Soil Testing: Once You Have the Sample

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

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

5. Where were the subsamples taken?

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

6. Is this for annual cropland, lawn/garden, or pasture/hay?

See table 1.

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

See table 1.

8. 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 #9).

9. Are there soil issues?

White crusting on soil surface, issues with 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.

10. Do you suspect micronutrient issues?

If N, phosphorus (P), potassium (K) and sulfur (S) fertilization and/or NPK 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/nutrientdeficient/) boron (B), chloride (Cl), copper (Cu), iron (Fe), and/or zinc (Zn) may be deficient. Consider confirming these deficiencies with plant tissue tests.

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 S deficiency, while N deficiency appears as yellow older leaves. Soil tests for S 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, etc.) is necessary to calculate N fertilizer rates. Home garden and cropland 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), The Soil Scoop Fertilizer Rate Calculations, and Home Garden Soil Testing & Fertilizer Guidelines (MT200705AG) for details.

14. 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 our crop specific soil nutrient management Extension publications (https://store.msuextension.org/).

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 MT guidelines. See the list of regional soil testing labs. For consistency, use the same laboratory over time.

16. Additional decisions to make

Phosphorus

Request Olsen P because MT guidelines are based on Olsen P test values. Bray tests can be highly inaccurate when pH > 7 and Bray test values cannot be converted to Olsen P values. However [(Mehlich-3 P/2.05) – 14] ≈ Olsen P  (Dari et al. 2019).

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.