The topsoil of golf course putting greens typically is sand, as this substrate promotes rapid drainage of water and is relatively insensitive to compaction by foot traffic. A disadvantage of a sandy topsoil is that it does not store much water, causing problems for the grass growing on the greens. This issue can be partially eliminated by installing a layer (horizon) of gravel beneath the sandy topsoil during construction of the putting green. Explain why the gravel layer helps the retention of water in the sandy topsoil.?