In what must have been a long process of trial and error, they learned how to build rafts of rushes and reeds, lashing the stalks together with tough roots. Then they dredged up soil from the shallow bottom of the lake, piling it on the rafts. Because the soil came from the lake bottom, it was rich in a variety of organic debris, decomposing material that released large amounts of nutrients. These rafts, called Chinampas, had abundant crops of vegetables, flowers, and even trees planted on them. The roots of these plants, pushing down towards a source of water, would grow though the floor of the raft and down into the water.