Soil, topsoil, turf underlay and sand are all terms used which make reference to a form of growing media used for the establishment or maintenance of a turfgrass. Soils may be an existing/native soil, manufactured or a blend. In short, two specific uses exist; what goes underneath the turf and what's goes on top.
Commonly referred to as turf underlay or (top)soil is the growing media present within the rootzone for the turf to establish and grow. Such material is placed over a subgrade, rubble or poor quality B horizon soil. Providing a sufficient layer of good quality topsoil mean your turf can readily access nutrients, moisture and assist the grass with drought avoidance.
Washed sand or high quality topsoil is also used as topdressing material. Topdressing is undertaken to maintain or improve surface levels and or to reduce organic matter content or thatch (dead plant material) within the turfgrass.
Depending on your selection of growing media and topdressing material, the implications on turf quality and future management of your lawn are significant. Choose carefully or be at its peril.
A strong root system is the best defence a lawn can have against weeds, wear, drying out and pests and diseases. Roots exploit available soil moisture and nutrients, thus supporting the growth of visible leaves, stems and shoots. A robust root system will give a lawn the vigour required to keep weeds at bay and recover rapidly from wear, and pests and diseases with normal maintenance.
Your chosen soil should at a minimum meet the following criteria:
The key to sustaining a healthy lawn is by providing a good quality top(soil), installed at a consistent sufficient depth above the subgrade, rubble or poor quality B horizon soil. Many lawns or turfed recreational areas are installed on poorer quality soil because the existing topsoil has been stripped away during the construction or earthwork phase of the project.
In unrestricted conditions high proportions of the turfgrass root system can be found at 20-30 cm. A soil of this depth, whilst it might be ideal for plant growth, is prohibitively expensive for many home owners. Many of the benefits of such a "soil tank" can be realised with a 10-15 cm turf underlay using good quality soil. However for shallow soils (claypan and rock), aim for a minimum depth of 20 cm. At times, underlying drainage issues will need attention prior to installing topsoil.
Increasing the soil depth from 10 to 20 cm over an area of 150 square metres means that the soil can hold an additional 19,000 litres of water. Adding an additional 10 cms of depth (taking it to 30 cms) over that area increases the water held by an additional 10,300 litres.
As a general rule of thumb, for a minimum of 50 mm of turf underlay, this equates to about 1 cubic metre of soil for every 20m2 of turf area.
Topdressing materials can range from 100% washed sand to say 70% sand and 30% organics. Topdressing applications can range from as little as 1 mm deep for thatch control to a suggested maximum of 10 mm deep for filling low spots and very minor undulations with a turfed area. It is recommended that all topdressing material be “worked into" the turf canopy, to ensure a minimum 50% to 90 % of the plant leaf is exposed prior to the completion of work and before the turf is irrigated. This is to ensure the turf is not smothered and can still receive essential sunlight and not "cook" under the topdressing material. To facilitate the topdressing material to enter the turf canopy, this can be done by either a modified drag mat made out of plastic, synthetic turf or brushes.
As a general rule of thumb, 1 cubic metre of washed sand provides a healthy topdress for 100m2 of lawn.