HowTo – Collecting and analyzing soil samples for physical analysis (75%)

Priority: low (soil data are low priority)
Updating: rare after maturity

This Howto covers how the project should collect soil samples for bulk density and soil organic carbon determination. This includes field tasks and lab tasks. This is applicable to categorical sites only since we do not consider on-property pesticide use for long term sites or lake watersheds.

Change log:

When Who Comment
2024 04 30 Sp17 Began.
2024 07 17 Sp17 Incorporate first round of field practice. Soil organic carbon method in part 8 is under testing.

Related howtos:

Summary: We collect a specific volume of soil from multiple locations at a categorical site where pesticides are typically applied, correlated with where we collect groundwater samples. We dry and weigh the volume of soil, then ash it to destroy organic compounds and weigh again. The results provide bulk density and organic carbon content data for pesticide leaching models.

Per each of the within- and downgradient- sampling points of selected categorical sites, the diagram below arrays the steps.

graph TD;
   WSL[/Downgradient<br>or within<b>water sample<br>location/]
   PUA[/Pesticide use area/]
   SSL[/Replicate soil<br>sample locations/]
   
   SS[Volume-preserving<br>replicate<br>soil samples]
   TDB[(Project tabular database)]
   GDB[(Project<br>geographic<br>database)]
   FL[(Field log)]-->TDB
   LL[(Lab log)]-->TDB
   
   SSL --> GDB
   WSL-->Where((Select<br>replicate soil<br>sample locations))-->SSL
   PUA-->Where
   SSL-->Collect((Collect replicate<br>soil samples))-->SS-- observations-->FL
   FL-->GDB
   SS--volume-->FL--volume-->LL
   SS-->Dry((Dry whole<br>volume-preserving<br>soil samples 105C))
   Dry-->Dried[Whole 105C<br>dry subsample]--weight-->LL
   
   Dried-->Subsample[TOC subsample]--weight-->LL
   Subsample-->Ash((Ash<br>subsample<br>500C))
   Ash-->Ashed[500c ashed<br>soil subsample]--weight-->LL

(Circles - processes; cylinders - data collections; slanted rectangles - external entities; rectangles - samples and subsamples)

The soil samples are replicated because of expected spatial variability within pesticide use areas, and the possibility of multiple use areas influencing a categorical site sampling point.

1. Objectives

  • Measure shallow bulk density of soil where pesticides are applied.
  • Measure shallow soil organic carbon content of soil where pesticides are applied.

2. Quality assurance considerations

  • Keep from mixing up samples by proper labeling and logging in field.
  • Check the weighing scales periodically against standard weights.
  • Ensure that all soil from a specified-volume sample is dried then weighed.
  • Clean drying and ashing pans between samples.

3. Safety considerations

  • Sample drying at 105C for drying prior to first weighing: Heatproof gloves.
  • Sample ashing at 500C for combustion of organic matter: Cool to 150C before removing crucible from oven. Heatproof gloves, and handle the drying vessel with metal tongs.
  • Grinding subsamples: eye protection.
  • Nitrile gloves for handling cool soil.

4. Supplies and Equipment Needed

4.1 Equipment

Field equipment:

  • Constant volume aluminum soil core sampler, sharpened edge beveled outside, 5 cm diameter, 5 cm long. Volume is 98 cm3.
  • Soft faced mallet (rubber, plastic) to strike the sampler gently into soil. Piece of wood to put over the core sampler if harder pounding is needed.
  • Knife or machete to slice off soil from the bottom of a removed core.
  • Narrow metal putty or icing knife to scrape out soil sample from sampler.
  • Garden trowel or short handled pickaxe, to shove under the sampler when in soil and lever it out.
  • Waterproof field logbook and Jetstream pen. To record location and relationship to water sampling location.
  • Cardboard or plastic box, or sturdy bag, to carry collected samples.

Lab equipment:

  • Drying dishes: Aluminum pans.

  • Ceramic combustion crucibles: capable of withstanding >500C temperatures.

  • Drying oven set to 105C.

  • Ashing oven set to 500C.

  • Soil grinder. Brushes and compressed air for cleaning between samples.

  • Desiccation cabinet to cool hot soil samples and store until weighing.

  • 8mm sieve, borrowed from Local Roads.

  • 2mm sieve mesh (#10) for grinding before ashing.

  • Heat safety gear protecting hands, feet, and body from 500C temperatures.

  • Eye protection during grinding.

  • Scale for weighing hot samples and containers, and taring containers. Sensitive to nearest 0.001 grams. (B68 lesser scale.)

  • Lab logbook and pen.

4.2 Expendable supplies

Field

  • Sample bags: one quart freezer ziplocks, whirlpacks, etc. Need to remain closed while carrying in field until brought into lab for drying and weighing. Need to be thick enough to resist puncturing.
  • Preprinted labels for sample bags: waterproof as used for water sample bottles. Packaging tape to retain labels.
  • Paper towels for wiping out the core sampler between samples.
  • Deionized washwater for sampling rings between samples.

Lab

  • Desiccant crystals. (lab stock item, Dririte or similar. Prefer with color indicator.)
  • Deionized water. (on tap in lab)
  • Nitrile gloves for handling cool soil.

5. Steps in field, per trip

In general, decide which sites merit soil sampling. Since this is for TGUS model application, the applicable sites are ones where there is an outdoor areal application of at least one pesticide, as opposed to single linear or point application. Greenhouses and rights of way are least relevant; sod farms, golf courses, fruit and vegetable farms, and vineyards are most relevant. One of three outdoor nurseries (Nur-2) and the Turf-1 site are relevant; two nurseries Nur-3 and Nur-5 are likely irrelevant due to pesticide use patterns.

Before a trip:

  • Decide on general sampling locations per site.
  • Preprint labels with identification of sampling site and location within site. Mark them with Site code, associated sampling point code, replicate letter A or B or C, and date. Affix labels to sample bags; note that waterproof labels used for bottles do not stick well to ziplock bags; add clear packaging tape on top of the label. Group empty sample bags by site into larger bags, buckets, etc.
  • Clean sampling rings and sampling tool.

During a trip, per site:

  • Obtain permission from owner to collect soil samples at specific places in site, corresponding to pesticide use areas that relate to water sampling points within or downgradient from pesticide use areas.
  • Select two or three soil sample points per each water sampling location. Go to them in turn and collect the sample. Refill the sample holes with native material.
  • Pack away all samples from the site in a box or heavy bag that protects them from puncturing.

6. Steps in field, per sampling point

This is largely consistent with Cornell Soil Health Lab sampling protocol for bulk density 1.

  1. Choose locations of the replicate soil samples. These should represent pesticide use areas upgradient from one or more nearby water sampling points. Identify these locations using the water sampling location, and a replicate letter A, B, C etc.

For each soil sample location, do steps 2-9.

  1. Scrape off loose organic matter from surface; we want to sample the mineral soil. If area is grassed, trim off a little grass (enough to fit the sampling ring) flush with surface using knife.

  2. Pound in ring sampler with mallet, using wood on top of the sampler and pounding on the wood to dampen blows. Avoid compressing the soil within the ring. Continue until the upper rim is flush with the surface and a little deeper so we can cut off an uneven top. Possible problems with insertion include high root density, excessive hardness of soil, coarse material that will not stay in the ring, very wet soil that will adhere to the ring when driving in. On a slope, insert ring perpendicular to the slope angle.

  3. Use garden trowel or small pickaxe to lever out the core sampler from one side, reaching under the ring sampler by an inch or so.

  4. Slice off soil, emergent plants, and roots from the removed sample, top and bottom, so that the sample is close to a full cylinder. Be careful not to compress or drop out soil that was inside the dimensions of the cylinder; we want that exact volume in-situ. Note any volume shortage in logbook.

  5. Press out soil from sampler into labeled sample bag. Scrape or brush all soil adhering to sides into bag, close bag.

  6. Refill the hole.

  7. Record in logbook the estimated location of each soil sampling point such as by taking a photo with GPS tagging on in smartphone. Make a note in field log about replicate collection: date, approximate time, site code, water sampling point code, replicate A B or C, position relative to pesticide use area and position relative to water sampling point. Make notes about the field conditions, wet/dry/etc, stoniness.

  8. In preparation for the next sample, clean the sampling ring with wet rag or wet brush. This is to prevent organic matter carryover from sample to sample. If using brush, clean brush after each use with tap or deionized water.

No preservation is required in the field or lab before testing within a few months after collection. We want to preserve the entire dry weight thus preservation is simply hanging on to all of the soil in the bag.

Continually inspect sample bags to ensure that they are not damaged before analysis. Ensure zippers are fully zipped so no soil leaks out. Protect from puncturing.

Ensure that identifying labels do not come off. Package mailing tape over the label has worked.

7. Steps upon return to lab from field

Store full sample bags where they will not be punctured before analysis. If any sample bags were marked with pens instead of labels, verify and correct readability.

8. Steps in lab for analysis, per batch of samples (under testing 2024-07-18)

This is derived largely from Cornell Soil Health Lab protocols for Bulk Density 2 and Loss On Ignition 3, convertible to Soil Organic Matter percentage. Also relevant are the Methods of Soil Analysis books4 5 and a paper by Schulte and Hoskins6.

Schedule drying and ashing ovens usage in advance. While using an oven, mark log sheet on oven door with contact info, temperature, start time and expected end time.

On oven launching day, prepare a table in lab logbook with one row for each sample to be processed in this session. Columns are drying pan number, sample identity (site ID, loccode of nearby water sample location, sample date, soil replicate letter), whole sample volume, whole sample weight, stones drying pan tare weight, stones 105C weight with pan, rest drying pan tare weight, rest 105C weight with pan, combustion crucible number, crucible tare weight, 500C weight with crucible.

Can use the computation spreadsheet (part 9 below) directly instead of a paper lab log.

Note: Tare-weigh the empty drying and ashing vessels at the obvious points. Take obvious precautions about handling of hot items.

For each sample:

Bulk density:

  1. Weigh whole sample (having known original volume; unless specified in the field log or on the bag, the volume is 98 cm3).
  2. Divide sample into two drying pans using 8mm sieve to separate stones from rest. (Clean sieve between samples to remove any residual organic matter and dust.) Large organic matter should be left in “rest”, chopped coarsely with sharp knife.
  3. Weigh two components of sample separately.
  4. Dry stones and rest of sample separately at 105C for 24 hours or until weight stabilizes. Note: Soils with organic matter content above 50% should be processed separately and dried at 60C until weight is stable.
  5. Cool samples in desiccator until moment of weighing.
  6. Weigh final dried weights of both components of sample.
  7. Compute bulk density with and without stones:

bd with stones = (final weight of dry stones + final weight of rest of sample (both minus tares))/original volume of sample; grams cm-3 bd without stones = (final weight of rest of sample (minus tare))/(orginal volume of sample - estimated volume of stones)

Stone volume: drop into partially water filled graduated cylinder and observe change in total volume.

Organic matter:

  1. Draw SOM subsample ~5g of non-stones part, into small clean pie pan (temporary).
  2. If not proceeding immediately after bulk density, re-dry the subsample at 105C for 2 hours.
  3. Grind subsample to pass through mesh size 10 (2mm). Grind Into tared ashing crucible.
  4. Weigh dry/ground SOM subsample.
  5. Ash dry SOM subsample at 500C for two hours. (preheat oven before using)
  6. For safety, move samples to desiccator and cool until temperature reaches 150C.
  7. Weigh ashed SOM subsample.
  8. Compute: %LOI = 100(1-ashed weight/dry weight) %OM = (%LOI0.7)-0.23

We are using 5g per subsample to allow using a weighing scale having 0.001g resolution. 0.001g is 0.02% of 5g. We expect organic matters from the sample soils to be 1-75% and wish to reach a precision of 0.1%.

Clean sieves, pans and crucibles between consecutive uses, put away after last use. Clean with deionized water, clean brush, and compressed air to dry.

9. Transcribe from lab log to spreadsheet

Note: Data may be entered directly into spreadsheet instead of into the paper lab log first.

The computational spreadsheet should have columns per each of the columns in the lab logbook table, plus columns for weights minus tares that contain formulae for differences between sample+tare minus tare. Also add columns at right for bulk density and organic matter computations. There can be one spreadsheet per year, or one for the entire project, with fresh data batches appended.

  • Use formula in section 8 to compute bulk density as 105C dry weight in grams (minus 105C pan tare) divided by sampler volume 98 cm3. (In some cases the sample bag may note a deficiency in volume; use such lower value if provided.)
  • Use formulae in section 8 to compute Loss On Ignition as 105C dry weight of ground subsample minus final 500C weight, dividing the difference by the final 500C weight. Then convert to %OM using the regression equation 7.

10. Data transcription to project database

This is part of the project tabular database, in a table named soil_samples.

The soil samples are identified (keyed) by the site_code, location ID (loccode) within site, a replicate letter, and a sample collection date. We are not expecting to sample at a site more than once, thus the date may not be a significant part of the identifier.

The data recorded about a sample are a subset of the computation spreadsheet columns: the sample and replicate identifier columns, an optional remark, sampled volume, three weights, and two derived values. Weights are the whole original sample’s weight after 106C drying, weight of the TOC subsample before ashing, and weight of the TOC subsample after ashing. Derived values are the bulk density and fraction organic carbon.

Note that these data are unrelated to EQuIS.

References

Other references:

FAO. 2023. Standard operating procedure for soil bulk density, cylinder method. Rome. https://doi.org/10.4060/cc7568en

David, Mark B. 1988. Use of loss-on-ignition to assess soil organic carbon in forest soils. Communications in Soil Science and Plant Analysis. Volume 19, Issue 14, Pages 1593 - 1599. November 1988.

Isabella Bisutti, Ines Hilke, Michael Raessler. 2004. Determination of total organic carbon – an overview of current methods. TrAC Trends in Analytical Chemistry, Volume 23, Issues 10–11, November–December 2004, Pages 716-726.

H. Zhang and J.J. Wang. 2014. Loss on Ignition Method. pp 155-157. In: Sikora F.J. and K. P. Moore (eds). Soil Test Methods From the Southeastern United States. Southern Cooperative Series Bulletin No. 419. URL: https://aesl.ces.uga.edu/sera6/PUB/MethodsManualFinalSERA6.pdf

Footnotes

  1. Cornell University, School of Integrative Plant Sciences. Soil Health Manual Series: Add-on Test: Soil Bulk Density (BD).Fact Sheet Number 22-17. URL: https://bpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/7/9922/files/2022/03/22-17_Soil_Health_Fact_Sheet_Bulk_Density.pdf↩︎

  2. Cornell University, School of Integrative Plant Sciences. Soil Health Manual Series: Add-on Test: Soil Bulk Density (BD).Fact Sheet Number 22-17. URL: https://bpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/7/9922/files/2022/03/22-17_Soil_Health_Fact_Sheet_Bulk_Density.pdf↩︎

  3. Cornell University, School of Integrative Plant Sciences. Soil Health Manual Series: Soil Organic Matter. Fact Sheet Number 16-08. URL: https://bpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/7/9922/files/2021/11/08OM.pdf↩︎

  4. American Society of Agronomy and Soil Science Society of America. 1986. Methods of Soil Analysis: Part 1. Physical and Mineralogical Methods. Chapter 13: Bulk Density. G.R. Blake and K. H. Hartge.↩︎

  5. Soil Science Society of America and American Society of Agronomy. 1996. Methods of Soil Analysis. Part 3. Chemical Methods: Chapter 34: Total Carbon, Organic Carbon, and Organic Matter. D.W. Nelson and L.E. Summers.↩︎

  6. E. E. Schulte and Bruce Hoskins. Alternate Procedure: Loss of Weight on Ignition (Adapted from Storer[^Storer], 1984). In: The Northeast Coordinating Committee for Soil Testing (NECC-1312). 2011. Recommended Soil Testing Procedures for the Northeastern United States. Northeastern Regional Publication No. 493, 3rd Edition. URL: https://www.udel.edu/content/dam/udelImages/canr/pdfs/extension/factsheets/soiltest-recs/CHAP8.pdf .↩︎

  7. Cornell University, School of Integrative Plant Sciences. Soil Health Manual Series: Soil Organic Matter. Fact Sheet Number 16-08. URL: https://bpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/7/9922/files/2021/11/08OM.pdf↩︎