Clay and humus are special parts of the soil. They help keep water and plant food (plant nutrients) in the soil. Water and plant nutrients stick to clay and humus. Water sticks to all of the rock in the soil. But, water sticks best to clay. Water is taken into (absorbed) into humus like a sponge absorbs water. Humus holds a lot of water and plant nutrients. Clay and humus keep water and plant nutrients in the soil. Sand will hold only a little water in the soil. If the soil has too much sand, the water will flow down into the earth. The water that flows down takes plant nutrients down too. The plant roots can not get at the water and plant nutrients if they go too deep. It is best for soil to have a little bit of clay and humus in it for growing plants.