Do you know mulch? Mulch is a protective cover of material that is spread on top of soil. It is usually made out of organic material, like crop waste. Farmers may keep the remains of corn or other crops on top of the soil. This creates mulch on the soil surface. The plant remains help protect the soil against wind and water damage. Mulching is one of the best things people can do for their plants. It also helps keep the soil from getting dry, and reduces the need for watering plants. It also limits temperature changes in the soil. And it stops unwanted plants, or weeds, from growing. Organic mulch improves the condition of soil. As the mulch breaks down, it provides material which keeps the soil from getting hard. This improves the growth of roots and increases the movement of water through the soil. It also improves the ability of the soil to hold water. Organic mulch contains nutrients for plants. It also provides good environment for earthworms and other helpful living things in the soil. It is easy to find organic mulch materials. Cut-up leaves and small pieces of tree bark can be used. Grass cuttings are also a good mulch for plants. Mulch from news- papers works well in controlling weeds. The best time to add mulch depends on your goal. Mulch provides a thick barrier between the soil and the air. This helps to reduce temperature changes in the soil. As a result, mulched soil will be cooler than other soil in the summer. In winter, the mulched soil may not freeze as deeply as other soil. The best time is after the ground has frozen, but before the coldest weather arrives. Spreading mulch before the ground has frozen may attract small animals searching for a warm place to spend the winter. Delaying the spreading should prevent this problem. The animals will probably find another place to live. The following can be used as organic materials to make mulch EXCEPT   _  .
tree barks plastic grass cuttings cut-up leaves