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Administration Finance & Focus


The low down on good mulch

By Urban Harvest

mulchGardening in southeast Texas has many intricacies, oddities and details, but there is one practice that is uncomplicated, easy to do and tremendously valuable. We all need to mulch.

Mulch your vegetable and flower beds, and around shrubs and trees with 2-3 inches of high quality, biologically active mulch. You can do this in any month but if you do it in the late spring or early summer, you will particularly help all those plants trying to tolerate our long, hot summer that is to come.

Mulch and compost often are the results of when microbes make organic materials rot. Compost is a general term. Compost can be used as mulch, but in the garden, it usually is incorporated into the soil one way or another. The term "mulch," on the other hand, refers to where the material is placed.

Mulches are put on the surface for many purposes. A common purpose is to reduce evaporation and weeding, and in the south, to keep roots of plants cool and damp. Mulches can be made out of many substances other than rotted plant materials. There are rock mulches, cardboard and newspaper mulches, plastic mulches, and rubber ones.

However, mulches that aren't made from organic decomposition do not convey the many other benefits mulch can provide. Quality mulch provides calories that feed soil fungi and bacteria. These microbes make nitrogen in the mulch, as well as produce growth promoting substances near the roots. The life giving nitrogen doesn’t evaporate or wash away as a result of it being produced by the soil microbes. Additionally, microbes provide food for bacteria and fungus-eating nematodes and other larger microbes. The result is a soil rich in "microbe manure" "microbe blood meal" and "microbe bone meal." See Nature's Way Microorganisms Fact Sheet.

There are other positive results as well. A large number of harmful fungi, bacteria and nematodes are removed from the soil. Tight soils with an active decomposer food web gradually develop a spongy high-quality tilth. Such soils have higher oxygen content, roots have an easier time growing and water stays available to the roots much longer, so less irrigation is necessary.

These changes are, at this point, a solid part of the scientific literature, but the precise mechanisms still are somewhat speculative. What is known is that soil bacteria, and especially the actinobacteria, decompose plant material into humic acids, and these tend to bind mineral particles together. Earthworms and other macrobes make air tunnels in the soil, while consuming plant debris, creating worm castings in larger pellets. The increase in tilth makes the soil more aerobic and fungi proliferate. They bind these pellets together, and beneficial mycorrhizal fungi use these passageways to increase mineral uptake by plant roots. They also release a protein glomalin that further improves soil texture.

All of this assumes you are mulching with a material that soil organisms prefer. You can get quality mulch by making it, but if you need yards of it and have a small property, you probably need to buy it. There are many products out there from the awful to the wonderful. Many products contain substances that are harmful to beneficial soil microbes. See Nature's Way Resources mulch info. sheet for more information.

Look for products made from mixed shredded tree branches that have been composted for six months or more. One good sign is that some places are now posting their soil food web test results Nature's Way Soil Test Results In bags, such products will have air holes, and when delivered by truck will certainly almost be among the most expensive mulches you can buy. On the other hand, they may be the cheapest products out there, because you will only need to buy it every two years, your plants will retain moisture better and your water bill will not as high in the summer.

This column is produced by Urban Harvest. Learn about gardening classes, community gardens and orchards, farmers markets and more at Urban Harvest website.  Article is written by Bob Randall, Ph.D., who is the former executive director and a cofounder of Urban Harvest. He is also the author of "Year Round Vegetables, Fruits and Flowers for Metro-Houston". You may contact him at