Always try to clear away all weeds and rocks from where you intend to lay the turf and allow 100mm of depth for soil under the grass. This results in a very strong growth foundation for the grass to cement its roots.
The type of soil used for under the grass should be a proper turf soil. This soil is often mixed with fertilizers to help aid the grass in establishing over the first few weeks of growing. It is also possible to throw handfuls of chicken or goat manure pellets across the surface of the soil before laying the turf.
Keep the soil level under the expected level of the grass by 15mm to 40mm depending on the type of grass you have purchased for your lawn. Couch lawns will need a smaller depth to reach their final growing heights than say a Buffalo or Sir Walter type of grass. Use a stiff timber board or aluminium level such as a cementing level to flatten out the surface of the soil before laying the grass. Always walk around and check the levels and slopes of the soil as this will give you some idea as to how well your lawn will drain away excess water after the lawn has become established. Article Source: http://EzineArticles.com/5713381. Find more about Sir Walter Turf Sydney.