About our lab tests

Particle size distribution (sand, silt, clay, and stone content)

Particle size distribution is important because it affects the ability of a soil to store and transmit water. It therefore affects plant growth and production, soil, air, and gas movement (including greenhouse gases), carbon and nutrient loss and cycling, and soil drainage and infiltration.

Moisture release and water storage

Moisture release analyses relate soil water content to matric potential. Undisturbed core samples are saturated and subjected to controlled draining up to a matric potential of –1,500 kPa. Macroporosity and air capacity are a measure of the larger soil pores, which gives an indication of soil physical health, including soil compaction. Macroporosity is used in regional and national soil quality monitoring, although the use of terminology can vary.

There are several measurements of water storage that are useful for plant production, and for farm and environmental modelling. Available water capacity (AWC) describes the amount of water that can be stored that is potentially available for plant growth. The AWC of a soil is important for managing irrigation, effluent application, and cultivation, and to estimate soil water balances for modelling crop water needs and environmental effects (such as nutrient leaching). AWC is generally considered to be between field capacity (–10 kPa) and the permanent wilting point (–1,500 kPa). Readily available water capacity (RAWC) is the difference between field capacity (–10 kPa) and the ‘stress point’ (–100 kPa), at which a plant becomes stressed. RAWC and AWC are useful for irrigation management.

Hydraulic conductivities

Saturated hydraulic conductivity is the rate at which soil conducts water when saturated (i.e. all soil pores are full). Unsaturated hydraulic conductivity is the rate at which soil conducts water when pores down to a certain size are non-conducting (empty). Saturated and unsaturated hydraulic conductivities are useful to help evaluate soil pore connectivity and the functionality of selected sizes of soil pores.

Bioretention media performance testing

The following are often included in engineering specifications for bioretention media because they determine the performance of the device (i.e. the volume of water treated and quality of treatment):

• permeability/infiltration (mm/hour measured in the field and/or the lab)
• water-holding capacity (informing mm of water retention and detention)
• particle size distribution.

Specifications for green roofs often have a maximum operating weight measured at air capacity. Methods are defined in NZAS, ASTM or FLL guidance.