How To Read A Soil Test
Learn how to read a soil test report — pH, organic matter, phosphorus, potassium, CEC, and base saturation. Practical guide for corn, soybean, and hay producers.
A soil test is the most underused tool in row crop and hay production. Most farmers get one, look at the pH number, call the co-op, and never touch it again. But the full report — pH, organic matter, phosphorus, potassium, calcium, magnesium, CEC, and base saturation — tells you exactly what your ground can and can’t do before you buy a dollar of fertilizer.
This guide walks through each number on a standard Midwest and Southern soil test report and what it means for your fertility program.
The soil test report — what’s on it
Most land-grant university labs return the same core metrics. A standard report typically includes:
- pH and buffer pH (lime requirement index)
- Organic matter (%)
- Phosphorus (P) and potassium (K) — often Mehlich-3 or Bray P1
- Calcium (Ca) and magnesium (Mg)
- Sulfur (S) on some panels
- CEC (cation exchange capacity) and base saturation (%)
- Optional micronutrients: zinc, boron, manganese
Know which extraction method your lab uses — Mehlich-3 and Bray P1 interpret phosphorus differently. The recommendation sheet from your Extension office should match your lab method.
Soil pH — the number that controls everything else
Optimal range: 6.0–6.8 for most row crops; 6.5–7.0 for alfalfa.
pH controls nutrient availability across the whole profile. Phosphorus availability peaks near 6.5. Below 5.5, aluminum and manganese toxicity becomes a yield limiter on many soils.
Buffer pH is separate from active pH. Labs use it to calculate how much lime you need to raise pH to your target — not just how acidic the soil is today, but how much reserve acidity it holds.
Both too acidic and too alkaline reduce yield. If pH is below 6.0 on corn ground, lime comes before fertilizer — otherwise you are paying for nutrients your plants cannot access efficiently.
Use the fertilizer rate calculator lime section to convert your lab’s lime recommendation to tons per acre for your lime source’s ECCE.
Organic matter — why it’s more than a number
| OM % | Typical interpretation |
|---|---|
| < 1% | Poor — sandy or heavily eroded |
| 1–3% | Typical row crop |
| 3–5% | Good |
| > 5% | Exceptional — high residue, pasture, or peat influence |
Each 1% organic matter holds roughly an inch of plant-available water and mineralizes about 20 lb N/acre/year as it breaks down. Building OM is slow — cover crops, reduced tillage, and manure help, but expect years, not one season.
Organic matter also drives CEC. OM has very high cation exchange capacity per unit weight, so improving OM improves your soil’s nutrient-holding ability over time.
Phosphorus (P)
Labs report P in ppm (parts per million). Two common index systems:
- Mehlich-3 — widely used in the Midwest and South
- Bray P1 — common in the Northern Plains and some Corn Belt labs
General Mehlich-3 sufficiency (corn):
| Level | ppm | Action |
|---|---|---|
| Low | < 15 | Build soil P — higher application rates |
| Medium | 15–30 | Maintenance + crop removal |
| High | > 30 | Maintenance only — avoid overapplication |
Phosphorus does not leach like nitrate, but it runs off in surface water. High soil P with no crop need is money on the ground and an environmental liability.
Your agronomist or county Extension office provides a rate recommendation for your specific lab method. Convert that recommendation to product rate with the fertilizer rate calculator.
Potassium (K)
Potassium is reported in ppm or lbs/acre (1 ppm ≈ 2 lbs/acre in the top 6–7 inches).
Corn sufficiency: roughly 120–160 ppm, but the right number depends on soil CEC. High-CEC soils hold more K; sandy low-CEC soils need more frequent, lighter applications because K can leach on coarse textures.
Hay and high-yield corn remove large amounts of K — it is the most commonly deficient major nutrient in intensive cropping systems. A 200 bu corn crop removes roughly 50–60 lb K₂O/acre in the grain alone.
Calcium and magnesium
Ca and Mg appear as ppm and as % base saturation on many reports.
- Calcium deficiency is rare on properly limed soils — liming supplies Ca
- Magnesium deficiency is more common on sandy, leached soils, or when K is very high (K and Mg compete for uptake)
- Dolomitic lime supplies both Ca and Mg; use it when Mg tests low
- Calcitic lime when Mg is adequate and you only need Ca
Cation Exchange Capacity (CEC)
CEC measures the soil’s ability to hold positively charged nutrient ions (Ca²⁺, Mg²⁺, K⁺, Na⁺, H⁺). Units are meq/100g or cmol/kg — the same thing.
| CEC (meq/100g) | Typical soil |
|---|---|
| < 5 | Sandy, low OM |
| 5–15 | Typical row crop |
| 15–25 | Heavy clay or high OM |
| > 25 | Heavy clay soils |
High-CEC soils hold more nutrients and forgive timing mistakes. Low-CEC soils need split applications and lighter rates per pass.
Estimate CEC and base saturation from your test data with the CEC calculator.
Base saturation — reading the balance
Desired ratios (approximate):
| Cation | Ideal % of CEC |
|---|---|
| Calcium | 65–75% |
| Magnesium | 10–15% |
| Potassium | 2–5% |
| Sodium | < 2% |
| Hydrogen (acid) | remainder |
Not all labs report base saturation, but understanding it helps interpret Ca, Mg, and K in context. If K% is high but Mg% is low, cattle on lush spring pasture may show grass tetany symptoms — even when absolute soil Mg is not severely deficient, because high K suppresses Mg uptake.
What to do with your soil test results — a 4-step process
- Flag pH below 6.0 — lime first, everything else follows
- Identify deficient nutrients below sufficiency range for your crop
- Get an application rate recommendation from Extension, a certified crop advisor, or your co-op agronomist
- Convert recommendation to product rate with the fertilizer rate calculator
The fertilizer rate calculator converts lbs N/acre, lbs P₂O₅/acre, or lbs K₂O/acre from your recommendation into application rates for anhydrous, urea, DAP, MAP, potash, and blends.
Know what your fertility investment needs to return? Pair rates with the crop breakeven calculator. Financing a major lime or tile project? Run scenarios through the farm loan calculator.
How often should you soil test?
- Every 3–4 years for established row crop fields
- Every 2 years for intensive hay production or large amendments
- Every year for vegetable production or new ground
- Test at the same time of year and same depth each time for valid comparison
Soil test FAQ
What is a good soil test result?
One where pH is in range and major nutrients meet sufficiency levels for your target crop — not necessarily the highest numbers on the sheet.
What does CEC mean on a soil test?
Cation exchange capacity — your soil’s nutrient-holding bank account. Higher CEC holds more Ca, Mg, and K.
What pH should my soil be for corn?
6.0–6.8 is the standard target range for most Corn Belt production.
How do I fix low phosphorus on a soil test?
Build soil P with recommended application rates from your Extension office, then switch to maintenance rates once levels reach sufficiency.
What is buffer pH on a soil test?
A separate index from active pH used to calculate lime requirement — how much reserve acidity the soil holds.
How accurate are soil tests?
Very accurate for the sample you send. Sampling error (wrong depth, too few cores, unrepresentative spots) causes more problems than lab precision.
Should I get a soil test before buying land?
Yes. Soil fertility and pH problems are expensive to fix and easy to hide in a listing. A $20 test can save thousands in unexpected lime and fertilizer.