For The Answer
By Calvin R. Finch, Ph.D., Conservation Director, San Antonio Water System, and Horticulturist
Week of November 24, 2003
COMPOST & MULCH
Two of the most important xeriscape principles are to enrich the soil with organic material and to cover the soil with mulch. If you read this article every week, you know that I say compost is the best organic material with which to enrich soil and it is also good mulch. Sometimes I overlook the fact that not everyone understands exactly what compost or mulch is.
Compost is decomposed organic material. It is the end product of the breakdown over time of materials like leaves, stems, fruits, vegetables, weeds and even wood. The final product is a homogenous, clean material that looks like coarse, crumbly soil.
The decomposition process can be fast or slow. Fast decomposition occurs when you add equal parts of brown material (as a carbon source) and green materials (as a nitrogen source), and add moisture and air. If you piled and mixed dry oak leaves from the lawn with an equal part of fresh grass clippings and it rained every week (or you wet the pile), the pile would decompose so fast that you could feel the heat (160 to 180 degrees F.). You would have compost in six or seven weeks.
If you just piled the leaves up and you didn’t add a nitrogen source or moisture, it could take over a year for the leaves to decompose. Some gardeners speed up the process by adding lawn fertilizer to the pile as a nitrogen source and by turning the pile over every few weeks to incorporate air into the mix.
The best raised bed gardens are made up of one-third compost. Compost spread two to four inches deep and tilled into the soil increases drainage and water-holding capability of the soil for a lawn or garden. Spread it one-half to one inch deep before or after lawn aeration and you increase its health remarkably. St. Augustine grass on a six-inch bed of soil that is one-half organic material is relatively water efficient. Zoysia or Bermuda grass on such a bed will be very effective.
Compost has some nutrients (a small amount of nitrogen, phosphorous, potassium and trace elements) but its major value is for texture and soil structure. Roots prosper in a soil enriched with compost.
Compost also makes good mulch. Mulch is a layer of material on top of the soil that insulates the soil from the air and sun. The soil stays cooler, water does not evaporate as quickly and weeds do not grow as readily. Cool soil means that roots function better and a lower evaporation rate means that less water is needed.
Many organic materials work for mulch. Leaves are great—pecans, mulberry, elm and deciduous oaks in the fall and live oaks in the spring. A coarse, shredded brush from the City of San Antonio’s Solid Waste Department’s Brush Site at 1800 Bitters Road is free to San Antonio residents and works well. Pecan shells are attractive as mulch and protect the soil as does bark, cedar, cypress, cocoa shells and sawdust. For more information on the City’s Brush Site call 522-8831 or 311.
Compost works as mulch but since it is so valuable as soil enrichment we generally save it for that use and utilize the other materials for mulch. Mulch applied to a soil and replenished each year gradually decomposes just like the compost pile. It replicates nature where the bottom of the organic layer in the woods or prairie is compost and the top is newly laid raw material.
Compost can be used as mulch. What about using mulch for soil enrichment? In some cases, this works fine. Leaves or grass clippings incorporated into the soil improve soil texture and plant performance. With coarser materials like sawdust or shredded brush, however, they create a temporary nitrogen shortage while the materials break down. The texture can improve but the soil becomes nitrogen deficient. To counteract the problem extra nitrogen fertilizer must be added. The best strategy is to use coarse materials as mulch above ground and relatively fine materials like compost in the soil.
For those of you who do not have the time to make your own compost or need more mulch than your trees can provide, buy it. Small quantities are available in bags at your favorite nursery or buy it in bulk from the organic material specialists like Fertile Garden Supply, Living Earth Technology, New Earth Technology, Garden-Ville or Keller Material. They will load your truck or deliver it to you and also have bagged material.