To read a soil test report, check which extraction method and units the lab used, then read pH and buffer pH, then compare phosphorus and potassium to the critical level for that method. Use CEC to choose the right potassium target. Below the critical level, build. Inside the range, replace crop removal. Above it, skip.
That order matters more than any single number on the page. A soil test value is not a count of the pounds of nutrient in your ground. Ohio State's fact sheet on interpreting soil tests puts it plainly: the value "represents the plant-available nutrient, not the total amount," and the total is "several times higher." The number is an index. It only means something next to the field trials that calibrated it.
This guide assumes you already have a good sample and a lab report in hand. If you don't, start with our guide to soil testing, labs and how often to test. This post ends where the report hands you a rate like "60 lb P2O5 per acre." Turning that into pounds of DAP or potash is covered in our fertilizer application rate guide.
Find the method line before you look at a single nutrient. The same field can read 12 ppm phosphorus on two tests and mean two different things. The University of Minnesota Soil Testing Lab gives the cleanest example: "For corn, an Olsen P result of 12 ppm is considered high, but a Bray 1 P result of 12 ppm is medium."
The three phosphorus extractants you are most likely to see:
| Method | Where it's used | What to know |
|---|---|---|
| Mehlich-3 | Now the default extractant in the Tri-State guide (OH/IN/MI) | Reads about 35% higher for P than Bray P1, and about 14% higher for K than ammonium acetate (Tri-State Bulletin 974) |
| Bray P1 | Acidic and neutral soils | Iowa State calls it "unreliable in calcareous soils." UMN uses it at pH 7.4 or below |
| Olsen | Calcareous and alkaline soils | UMN switches to it above pH 7.4. Oregon State pairs it with the alkaline soils of eastern Oregon |
Do not use that 35% figure as a conversion factor. It is an approximate average, not a formula. The rule is simpler: only read a number against a chart built for the same extractant. Iowa State goes a step further and notes that Mehlich-3 needs different interpretations depending on whether the lab measured it by colorimetry or ICP. Same extractant, different instrument, different scale.
Then check the units. Labs report in ppm (parts per million, the same thing as mg/kg) or lb/acre, and the conversion is a reporting convention:
Because lime comes first. pH controls how much of the phosphorus in your soil a crop can actually use, so a P recommendation on a field that needs lime is answering the wrong question. The water pH tells you where the soil sits now. The buffer pH tells the lab how much reserve acidity it has to neutralize, which is what sets the lime rate.
We cover what pH means in our guide to testing soil pH, and buffer pH and lime requirement in depth in our post on variable rate lime and pH mapping. For this report, the takeaway is one line: if the lab recommends lime, that is the first check you write.
Every calibrated soil test has a critical level: the value below which a crop is likely to respond to fertilizer. Above it sits a maintenance range, and above that, a level where fertilizer is unlikely to pay. The Tri-State Fertilizer Recommendations (Bulletin 974, 2020), used across Ohio, Indiana and Michigan, set these Mehlich-3 maintenance ranges:
| Crop | P maintenance range (Mehlich-3) |
|---|---|
| Corn, soybean | 20 to 40 ppm |
| Wheat, alfalfa | 30 to 50 ppm |
Bulletin 974 turns those numbers into three decisions:
Crop removal is how you size the maintenance rate. Tri-State uses 0.35 lb P2O5 per bushel of corn grain, 0.80 for soybeans and 0.50 for wheat. A 200 bu corn crop hauls off about 70 lb P2O5 per acre.
These are Tri-State numbers, printed here because they show how one major regional standard reads, not because they are national truth. Kansas sets its critical P at 20 ppm for most crops and maintains it at 20 to 30. Iowa State revised its category boundaries in 2023, widening the optimum range. Use your state's guide: K-State MF-2586, Iowa PM 1688, or your land-grant lab's current charts. Be careful with older Iowa tables that still turn up in search. They were revised in 2013 and again in 2023.
Potassium works the same way with one extra step: you need the CEC to know which target applies. Tri-State's Mehlich-3 K maintenance ranges for corn, soybean, wheat and alfalfa:
| Soil | CEC (meq/100 g) | K maintenance range (Mehlich-3) |
|---|---|---|
| Sandy | Less than 5 | 100 to 130 ppm |
| Loam and clay | More than 5 | 120 to 170 ppm |
Sandy soils get a lower target for a practical reason. Ohio State's interpretation fact sheet: "Recommended K rates for soils with a CEC less than 5 are lower due to the potential for K to leach below the root zone." Building K on sand means paying for potash that ends up below the roots.
Crop removal for K is where soybeans surprise people. Tri-State lists 1.15 lb K2O per bushel of soybeans against 0.20 for corn grain. A 60 bu soybean crop takes about 69 lb K2O per acre, while 200 bu corn takes about 40.
Cation exchange capacity is the soil's ability to hold positively charged nutrients like calcium, magnesium and potassium. Ohio State calls it "the potential of available nutrient supply, not a direct measurement of available nutrients." It is reported in meq/100 g (some labs write cmol/kg, which is the same number).
CEC follows texture and organic matter. Ohio State's typical ranges:
| Soil texture | Typical CEC (meq/100 g) |
|---|---|
| Sands | 3 to 5 |
| Loams | 10 to 15 |
| Silt loams | 15 to 25 |
| Clay and clay loams | 20 to 50 |
| Organic soils | 50 to 100 |
Here is the part most readers get wrong: CEC is not a target. Ohio State again: "We cannot alter CEC by management. Organic matter (OM) can be changed but increasing OM requires many years." Liming an acid soil can nudge the measured number up for a while, but that is a quirk of the test, not new exchange sites. CEC describes your dirt. On a fertility report, its practical job is to pick the potassium row in the table above.
High CEC isn't automatically better, either. It means more buffering, which also means it takes more lime to move pH.
Ohio State's fact sheet on calculating CEC (ANR-81) walks through a sample report. Here are its numbers. To finish the walk-through, we've added a Mehlich-3 phosphorus line of 30 lb/acre and assumed the K value is Mehlich-3 too. Those additions are ours and are labeled.
Step 1: method and units. Our added P line is in lb/acre, so divide by 2: 15 ppm Mehlich-3 P.
Step 2: pH. Buffer pH is 6.6. That means there is reserve acidity, so check the lime line first.
Step 3: build the CEC. Most labs estimate CEC by "summation": convert each cation from ppm to meq/100 g, add them up, then add the acidity estimated from buffer pH. Ohio State's divisors are Ca 200, Mg 120, K 390 and Na 230. Acidity is 12 x (7.0 - buffer pH), or zero if buffer pH is 7.0 or higher.
| Line on the report | Value | Divide by | meq/100 g |
|---|---|---|---|
| Calcium (Ca) | 2,000 ppm | 200 | 10.0 |
| Magnesium (Mg) | 240 ppm | 120 | 2.0 |
| Potassium (K) | 100 ppm | 390 | 0.26 |
| Sodium (Na) | 20 ppm | 230 | 0.09 |
| Bases subtotal | 12.4 | ||
| Acidity, buffer pH 6.6 | 12 x 0.4 | 4.8 | |
| CEC | 17.2 |
Ohio State's results: base saturation 72%, calcium saturation 58%, magnesium saturation 12%. Potassium saturation works out to 0.26 / 17.2, or about 1.5%.
Step 4: phosphorus. 15 ppm Mehlich-3 P is below Tri-State's 20 ppm floor for corn and soybeans. Decision: build, or at least apply crop removal.
Step 5: potassium. CEC is 17.2, well above 5, so the loam and clay row applies: 120 to 170 ppm. K is 100 ppm. Decision: build.
Notice what drove that potassium call. It was the sufficiency chart, not the 1.5% K saturation. A lab that works from ratios might reach the same answer on this report by a different road, and a very different answer on the next one. Which brings us to the ratios.
No. Those "ideal" ratios come from the base cation saturation ratio (BCSR) idea, which traces back to William Albrecht's work at the University of Missouri. The research has not held it up. A 2021 forum paper in Agronomy Journal led by soil fertility researcher Steve Culman says "there is widespread agreement among soil fertility scientists that BCSR is not a legitimate practice of soil management." A 2007 review in the Soil Science Society of America Journal found no credible evidence to justify its use.
Base saturation still earns its place on the report as a description. Ohio State says it "further describes the relationship of soil cations." Read it, but don't spend money moving it. Lime to fix pH, and fertilize P and K to the sufficiency chart.
Some labs print ratio targets by default. That doesn't make them a bad lab. It means you should check which philosophy sits behind the recommendation line before you act on it.
If your pH is above about 7.3 and the report shows huge calcium and a CEC that doesn't match your soil, the number may be inflated. K-State researchers compared Mehlich-3 and ammonium acetate calcium and found they "diverge exponentially at a soil pH of 7.3 and higher," and concluded Mehlich-3 "should not be used to assess the plant availability of soil-Ca or estimate cation exchange capacity (CEC) in soils with a pH above 7.3."
What happens is simple. The extractant dissolves free lime in the sample, the lab counts that calcium as exchangeable, and the summation CEC climbs. You haven't found more exchange sites. You've measured limestone. This matters on the alkaline, often calcareous ground east of the Cascades, where Oregon State steers growers toward the Olsen test for exactly this kind of soil.
Usually because they follow different philosophies, not because one of them did the chemistry wrong. K-State's MF-2586 (January 2024) defines the two main ones:
The build part can be large. K-State's P build formula uses 18 lb P2O5 for each ppm of soil test increase. On a Kansas field at 12 ppm, building to 20 over four years is (20 - 12) x 18 / 4 = 36 lb P2O5 per acre per year, on top of crop removal. That 18 lb figure is a Kansas calibration, and MF-2586 notes the amount "differs according to soil type." Don't carry it to other states.
Labs also disagree on the chemistry itself. Iowa State reported in 2023 that "the bias in test results among laboratories continue being substantial." Oregon State's EM 9423 recommends a lab accredited by NAPT-PAP or ALTA and gives the most useful advice in this whole post: "Once you select a reliable soil testing lab, try to stay with it as long as possible." A trend line means something only when it comes from the same lab and the same method.
If your land-grant lab ran the test, its recommendation is a sound default. If a national commercial lab ran it, read the raw numbers yourself against your state's chart. That is how you catch a build-heavy or ratio-based recommendation.
CEC tracks texture, which gives you a free cross-check. Pull up your field in the Soil Profile Quicklook, which shows the NRCS-mapped texture class, sand, silt and clay percentages, drainage class and available water capacity for the spot you pick. It doesn't read lab reports or show your P, K, pH or CEC. What it gives you is the soil you'd expect to find.
If your report says CEC 4 but the map shows a silty clay loam, something is off. Maybe a sandy spot got sampled, a bag got mislabeled, or the summation CEC is inflated on a high-pH soil. If the report and the map agree, you can trust which potassium row applies. If you want to confirm texture with your own hands, our jar test guide shows how. Texture and CEC can change a lot across one field, which is the case for EC mapping and management zones.
Once the report gives you a rate in lb P2O5 or K2O per acre, the Field Boundary & Input Calculator measures your true acres and totals the fertilizer for the whole field. For converting nutrient pounds into product pounds, use the fertilizer application rate guide.
The report also lists nitrogen, sulfur, micronutrients and organic matter. Those are outside this post. Bulletin 974 notes that most Tri-State soils supply enough calcium, magnesium, sulfur and micronutrients, so P, K and lime are where most of the money gets decided. Default recommendations in Tri-State and K-State also assume grain crops. If you grow hay, pasture or vegetables, use your state's crop-specific chart.
Start with the method line (Mehlich-3, Bray P1 or Olsen) and the units (ppm or lb/acre). Read pH and buffer pH next, since lime comes first. Then compare phosphorus and potassium to your state's critical level for that same extractant, using CEC to pick the potassium target. Below the critical level, build. Inside the range, replace crop removal. Above it, skip fertilizer this year.
There is no good or bad CEC, because it reflects soil texture and organic matter rather than fertility. Ohio State lists typical ranges of 3 to 5 meq/100 g for sands, 10 to 15 for loams, 15 to 25 for silt loams and 20 to 50 for clays. You can't change CEC much by management. Its main job on a report is deciding which potassium target applies.
It depends on the extractant and your state. In the Tri-State guide for Ohio, Indiana and Michigan, the Mehlich-3 maintenance range is 20 to 40 ppm for corn and soybeans and 30 to 50 ppm for wheat and alfalfa. Below that range, crops are likely to respond to phosphorus. Bray and Olsen numbers need their own charts, so never read them against a Mehlich-3 table.
Multiply ppm by 2. This convention assumes the top 6.7 inches of an acre of soil weighs about 2,000,000 lb, so 1 ppm equals about 2 lb. Divide lb/acre by 2 to go back to ppm. Separately, soil tests report elemental P and K while fertilizer uses oxides: multiply P by 2.29 to get P2O5 and K by 1.2 to get K2O.
No. Target ratios like 65% calcium, 10% magnesium and 5% potassium come from the base cation saturation ratio idea, which research reviews have found little support for. A 2021 Agronomy Journal paper said soil fertility scientists widely agree it is not a legitimate management practice. Read base saturation as a description, then lime for pH and fertilize P and K to sufficiency levels.
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