The measurement "tonnes" is actually a measurement of weight. The measurement "cubic metres" is a measurement of volume. Thus, for different materials of different density - that is a greater weight by volume - the conversion factor will be different.
Landscape design can be a very wide and varied undertaking. There are many different aspects to be considered, some of which you may understand more about than others. Here are a few things to consider, but remember that your landscape architect is your best point of reference for concerns regarding your specific landscape ideas.
Landscaping is usually undertaken in order to improve the aesthetics of your outdoor spaces. Landscaping supplies come in many shapes and sizes. To determine which suits your home is often a question of your own personal taste, though there are some features that you should be aware of too.
When you buy soil from a plant nursery or garden supplies store, you often buy what is called "topsoil". Topsoil is usually a premixed collection of decaying organic matter and microorganisms that are full of nutrients to support plants and retain moisture. It is very similar to mulch, which is an earlier stage in the process. Many organic gardeners and farmers will tell you there is no need to ever buy soil, but there are many circumstances where it might be preferable.
Landscaping is a topic where there is information in abundance, as it is both a profession and a popular pastime for many people. Tips and tricks and general landscape ideas can be found in a variety of places.