Soil characterisation and sampling
Soil characterisation can generally be divided into a field and laboratory part. Throughout, it follows scientifically accepted guidelines and methodologies. Depending on the question or purpose of the work, different characteristics of the soil can be investigated – from rapid, visual assessments of soils in the landscape to month-long analyses of individual soil particles in the laboratory.
Soil characterisation in the field enables the scientist to capture the soil in its original state and spatial setting. Before examining the soil at a location, observations of topography, vegetation, geology, hydrology and climatic conditions are made as they are known to affect the soil characteristics beneath. Such an assessment may include documenting the shape/angle of the slope, indicators of soil erosion, agricultural management, plant communities, and current weather conditions. This assessment is normally supported by photos, and the geographic location (GPS) o the site is recorded.
To characterise the soil material beneath the surface, a wide range of methods can be applied. They can be divided into invasive and non-invasive methods.
1) Excavating soil pits
Soil pits allow the scientist to access a two-dimensional, undisturbed profile of the soil. This is ideal for a morphological assessment of soil colour patterns, the physical soil structure, the particle sizes of the soil material (clay, silt, sand, coarse fragments), and plant root characteristics. This information allows the identification of the soil horizons that comprise the soil profile. Soil profiles allow easy access to take samples with good control over sampling depth and the horizons sampled, both for disturbed and undisturbed samples. Disturbed soil samples (also called ‘grab’ or bulk samples) are mostly used to measure chemical and biological properties of the soil. Undisturbed samples are primarily collected for soil physical assessments, like measuring soil density or soil water characteristics. Undisturbed samples are usually collected using large-diameter metal rings that are carefully carved into the soil with a knife while avoiding vibrations or compression to not disturb the soil’s physical structure.
Augering is a rapid means of accessing soil material without excavating. By drilling into the soil, usually by hand, soil samples can be brought to the surface. While the method is much faster than excavating a soil pit, it also physically disturbs the soil samples and sample sizes are small. This limits the level of detail of the soil assessment compared to that of soil profiles.
3) Soil coring
Like augering, taking soil cores is a time-efficient method to access the soil beneath. Usually driven into the soil using a machine-driven hammer or hydraulic system, the soil is collected within a hollow metal rod/core, normally at least 0.5 m long. If large enough in diameter, the samples retain more of their structural configuration compared to auger samples and allow for a more detailed description of the soil properties, albeit not as good as with undisturbed samples or soil profiles.
These methods do not necessarily require physical access to the soil beneath the surface. Soil information is obtained by exposing the soil to electromagnetic waves. The way soils absorb, reflect or transmit these waves can be measured by sensors. These measurements can then be linked to soil characteristics. One of the greatest advantages of these methods is that they can give us two- or three-dimensional images of soil characteristics across the landsurface and belowground, not just at a point location. However, since very different soil characteristics can give very similar sensor responses, some ‘groundproofing’ by soil pits, augering and coring is often necessary to allow for a confident interpretation of the sensor data. The field of applying non-invasive technologies in soil science is developing rapidly as our capacity to analyse and store large amounts of digital data increases. These techniques are promising tools to greatly advance the way we make soil maps.
Under controlled laboratory conditions, more detailed measurements of the soil samples can be conducted. Our MWLR laboratories in Palmerston North and Hamilton can analyse a wide range of soil properties.