Soil Sampling: Recommended Guidelines
Sample Collection
Soil sampling is a particularly difficult task when attempting to get a representative sample. It is always important to consult standard operating protocols (SOP’s), colleagues, and private investigators to ensure that you are collecting, storing, and preparing your samples correctly.
Depending on the analysis to be completed, a typical sample size of 500g is collected. Depending on your sampling design, this sample could be representative of a whole field, or a single location within a field. It is important to consider this when taking your sample to know how much to take, how well to mix it, where to take from, and the proper protocols to follow between a sampling location.
For example:
- If the area covered by the sample is not uniform, the chemical analysis may not accurately reflect the nutrient status of specific sites. Factors that need to be considered when sampling soil include the depth and time of sampling.
- Proper sampling depth is affected by the crop being grown, past cropping, depth of plowing and also the nutrient of interest. Subsoil samples are important for most crops.
- Standard sampling times should be used due to the difficulty in comparing samples taken at different times (i.e. samples taken for nitrate and ammonium will be different a couple of days apart).
- The fertility level of a field will vary over the course of the year and interpreting results for samples taken at different times of the year will be very difficult.
- Sampling between crops rows will give more consistent results.
- When sampling soils, the area should be subdivided into as homogeneous sections as possible (i.e. when establishing trials, select uniform areas with similar slope, aspect, drainage, pest pressures).
- Sub-samples should be small enough that the composite sample will be of a size that can be completely processed for analysis.
- The depth of the sampling is determined by the elements of interest and existing knowledge about the soil profile (i.e. aggregate stability and microbial samples are typically taken at 0-7.5cm, and nitrate and ammonium are typically taken at the rooting depth of your crop which can vary and can be sectioned into 0-15cm and 15-30cm depths).
- Samples for cultivated crops are taken from the plow layer and are typically broken into smaller sections(0-15cm and 15-30cm)
- Pasture and sod crop soil samples should be collected from 0-10cm
- Samples for nitrate, soluble salts and available micronutrients should be taken at the root depth (typically 0-15cm and 15-30cm)
- Samples for aggregate stability and microbial activity are typically taken at 0-7.5cm
- Care should be taken to clean tools and equipment between fields to limit the spread of crop diseases and pests.
- Tools and gloves should be sterilized between sampling for microbial analyses.
Transport
Some samples such as soil sampling for biological activity (phospholipid fatty acid analysis (PLFA), enzyme analysis, etc.), plant-available nitrogen (nitrate and ammonium), and other analysis require keeping your samples cold during transport. Large coolers used in conjunction with ice/cold packs and a thermometer are recommended to keep the samples cool. Keeping your cooler in a shady place during longer sampling days is recommended.
It may be important to preserve the integrity of other samples that may be more fragile or prone to damage such as soil structure or bulk density samples. In this case, bubble wrap, foam, tinfoil, or hard plastic containers may be recommended.
Storage and Processing
Cold Storage
Depending on the analysis being completed, to preserve samples, it may be pertinent to store samples in the cooler (4°C) or the freezer (-4 to -10°C). For example, samples that should be placed in the cooler are:
- Soil structure samples (3D scanning, aggregate stability, soil water retention, X-ray diffraction)
- Soil chemical samples (plant-available nitrogen (ammonium or nitrate))
Samples that should be placed in the freezer are:
- Soil biological samples (phospholipid fatty acid analysis)
In order to save space when resources are limited, placing samples into labeled large plastic totes or cardboard boxes is recommended.
Drying Recommendations
Once samples have been collected, depending on the type of sample, they must be processed promptly to prevent any changes that might affect the analysis. Break up large chunks of soil and spread out to air dry where the sample will not be contaminated, particularly by fertilizer dust. The sample may also be placed in a forced-air oven set between 35°C and 55°C to become ‘air-dried’.
Grinding Recommendations
Depending on your sample type, the soil may need to be ground and sieved to <2mm. Soil should be crushed using a rolling pin then passed through a 2mm mesh screen.
Exceptions
- Analysis of Carbon, Nitrogen (including Total Kjeldahl Nitrogen), Calcium Carbonate, and total elements (Arsenic, Cadmium, Copper, Chromium, Iron, Lead, Manganese, Molybdenum, Nickel, Phosphorus, Selenium, Zinc) requires that the sample be ground to a powder fine enough to pass through a 60 mesh screen. Grinding with mortar and pestle, mechanical grinder, or coffee mill may be necessary to powder the sample, especially with soils containing sand.
- Soils with a high percentage of sand or organic material may require grinding to pass through a 60 mesh screen to achieve sample homogeneity.
Related Virtual Soil Science Learning Resources
References
Refer to references on this page.