Clay is an excellent material for water retention. It is slow-draining and has an exceptional ability to hold nutrients. Because of its tiny particles - clay particles are typically less than .002mm in diameter - it warms slower than loose topsoils. Clay also retains heat well, making it an ideal additive to gardens growing various tomatoes and peppers, peas, and leafy vegetables. Flowers, such as selenium, roses, and asters, all thrive in clay soil. Most ponds and culverts contain some mixture of clay and other rocks and soils.