Soil Bulk Density - Deep Cores

Active

In use from 2001-01-01

Abstract

Soil bulk density is a measurement of the dry mass of soil per unit soil volume (g/cm,sup>3) and thus includes the combined volume of solids and pores that may contain gases or water, or both. Bulk density can affect many soil processes, such as water passage, heat transfer, aeration and root growth. It is also a necessary factor to convert mass-based units to area-based units.

This protocol describes bulk density analyses associated with deep (1 m) soil cores sampled at decadal intervals in the Main Cropping System Experiment (MCSE) and elsewhere to examine changes in soil carbon with depth. Cores are delineated into soil horizons and bulk density is determined either by horizon (prior to 2013) or by depth section (since 2013). For each core section (either horizon or depth interval), dried subsamples are pulverized and subsampled for carbon and nitrogen analysis, and the remainder or a portion thereof is archived.

Sampling frequency: Usually every 10 years

Protocol

Materials:

    • Intact core sample in PVC or PETG tube (since 2013, 7.5 cm diameter x 1.2 m long), such as liners (600517) and end caps (#207439) available from Geoprobe Systems (Salina KS)
    • # 10 and # 20 paper bags (pre-labeled)
    • Meter stick
    • 4 mm sieve (one for each treatment)
    • Data sheets
    • Drying oven @ 60 ° C
    • Balance
    • Mill or soil grinder (e.g., SPEX SamplePrep 8530 Shatterbox® )
    • Pint jars (pre-labeled)

General Procedure:

  1. Pre-label paper bags with sample information (e.g., date, plot, station, core#, depth section).
  2. Enlist a soil taxonomist to delineate horizons (Ap, Bt, Bt2, etc) and mark on core liner. To eliminate gaps in cores, tap the bottom of the core lightly against the floor prior to delineation and handle cores gently once they are marked.
  3. Measure and record the length (in cm) of each horizon and the diameter (in cm) of the corer used; measurements are used to determine the volume of soil in each section. Make note of cores that are not full, often due to compaction during sampling, and adjust the depth horizon length accordingly.
  4. Lay core horizontally on cutting board. Lift core slightly to remove end caps. Carefully cut open core liner using cutter.
  5. Cut and remove the 0-10 cm section of the core. Place in tared paper bag, weigh and record mass. This is the wet weight of the entire core section (WWsection).
  6. Sieve the section into a catch tub using the designated 4 mm treatment sieve. Weigh rocks (gravel) >4 mm, record their total mass (g), then discard. There is no need to determine water content nor archive rocks. This is WWgravel.
  7. Mix sieved soil and subsample to fulfill any additional sample requests. Transfer the remaining sieved core section, or a subsample thereof, into a tared, labeled paper bag. Weigh bagged soil and record mass. This is the subsample Wet Weight (WWsub) used to determine the Soil Water Content (SWC).
  8. Brush the sieve and tub to clean. Repeat steps 5-8 for other sections (10-25, 25-50, and 50-122 cm) of the core. Repeat entire procedure for all cores.
  9. Dry sieved soil samples and 10 empty paper bags (for each size used) in forced-air drying oven at 60 ° C until a subset of bags no longer loses mass, or for at least 48 hours.
  10. Remove samples and paper bags from oven and allow to cool for at least 15 minutes.
  11. Weigh bags with soil and weigh empty paper bags. Subtract the mean weight of the 10 empty bags from the weight of the soil sample + bag weight. This is the subsample dry weight (DWsub) used to determine the SWC.
  12. Remove approximately 100g of the dried soil from each bag and store in a labeled Whirl-pak bag to be pulverized for carbon and nitrogen analysis.
  13. Place remainder of the dried soil (not pulverized) in labeled glass jar for archiving. Organize jars in original boxes in the order: Treatment-Replicate-Station-Core-Section.

Calculations:

1. Determine wet weight (g) of soil in core section:
WWsoil = WWsection – WWgravel
2. Determine gravimetric soil water content (SWC, g water / g dry soil) of section subsample:
SWC = (WWsub – DWsub) / DWsub
where:
WWsub = wet weight of soil subsample (g)
DWsub = dry weight of soil subsample (g)
3. Determine the dry weight of soil (g) in the entire core section according to the relationship DWsoil/ WWsoil = 1 / (1+ SWC):
DWsoil = WWsoil / (1+ SWC)
4. Calculate total bulk density (g/cm3) of core section (includes gravel):
BDtotal = DWsoil + WWgravel / V
where:
V = volume of core section in cm3 = cylindrical volume = (π) (r2) h
5. Determine gravel bulk density (g/cm3) of core section:
BDgravel = WWgravel / V
6. Determine gravel-free BD, used to determine C and N content of soil section:
BDgravel-free = BDtotal – BDgravel

Changes to Protocol:

  1. Prior to 2013, soil sections were horizon based. Beginning in 2013 soil sections are depth based (0-10, 10-25, 25-50, and 50-122 cm) because of inconsistent horizon designations between the 2001 and 2010 samplings.
  2. Prior to 2010, soil cores were 6 cm diameter ID x 1 m depth; beginning in 2010 soil cores were 7.6 cm diameter ID x 1.22 m (48 inches) depth.
  3. Prior to 2010, the entire section was ground and after subsampling for carbon and nitrogen, was archived. Since 2010, a subsample of each section is pulverized for carbon and nitrogen, and the remainder of the dried section (or a portion thereof) is archived.


Date modified: Wednesday, Feb 20 2019

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