Standard Operating Procedures - Sampling Soil Characteristics

KEY DETAILS

Principal Investigators
Dr. Jenny Farmer, Dr. Nicola Stevens
Date
8 July 2025
Version
1.0.0
Programme
Rangelands Carbon Programme
Key partners
University of Oxford, ICRAF, University of Liverpool, University of Cape Town
Contact email
jfarmer@naturalstate.org

1. PREAMBLE

Natural State’s research methods and activities are detailed by a set of accepted Standard Operating Procedures (SOPs). These documents describe the steps involved in all Natural State research methodologies, from data collection to data processing and storage. Each SOP documents key methodological details for a specific data type. The objectives and background of the projects or surveys these methods are used for, features of the study area where these methods are applied, and details on survey and sampling designs for these methods may be found in survey Design Documents (DDs), which are available in the Related Documents section below or may be perused in the main NS Design Documents documentation page.

2. GLOSSARY

Natural State Glossary

3. METHODS OVERVIEW

3.1 METHOD AIMS

The soil sampling methodology is designed to:

  1. Measure soil bulk density across plots to assess soil compaction and mass per unit volume.
  2. Collect soil samples across plots in preparation for laboratory analyses.

3.2 METHODOLOGICAL BACKGROUND

Soil carbon is calculated using two complementary methodologies:

  • Bulk density sampling, which uses undisturbed soil cores to determine soil mass per unit volume.
  • Soil Carbon sampling, which uses auger cores to collect and composite samples from three depth intervals.

4. SAMPLING PREPARATION

The equipment mentioned in the list below needs to be gathered, checked and packed before sampling begins. If any sensors need to be configured prior to commencing sampling this will be documented below the equipment list.

4.1 EQUIPMENT LIST

  • Trowels
  • Jembe
  • Bulk density rings
  • Hammer
  • Corer and ring cores
  • Knife
  • Auger
  • Tape measure
  • Bucket
  • Plastic bags (ziplock)
  • QR code stickers
  • Marker pen
  • Sack or plastic sheet

4.2 DEVICE CONFIGURATION

  • The S123 app needs to be loaded onto the tablet and the Soil Carbon Survey needs to be loaded.
  • Station locations need to be loaded onto the handheld GPS.

5. SAMPLING PROCEDURES

Field sampling consists of two complementary methods: Bulk density sampling and soil Carbon sampling described below. These methods require the establishment 50 m x 50 m plots. Each of these plots will be divided into four subplots. Begin by navigating to the plot and establish the extent of the 50 m x 50 m plot by running a 50 m measuring tape 25 m north and 25 m south and by running a second one 25 m east and 25 m west, intersecting at the plot centroid.

5.1 Bulk Density Sampling

  1. Identify sampling locations: Within each plot, identify five sampling points — one in the center of the entire plot and one in the center of each of the four subplots.
  2. Insert the bulk density corer: At each location, place the bulk density corer vertically onto the ground surface and hammer it in until the top of the ring is flush with the soil surface. The target depth is approximately 10 cm.
  3. Insert the bulk density corer: At each location, place the bulk density corer vertically onto the ground surface and hammer it in until the top of the ring is flush with the soil surface. The target depth is approximately 10 cm.
  4. Extract the core: Carefully excavate around the corer using a jembe or trowel to avoid disturbing the soil column inside the ring. Once exposed, use a knife to trim the soil flush with both ends of the ring.
  5. Transfer the sample: Gently remove the soil from the ring and place it into a labelled ziplock bag. Avoid soil loss and cross-contamination during transfer.
  6. Label the sample: Each ziplock bag must be clearly marked with the following information:
    • “Bulk Density”
    • QR code
    • Location details (plot name/number)
    • Subplot number
    • Date of sampling
  7. Repeat: Complete steps 2–6 for each of the five sampling points per plot.

5.2 Soil Sampling

  1. Prepare subplot for augering: Within each subplot, identify three auger sampling points. These should be distributed to capture spatial variation but remain within the subplot boundaries.
  2. Collect samples by depth: At each of the three points, use a bucket, auger, and tape measure, to collect soil samples from the following depths:
    • Sample 1: 0–15 cm
    • Sample 2: 15–30 cm
    • Sample 3: 30–50 cm

    Working one depth at a time, auger to the target depth, remove the soil, and place it on a clean sack or plastic sheet.

  3. Mix and subsample:
    • For each depth (0–15 cm, 15–30 cm, 30–45? 50 cm), place the soils onto a plastic sheet or into a large bowl and combine the soils from all three cores collected within the subplot at the same depth using a clean trowel. Ensure the trowel is wiped completely clean before moving on to the subsequent depth.
    • Mix thoroughly to create a composite sample.
    • Take a subsample using the trowel (approximately 2–3 scoops) and transfer it into a clean, labelled ziplock bag.
  4. Label the sample: Each ziplock bag should include the following:
    • “Soil Carbon sample”
    • QR code
    • Subplot number
    • Sample depth (e.g., 0–15 cm)
    • Date of sampling
  5. Repeat: Complete steps 2–4 for each of the three depth intervals in all four subplots per plot.

NOTE: Soil samples should be kept as cool as possible on site and not ‘sweat’ in the bag. While collecting keep samplies in the shade or in the vehicle or at least open the ziplock. This is especially important for soils that are very moist. Once all samples are ready from a site, roll up the ziplocks and seal ready for transport to next plot or base camp.

6. POST PROCESSING

This section details all steps that need to be followed after returning to the office from the field to ensure that samples are properly processed and stored and that data is uploaded and saved to the correct location.

6.1 SAMPLE PROCESSING AND STORAGE

Samples should be handed over to the lab using the Lab Sample Log. All samples should be airdried immediately - the ziploc bags should placed in a safe, sheltered location and left open for enough days to allow the soil to dry. Bags then need to be closed for storage. Soil samples should be stored in a cool and dark location (out of direct sunlight). Before CNS analysis oven dry the samples to 60°C.

6.2 DATA ENTRY AND UPLOADS

7.1 DESIGN DOCUMENTS

DD - Carbon Pool Plots

7.2 OTHER RELEVANT SOPS

7.3 DATA ELEMENTS

8. REVISION AND VERSION HISTORY AND DESCRIPTION

v1.0.0 Original SOP from July 2025.