Soil testing can help you manage nutrients and amendments for optimum soil health, crop production, and pasture quality. Accurate and reliable test results depend on good sampling techniques. This article provides tips on how to get the most out of on-farm soil testing.
Why Test Your Soil?
A soil test provides valuable information on the capacity of your soil to deliver plant nutrients and support satisfactory yields and quality. When you begin farming a new field, you need to know your “starting point” so that you can make sound management decisions to build and sustain fertility. For example, a biologically depleted soil may be deficient in phosphorus and potassium — or very high. The former will benefit from generous manure or compost applications, while the latter requires a different approach to restoring fertility.
Testing your soil every one to three years helps you track trends and adjust inputs. For example, decreasing soil organic matter may warrant adding organic amendments. At the same time, increasing a micronutrient from low to optimal levels indicates that you no longer need to supplement it.
Soil testing can help improve crop yields by identifying potential deficiencies before planting. It can also save money and protect water quality by preventing overapplication of nutrients.
What is a Soil Test and What Does it Tell You?
Soil tests offered through extension services or private labs provide a snapshot of the chemical condition of the topsoil at the time of sampling.
Some labs also offer texture analysis, which identifies the proportions of sand, silt, and clay in your soil. Texture is an inherent (given) soil property that plays a significant role in your soil’s capacity to hold moisture, nutrients, and SOM and, therefore, in informing soil test interpretation and management strategy.
The testing lab uses an extractant that mimics how crop roots absorb nutrients to obtain an estimate of plant-available nutrients. Results are rated on a scale starting from very low (VL-critically deficient) to very high (VH-ample to excessive, may reduce yield if extremely high) and the results describe the steps necessary to mitigate for each of the readings. The readings between are Low (L), Medium (M), High (H), Optimum (O), and Sufficient (S).
Most soil test reports provide recommendations for lime, phosphorus, potassium, and micronutrients based on test results. Nitrogen recommendations are based on the crop grown and yield goals.
Information You May See on Your Soil Report
pH: 7.0 is neutral, < 7 is acidic, > 7 is alkaline
Major and secondary nutrients: phosphorus (P), potassium (K), magnesium (Mg), calcium (Ca), and sulfur (S), measured in parts per million or pounds per acre (1 ppm ~2 lb/ac)
Micronutrients: boron (B), copper (Cu), zinc (Zn), manganese (Mn), and iron (Fe), measured in parts per million or pounds per acre (1 ppm ~2 lb/ac)
Soil organic matter % (SOM): index of soil health and an essential source of plant-available nitrogen (N)
Cation exchange capacity (CEC): the negative charge on soil clays and SOM that hold K, Mg, Ca, and other positively charged (cation) nutrients in plant-available form
Plant-available soil N — nitrate (NO3-) and ammonium (NH4+): typically not reported in routine soil tests as their levels fluctuate rapidly and can be difficult to interpret
Additional Tests (extra fee): nitrate-N, additional micronutrients, soluble salts, or heavy metals
How to Test Your Soil
When to Sample Your Soil for Testing
The best time to take a soil sample is in the fall after harvest or early spring before planting. Sample a new field several months before planting so you have time to correct pH and nutrient deficiencies before your first crop. Repeat tests annually until desired levels of nutrients, pH, and SOM have been attained, then again every two to four years to monitor long-term trends.
Whenever practical, take samples in the same season and at the same point in your production cycle each year. For example, if your initial test takes place in March before tillage, do subsequent years’ tests at that time. This will make it easier to compare successive test results to track trends and adjust inputs and management practices.
How to Select and Map Out Areas to Sample
Each soil test sample should consist of multiple soil cores representing a relatively uniform production area. Sampling just a single spot within a field or garden can give misleading results, and pooling cores from areas of contrasting soil types or management history will mask essential differences in soil fertility.
A few examples of areas that should be sampled separately include vegetable gardens, cornfields, pastures; open field vs. high tunnel; a field in long-term organic management vs. an adjacent transitioning one; upland vs. bottomland, and much more. Scan the QR code above for more details.
How to Take a Soil Sample
Take multiple soil cores from spots scattered evenly across the area you are testing — at least 10 cores for a high tunnel or small market garden and 15-25 cores for fields larger than one acre, avoiding atypical spots such as a cowpie, a wheel track, a waterlogged patch, or near a building or fence line.
- Gather Tools: Soil corer or shovel and trowel, plastic bucket (no metal), soil sample bags/boxes.
- At each spot, remove surface residue or foliage. Then use a one-inch diameter soil sampling tube or soil corer to take cores from the surface to a consistent depth (normally, six inches for cropland or pasture; for orchards, 12-14 inches deep near the driplines of trees.) If you do not have a corer, use a shovel to dig a hole at least six inches deep, leaving a smooth cut. Then, use a trowel to remove a uniform slice of soil from the surface to six inches.
- Combine cores in a clean plastic bucket. Mix cores thoroughly with a clean trowel. If the soil is wet, spread it in a clean plastic tray and let it air dry for one to two days. When using soil sample bags from the lab, add soil to the “fill” line. Otherwise, place one to two cups of soil in a labeled zipper-seal plastic bag or box.
Choosing a Lab and Submitting Your Sample for Analysis
Send soil samples to a reputable soil testing laboratory that will test for SOM, pH, phosphorus, potassium, calcium, magnesium, boron, and any other micronutrients of concern for your region and crop. University extension service labs vary widely from state to state in the range of nutrients reported and the format of their reports. Virginia Tech’s test is considered by many easy to read; some may find NC State and U Tennessee reports a little harder to interpret; and U Georgia reports a more limited range of nutrients.
Private labs that serve the Southeast U.S. and cover a wide range of nutrients, SOM, pH, and CEC include Waters and Waypoint Analytical (multiple locations).
Obtain and fill out the lab’s sample submittal form. Be sure to indicate the test package and any additional tests you would like done. Follow lab instructions for shipping.
Once you have found a satisfactory lab, stick with that lab for successive tests to monitor trends over time. Because different labs use different procedures, switching labs can result in differences that do not accurately represent changes in your soil.
| Soil texture | Depleted SOM | Desirable SOM |
|---|---|---|
| Loamy sand (>80% sand) | <1% | 2% |
| Sandy loam | 1-1.5% | 2.5-3.5% |
| Loam | 1.5-2.5% | 4-5% |
| Silt loam or clay loam | 2-3% | 5-7% |
How to Use Soil Test Results
Interpreting a standard soil test for organic or regenerative farming can be challenging. Don’t take nutrient recommendations too literally; pay attention to low or very high readings. A nutrient rated “L” or “VL” will likely limit production and should be applied. For a nutrient rated “H” or “O,” add just enough to replenish what is removed in harvest. Minimize or avoid inputs of a nutrient rated “VH” — especially if it is “off the charts.”
Monitor micronutrients and the three primary nutrients required for plant growth: nitrogen, phosphorus, and potassium (NPK). For deficiencies, apply once at the recommended rate, then monitor future tests to see if more is needed.
Maintain a favorable pH (6.0-7.5 for most crops) and apply lime for a pH below 6. Recommended rates depend on pH and CEC. To ensure sufficient Mg and Ca for crops, use calcitic limestone if Mg is very high and Ca is low and dolomitic limestone if Mg is below optimum.
A soil or crop consultant specializing in organic or ecological agriculture can help interpret soil tests. Organizations such as Florida Organic Growers, Carolina Farm Stewardship Association, and Georgia Organics can provide technical assistance or referrals to consultants.
Limitations of a Standard Soil Test: What it May Not Tell You
Standard soil test methods and recommendations are primarily based on research conducted in conventionally managed soils. They do not fully account for crops’ capacity to retrieve nutrients from deeper in the soil profile or soil microbes’ capacity to release nutrients from crop residues and SOM. In healthy, organically managed soils, crops may need less fertilizer than recommended. Overapplication of nutrients increases costs and may compromise soil health.
As research improves our understanding of soil life and biological nutrient cycling, scientists are developing new tools and methods to monitor soil health and the soil’s capacity to provide nutrients, especially N. In the meantime, farmers can supplement the information in a soil test report by observing soil and crop conditions, conducting foliar nutrient analysis to pinpoint crop nutritional status, and participating in side-by-side on-farm trials with and without a recommended nutrient input. In addition, soil biological evaluations are available such as the Solvita respiration test or the Haney soil health test. In conclusion, correct soil sampling and testing by a reputable lab can empower farmers to build healthy, fertile soils for profitable crop and livestock production.
Mark Schonbeck has 37 years of experience as a researcher, consultant, educator, and advocate for sustainable agriculture. As a Research Associate with the Organic Farming Research Foundation, Mark reviews organic agriculture and conservation research and develops educational materials on soil health, climate resilience, and crop, weed, and nutrient management for organic systems. He also provides individual consulting for organic farmers, taking a site-specific approach to soil test interpretation, soil health, and crop management.