This Soil Scoop provides questions a person needs to ask once a soil sample has been collected, but before it is sent to the lab.

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Questions to ask

1. Has the 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?

MT soil nitrogen (N) 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 The Soil Scoop Fertilizer Rate Calculations.

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 truck-mounted hydraulic probe. If it is not possible to get a sample to 2-foot depth, multiply the top 6-inch nitrate-N (NO3) by 3 (based on a small MT data set) to estimate 0 to 24-inch nitrate and to reduce the risk of over-fertilizing N at a financial and environmental cost.

Sample depth is also necessary to convert nitrate reported as ppm to lb N/acre to calculate N fertilizer rates (lb N/acre = ppm x 2 x actual depth in inches/6). Most labs do this conversion for you.

Table 1. Soil depth to sample for land uses and recommended soil analysis.
Depth (inches)
Soil Analysis
0-3

pH for acidity assessment and liming rate calculations

0-6 for annual crops and gardens

0-12 for hay and perennial pasture
pH1., organic matter (SOM), nitrate (NO3), phosphorus (Olsen P), potassium (K), Electrical conductivity (EC), sulfate (SO4; to add to 6-24-inch sulfate), cation exchange capacity (CEC)2., exchangeable Na, B, Cu, Cl (for cereals only), Fe, and Zn
6-12 for annual crops
Electrical conductivity1., exchangeable Na
6-24 or 6-36 for annual crops
Nitrate1., sulfate

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 Nutrient Management Plans)

2. Some labs might list "exchangeable bases" rather than CEC.

4. How many subsamples were collected?

Due to high variability in soil characteristics across a field, garden, or lawn, 10 subsamples per composite sample is a reasonable average.

5. Where were the subsamples taken?

Were they taken over a broad, rather than a small area of the field, lawn, or garden that the sample is to represent

6. Is this sample for land enrolled in an NRCS program?

See table 1.

7. Are you submitting because you’re seeing growth
problems?

If so, mention the specifics to the lab. For example, stunted plants may suggest testing for saline or sodic soils (see #8).

8. Are there soil issues?

White crusting on soil surface, water ponding on surface, poor drainage, extreme shrink/swell, crusting, cracking? Is irrigation water saline or sodic? Electrical conductivity (EC) is a measure of soluble salts in soil. Saline soils have EC > 4.0. Exchangeable sodium (Na) 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.

9. Do you suspect micronutrient issues?

If N, phosphorus (P), potassium (K) and sulfur (S) fertilization and previous soil tests seem adequate based on Fertilizer Guidelines for Montana Crops (EB0161) or Home Garden Soil Testing & Fertilizer Guidelines (MT200705AG), yet plants show nutrient deficiencies (https://landresources.montana.edu/soilfertility/nutrientdeficiency.html) boron (B), chloride (Cl), copper (Cu), iron (Fe), and/or zinc (Zn) may be deficient. Consider confirming these deficiencies with plant tissue tests.

10. 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.

 11. Do the plants look N deficient (uniform yellowing) but sufficient N was provided?

Uniform, yellow new growth may indicate S deficiency, while N deficiency appears as yellow older leaves. Soil S, determined by Ca(H2PO4)2 extract, in the top 2 feet is a better indicator of plant availability than in the top 6 inches. However, critical S levels and corresponding S fertilizer rates are not yet available. Use prior crop production performance, current visual symptoms, and tissue tests to assess deficiency. See The Soil Scoop Fertilizer Rate Calculations for fertilizer rate guidelines.

12. What is the land's history?

Knowledge of past cropping and management practices (dryland, irrigated, manured, legume/pulse in rotation, stubble, etc.) is necessary to calculate N fertilizer rates. Fertilizer N rates are adjusted based on soil organic matter (SOM). If the lab provides fertilizer recommendations, ask how they adjust their recommendations based on this information. See Developing Fertilizer Recommendations for Agriculture (MT200703AG), Home Garden Soil Testing & Fertilizer Guidelines (MT200705AG), and The Soil Scoop Fertilizer Rate Calculations) for details.

13. What is the intended crop (e.g., canola or home garden)?

Soil nutrient level guidelines vary by crop, and for many crops by the expected yield. Ask the lab to use MT based fertilizer rate guidelines, or do your own calculations based on soil test results and crop specific MSU Extension documents.

14. 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 MT guidelines.  A list of labs is posted at the MSU Extension Soil Fertility soil sampling website. For consistency, use the same laboratory over time.

15. Additional decisions to make

Phosphorus

Request Olsen P because MT guidelines are based on Olsen P test values. For approximate conversions: at pH < 7, [Olsen P ≈ (Bray P * 0.42) + 3.5]; at pH > 7, [Olsen P ≈ (Mehlich-3 P * 0.45) + 1.8]  (Mallarino 1995). 

Potassium

Request the ammonium acetate test for K.

Texture

The lab can do a texture test for an extra fee, or the ‘mason jar test’ can be done at home for free.

 

Soil sample checklist
Item
Notes
Date
 
Location
 
Garden/farm/forage
 
Depth increment
 
Tests
 
1.NO3, exch. K, pH, SOM, EC
 
1.Olsen P (rather than Bray)
 
2.B, Cu, Cl, Fe, S, Zn
 
CEC and exchangeable Na
 
Texture (or home test)
 
Prior crop
 
Past Management
 
Intended crop
 
Yield goal (if farm)
 

1. Required by NRCS

2. Ifevidence of deficiency in current or prior year.

For more information

Mallarino 1995. Comparison of Mehlich-3, Olsen, and Bray-P1 Procedures for Phosphorus in Calcareous Soils. https://www.agronext.iastate.edu/soilfertility/info/ComparisonofMehlich-3OlsenandBray-P1Procedures.pdf

The Soil Scoop at  https://landresources.montana.edu/soilfertility/soilscoop/index.html

MSU Extension soil fertility publications for a variety of crops, market vegetable farms, and home gardens 

USDA NRCS South Dakota. 2012. Sampling Soils for Nutrient Management.
 
Acknowledgement

We thank Adriane Good, Mike Schuldt and Juli Thurston, the Pondera, Custer and Sanders County Extension Agents, respectively, for review of this document.

Revised Sept 2025