Measuring Soil Porosity

Viti-note Summary: Soils need large pores and channels for Equipment adequate aeration and good drainage. • Equipment Shovel, fuse wire (0.1mm and 0.5mm Large pores that can be seen by the diameter), recording sheet and pen. • Timing human eye are known as macropores. • Method Mesopores and micropores are too small to be visible: they are respectively Timing responsible for storing plant available water and holding the water that is This measurement is best undertaken unavailable to plant roots. The movement when soil sampling is conducted. See of air through micropores is very slow. Taking soil samples. For good plant growth, the soil needs a balance of macro-, meso- Method and micropores. Soils with too many micropores will drain poorly and this Extract a moist sample of soil and on will result in waterlogging. Clay subsoils an undisturbed face on a clod, select a often restrict water movement to depth representative 25 mm2 section (i.e. 25 and have a low porosity. Therefore, the mm x 25 mm). Count all pores in the size porosity of the subsoil is often a good range 0.1-0.5mm using the 2 wires as a indicator of potential waterlogging of guide. Repeat 3 times at each site/depth. the surface soil. NOTE: IF THE TOPSOIL IS VERY CRUMBLY Soil management can modify the (FRIABLE), THEN THE MACROPOROSITY IS porosity of a soil. Tillage and trafficking, GOOD, EVEN THOUGH IT IS DIFFICULT TO particularly of wet soil, can destroy COUNT THE NUMBER OF PORES. macro- and mesopores, while cover Optimal values are given in the table crops and mulches can maintain and below. stabilise these pores. There is a simple field method to measure soil porosity by counting the number of pores on a face Other topics in this of a clod of soil. This is described below. Viti-Notes series include:

• Measuring the infiltration rate of water into soil using the ring Table 1. Irrigation classification and rate of soil water conductivity at varying soil pore levels. infiltrometer method *See Cockroft for details • A method for examining grapevine root systems Number of soil pores Soil water conductivity Irrigation classification (25 mm x 25 mm) (mm/h)* • Soil moisture monitoring • Measuring soil porosity 10 0.6 Poor • Measuring soil strength 15 1.3 Acceptable for irrigation • A method for assessing 20 2.3 “ soil structure • Taking soil samples 25 3.5 “ • Measuring soil pH 30 4.9 Good for irrigation • Measuring soil salinity 40 8.5 “ • Measuring organic carbon in soil 50 12.9 Excellent

Acknowledgement The Australian Wine Research Institute would like to acknowledge: • Cooperative Research Centre for Viticulture (CRCV) and all involved in the VitiNotes series (1996 – 2006).

Further information • Nicholas, P. 2004. Soil, irrigation and nutrition. Adelaide: Winetitles. • Cockroft B, 1970. Estimation of soil permeability from counts of visible pores, Australian Journal of Experi- mental Agriculture and Animal Husbandry 10 (45), pp 460-1.

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