Answer: The best way to improve a very sandy or gravelly soil for a lawn is to work organic matter and topsoil into it. These amendments must be mixed thoroughly with at least the top 4 inches of existing soil, and more deeply if possible. While organic matter or topsoil could be used alone, a sandy or gravelly soil is improved more if you add both organic matter and topsoil than if you add either material alone. Organic matter is depleted by microbial activity over time, while topsoil will be a more permanent amendment. However, be especially cautious in adding a clayey topsoil to sand, since the result can resemble concrete!
Extreme care is needed when buying topsoil. There are no regulations about the actual content of topsoil, hence many different materials are called topsoil in the landscape and building trades. Some "topsoils" have been found to contain excessive amounts of stones, clay, sand, wood, demolition debris, or other undesirable matter. Always inspect topsoil before you buy it. Insist on an accurate sample if you buy topsoil from someone who will deliver it for you. If a slightly moist sample of the topsoil seems adequate to make clay pots from, it is probably too high in clay. If it is extremely gritty, then it contains too much sand. Stones and other larger debris should be apparent by visual inspection. If possible, be on-site when the topsoil is delivered, so that a load can be rejected if it is not the same as the sample material you previously inspected.
There are several sources of organic matter. Peat moss, composted animal manure, composted leaves and yard waste, and composted brewery waste are just some of the materials available. Again, an inspection of organic matter before purchase is recommended, especially when a large load is to be delivered by truck to your site.
Once you've amended your sandy soil, it should support both turfgrass and flowers.
Best wishes with your landscape!
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