Mulches help to retain soil moisture and reduce weed growth. They fall into two categories: organic types that decompose naturally in the soil, and inorganic types that do not decompose and therefore must be removed after serving their pupose. The choice of an organic or inorganic mulch depends on the season and what purpose the mulch serves:
Organic mulches are used by most home gardeners. They are the best choice if the goal is to conserve moisture and reduce soil temperatures during the summer. They can also help to reduce soil crusting. A variety of materials can be used, such as bark chips, compost, ground corncobs, chopped cornstalks, grass clippings, leaves, manure, newspapers, peanut shells, peat moss, pine needles, sawdust, straw and wood shavings.
- Because organic mulches conserve soil moisture and reduce the soil
temperature by 8–10°F, they should not be used too early in the spring. If mulches are applied to cold garden soils, the soils will warm up more slowly and plant maturity will be delayed. On the other hand, organic mulches can reduce soil crusting.
- After the soil warms in spring, an organic mulch may be applied to a depth
of 2–4 inches around well established plants. Be sure that there is adequate moisture in the soil before applying the mulch.
- Mulches such as sawdust, wood shavings and corncobs can use up some of the
soil nitrogen as they decompose. To compensate, add 1–2 cups of 8-8-8 or 10-10-10 fertilizer to each bushel before applying them.
- Reduce weeds around the outside of the garden and between rows by putting
down several layers of newspaper and then covering them with leaves.
The greatest value of inorganic mulches is their ability to increase soil temperature very early in the growing season when the soil is cool. Because plastic film mulches serve as barriers to fruit rot and soilborne diseases, they are especially beneficial for crops that produce fruit on the ground, such as melons, cucumbers, squash, eggplant, okra and tomatoes. They also reduce nutrient leaching from rainfall and keep mobile fertilizer nutrients from being lost from the root zone.
Plastic mulches are easier to use if applied before planting. Soil moisture should be high before these filmlike materials are placed over the soil. Groundcloth is applied as strips, placing each over the prefertilized row to be planted, then burying the edges to prevent the wind from blowing it away. Short slits are then made with a pocket knife for depositing seeds or planting transplants.
Clear plastic increases soil temperature 8–14°F at a depth of 2″ and can raise it further if you use a double layer of plastic with a small air space between. The soil warms faster with clear plastic than with any other type of mulch. Because clear plastic does not exclude light, it stimulates germination of weed seeds as well as crop seeds. The plastic must be removed as soon as the seedlings emerge through the soil.
Black plastic increases soil temperature 5–6°F at a depth of 2″ provided that there is good contact between the mulch and the soil, and reduces the weed population.
Wavelength-selective plastic, also known as infrared transmitting (IRT) plastic, this permits heat to penetrate, but blocks the passage of light. It can be used from early spring through the growing season. It warms soil like clear mulch but without the accompanying weed problem.