Organic matter improves soil as a growing medium for plants. It helps release nitrogen, minerals, and other nutrients for plant use when it decays. A mulch of partially rotted straw, compost, or undecomposed crop residue on the soil helps keep the soil surface from crusting, retards water loss from the soil, and keeps weeds from growing.Practically any plant material can be composted for use in the garden. Leaves, old sod, lawn clippings, straw, and plant refuse from the garden or kitchen can b.