Soil Testing: Once You Have the Sample
This Soil Scoop is written primarily for Extension Agents or others who help landowners with soil testing. Here we provide questions you need to ask once the sample has been collected, but before it is sent to the lab.
Questions to ask
1. Has a sample been collected?
If a sample has not been collected, go to The Soil Scoop Soil Testing: Getting a Good Sample for assistance with sampling method including area, timing, sampling, depth etc. If a good sample was collected, continue to the next step.
2. What is the sampling date?
MSU fertilizer guidelines are based on spring soil test values. If this is a late-summer or fall soil sample, N rates based on this sample result will need to be adjusted. See (MT200703AG) or Home Garden Soil Testing & Fertilizer Guidelines (MT200705AG).
3. What is the sampling depth?
Depth depends on soil characteristic being tested and intended land use (Table 1).
Sampling to 24-inches will almost always require a hydraulic mounted probe truck.
If it is not possible to get a sample to 2-foot depth, multiply the top 6-inch nitrate-N
by 2.7 (based on a small MT data set) to estimate 0 to 24-inch nitrate-N and to reduce
the risk of
over-fertilizing nitrogen (N) at a financial and environmental cost.
Sample depth is also very important to convert nitrate-N reported as ppm to lb N/acre in order to calculate N fertilizer rates (lb N/acre = ppm x 2 x actual depth in inches / 6). Most labs do this conversion for you.
|Depth (inches)||Soil Analysis|
pH for acidity assessment and liming rate calculations
|0-6 for annual crops and gardens||pH1, organic matter (OM), nitrate-nitrogen (NO3-N), phosphorus (Olsen P), potassium (K), Electrical conductivity (EC), cation exchange capacity (CEC)2, exchangeable sodium (Na), chloride (Cl), zinc (Zn)|
|6-24 or 6-36 for annual crops||NO3-N|
1 pH, organic matter (OM), nitrate-nitrogen (NO3-N), phosphorus (Olsen P), potassium (K), Electrical conductivity (EC) are required by MT NRCS for planning and programs (EQIP, CSP, AFO/CAFO)
2 Some labs might list "exchangeable bases" rather than CEC.
4. How many subsamples were collected?
Use this as teaching moment if only 1 or 2 subsamples collected. Due to high variability across a field, garden, or lawn, in soil characteristics, 10 subsamples per composite sample is a reasonable average, though not always done.
5. Where were the subsamples taken?
Over a broad area of the field the sample is to represent, or in one small area?
6. Is this for annual cropland, lawn/garden, or pasture/hay?
See table 1.
7. Are you enrolled in an NRCS program?
See table 1.
8. Are you submitting because you’re seeing growth
f so be specific. For example, stunted plants may suggest testing for saline or sodic soils (see #9).
9. Are there soil issues?
White crusting on soil surface, issues with water ponding on surface, poor drainage,
extreme shrink/swell, crusting, cracking? Soil may be saline or sodic. Electrical
conductivity (EC) is a measure of soluble salts. Saline soils have EC > 4.0.
Exchangeable sodium percentage (ESP) > 15 indicates a sodic soil. Determining ESP requires exchangeable Na and cation exchange capacity (CEC). ESP = (exch. Na/CEC) x 100. See Salinity and Sodicity Management (4481-2), and Commercial Fertilizers and Soil Amendments (4449-10) for more information.
10. Do you suspect micronutrient issues?
If NPKS fertilization seems adequate based on Fertilizer Guidelines for Montana Crops (EB0161), yet plants show nutrient deficiencies, micronutrients such as zinc (Zn) and/or chloride (Cl) may be deficient.
11. Do you suspect low soil pH?
Test the top 0-3” in ‘good’ and ‘poor’ production areas. Plants in low soil pH are stunted, yellow/pink, with club roots. Legumes have poor or no nodulation. See Cropland Soil Acidification for more information.
12. Do the plants look N deficient (uniform yellowing) but sufficient N was provided?
Yellow new growth may indicate sulfur (S) deficiency, while N deficiency appears as yellow older leaves. Soil tests for S are not reliable and are generally not recommended. Rather, use prior crop production performance, current visual symptoms, and tissue tests.
13. What is the history on the land?
Knowledge of past cropping and management practices (dryland, irrigated, manured, legume/pulse in rotation, stubble,<br>etc.) is necessary to calculate N fertilizer rates. If the lab provides fertilizer recommendations, ask how they adjust their recommendations based on this information. See (MT200703AG) and The Soil Scoop Fertilizer Rate Calculations for details.
14. What is the intended crop and yield goal?
This is necessary to calculate fertilizer rates. Ask the lab to use MT based rate guidelines, or do your own calculations<br>based on soil test results and MSU Extension documents.
15. Do you have a preferred laboratory to perform the soil analysis?
Use a lab that is part of a performance and proficiency testing program such as the North American Proficiency Testing Program-Performance Assessment Program (NAPT-PAP) or Collaborative Testing Services Agricultural Lab Proficiency (CTS-ALP). The NRCS requires the use of labs that are part of NAPT-PAP or another NRCS-approved proficiency program. Consider using a lab that provides fertilizer recommendations based on MSU guidelines. Lists of labs are available in Interpretation of Soil Test Reports for Agriculture (MT200702AG), Home Garden Soil Testing & Fertilizer Guideline (MT 200703AG), and Soil Sampling and Laboratory Selection (4999-1). For consistency, use the same laboratory over time.
16. Additional decisions to make
It is important to specify the appropriate phosphorus test. Olsen P values are appropriate for soils with overall pH > 6, while Bray tests are appropriate for soils with overall pH < 7.3. Some labs may perform Bray tests unless the Olsen test is specifically requested. MSU guidelines are based on Olsen P test values. Bray test results cannot be exactly converted to Olsen P values, however Olsen P ≈ Bray-1 P/1.33 (Sawyer). Conversion factors become less consistent when pH is < 6.0 or > 7.3.
The lab can do a texture test for a fee, or the ‘mason jar test’ can be done at home “Mason jar” soil texture test for free.
|* NO3-N, exch. K, pH, OM, EC|
|* Olsen P (rather than Bray)|
|Zn and/or Cl|
|CEC and exchangeable Na|
|Yield goal (if farm)|
* Required by NRCS
For more information
Sawyer, J.E. Differentiating and Understanding the Mehlich 3, Bray, and Olsen Soil Phosphorus Tests.Iowa State University.
MSU Soil Fertility Extension publications http://landresources.montana.edu/soilfertility/publications.html
USDA NRCS South Dakota. 2012. Sampling Soils for Nutrient Management. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs141p2_036444.pdf
We thank Adriane Good, Mike Schuldt and Juli Thurston, the Pondera, Custer and Sanders County Extension Agents, respectively, for review of this document.