AGCSATECH UPDATE AGCSATECH UPDATE
Understanding
PWT’s
Continuing on from last edition’s look at greens profile specifications and sand selection, AGCSATech environmental agronomist Bruce Macphee looks at perched water table constructions, amendments and the importance of testing.
Above: The purpose of a PWT is to have a rapidly draining profile that can provide ideal growing conditions to support healthy turf growth
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n the last instalment of AGCSATech Update (‘On spec’ – ATM Volume 19.3, pages 36-38) we looked at aspects regarding greens profile specifications (the subgrade, drainage and gravel and intermediate layers) and the importance of sand selection. In this edition we will look further into the additional tests that should be completed before selecting an appropriate sand, amendment options and other points to consider when undertaking a greens construction project. First, let’s look at the theory of perched water tables (PWT), often one of the most difficult concepts to understand.
PERCHED WATER TABLES The USGA’s greens construction specification is based on a sand profile depth of 300mm. The purpose of a PWT is to have a rapidly draining profile that can still provide ideal growing conditions to support healthy turf growth. In a PWT construction, water is held in the rootzone layer due to the sharp change in porosity at the sand-gravel interface. There are a large number of pores in the sand that are not in contact with the gravel where the sand grains have formed bridges across the gaps created by the relatively larger gravel particles (Figure 1). Water contained in the pores of the sand here are exposed to air in the gravel layer and form menisci, where water molecules are more attracted to each other than the surrounding air. The forces of adhesion and cohesion also come into play here. Water molecules are held in the small pore spaces, adhering to the large surface area of the sand particles seemingly against the forces of gravity. This causes a saturated zone, or PWT, to form at the bottom of the rootzone layer where, in theory,
AUSTRALIAN TURFGRASS MANAGEMENT 19.4
turfgrass plants with a deep root system can access water while maintaining ideal playing conditions at the surface. Water entering the rootzone from irrigation or significant rain events will work its way down through the profile, increasing the height of the saturated zone (or PWT) to where the head or weight of water built up in the rootzone can no longer be held against the forces of gravity. At this point excess water will move freely into the gravel drainage layer, flowing across the surfaces where sand particles and gravel particles are touching. The moisture content of the upper rootzone quickly returns to field capacity with a saturated layer remaining at the sand–gravel interface. This is, of course, a very simplified version of the processes involved. Sands that conform to the USGA or conventional specification may have physical characteristics such as volumetric water at the higher end of the ideal range and consequently aeration porosity at the lower end of the desirable range. This would generally be the case if the particle size distribution had a higher percentage of fine and very fine sand particles. In this situation the depth of the PWT or saturated zone may extend higher in the rootzone layer, whereas a sand with a higher portion of coarse and very coarse sand would have a shallow PWT. In real terms, it is the pore size distribution and how tightly the particles are packed together that will determine the height a PWT will form in a rootzone media. This can be determined in a laboratory using a moisture release curve test which provides information on the air entry point into the profile at varying rates of compaction to simulate real life scenarios.