Clay soil can be a challenge for many gardeners. Clay soil absorbs water slowly and then holds the water releasing it slowly over a longer period of time. When the soil dries out, it becomes very hard and solid. Clay soil can be always lightened by adding coarse sand (the best results are by adding sand and compost and tilling it in). The roots systems of plants that thrive in clay are strong enough to break through heavy soils and handle the compaction that can occur. Many of these species also do well in other soil types as noted in each description.