario.
• Mehlich-3 extracts P, K and micronutrients in one step, so many commercial labs default to it. Read colorimetrically it tracks Bray-1 closely on Iowa soils; read by ICP it includes some organic P and reads higher, so Iowa publishes a separate ICP column. Minnesota has no Mehlich-3 guidelines and reports over-extraction on some high-pH soils.
• Potassium is almost always ammonium acetate or Mehlich-3, but Iowa State interprets field-moist samples on a different scale from dried samples, so check which your lab ran.

Minnesota's advice is blunt: labs that run one test and convert it to another by equation cause problems; use labs that run the tests calibrated for your region. OMAFRA's tables likewise apply only to accredited methods.

Interpretation categories and the build, maintain, draw-down logic

Categories turn the ppm value into a probability of yield response. Iowa State's PM 1688 (2013 revision; a 2023 update exists) gives the following for corn and soybean:

The management logic follows the categories. In very low and low, apply enough to reach full yield and build the soil test. At optimum, replace what the crop removes (Iowa uses 0.32 lb P2O5 and 0.22 lb K2O per bushel of corn). In high, no annual application is recommended, though about half of removal can go on over two years before the next test. In very high, apply nothing except a small starter. That is the build, maintain, draw-down cycle, and it is why "high" P on a report is good news, not a prompt to buy more.

Ontario's sufficiency approach, set for the Olsen test, reads: below 10 ppm P a profitable response is likely, 11–15 ppm moderately likely, 16–30 ppm a low starter rate may help corn and winter cereals, and above 30 ppm a response is unlikely. For ammonium acetate K the bands are below 60, 61–120, 120–150 and above 150 ppm. OMAFRA's rule of thumb is that roughly 35 lb P2O5/ac (39 kg/ha) and 20 lb K2O/ac (22 kg/ha) above crop removal raise the test by about 1 ppm.

Sulfur, zinc, boron and the other micronutrient lines

Sulfur is mobile; OMAFRA does not accredit a sulfur soil test and Iowa State does not publish one, while Manitoba's tables do take a soil sulphate-S value. Zinc has the best-calibrated micronutrient test: Iowa State's DTPA scale for corn and sorghum is low at 0–0.4 ppm, marginal at 0.5–0.8 and adequate at 0.9 ppm or more. Ontario reports a zinc index that already accounts for pH (below 15 likely deficient for corn, 25–200 adequate) and a manganese index on the same pattern for cereals and soybean. Those indices are not ppm and cannot be compared with a DTPA or Mehlich-3 number, and Mehlich-3 and DTPA values are not interchangeable either. OMAFRA says no reliable soil test exists for boron or copper; tissue testing and visual symptoms do that job.

Nitrate: a different kind of test

Nitrate moves with water, so the line is a snapshot. On the Prairies, Manitoba's tables assume a fall sample to 24 in (60 cm) and deduct 8 lb N/ac (9 kg/ha) from a spring sample for expected mineralisation. In Iowa, the late-spring soil nitrate test is taken 0–12 in (0–30 cm) deep when corn is 6 to 12 in (15–30 cm) tall, with 25 ppm the critical value above which added N is unlikely to pay, lowered to 20–22 ppm after wet or cold springs. Ontario uses nitrate only to fine-tune pre-plant or pre-sidedress corn and spring barley N, from 12 in (30 cm) cores. A nitrate value from a routine 6-inch fall P and K sample belongs in none of those systems.

Sampling caveats and common misreadings

Standard P and K interpretations assume a 6 in (15 cm) core. OMAFRA asks for at least 20 cores per composite, one sample per 25 ac (10 ha) at most, taken at the same time of year, ideally fall before a spring crop. The lab may analyse as little as 2 g of what you send, so field sampling, not lab error, is the weak link.

The misreadings that cost the most:

• Reading ppm as total supply. Multiplying ppm by two gives an approximate lb/ac of extractable nutrient in the top 6 inches, not the soil's reserve.
• Comparing a Mehlich-3 or Bray number against an Olsen table, or a US report against an Ontario table.
• Treating CEC, base saturation or a cation ratio as a fertilizer recommendation.
• Using a 6-inch sample for lime in long-term no-till, or for nitrate or sulfur.
• Assuming high K means add more K; high K can depress magnesium uptake instead.
• Acting on a single very low or very high value when the previous test was average: resample before you spend.

When in doubt, check your state or provincial guide for the extractant and category table that match your report.