Native Mulch Test Results

 

Many people understand the value of mulching, but some think the only acceptable material is pine bark. Even when the supply is hundreds of miles away and the price is exorbitant, people think that is their only choice. In San Antonio, citizens have been getting pine bark from more than 200 miles away. At the same time, they were dumping, burning, and burying a mulch material that was equally as good and in some ways better. At one dump site alone, over a million cubic yards has already accumulated. This mulch material is a native tree trimming that has been run through a shredder/chipper. Some people don't like it because it is a local product and others say it doesn't look good.

At Garden-Ville, we decided to see if this native material could be made acceptable. A specially designed grinder was purchased to regrind it. The new machine turned out a uniform, nice looking material, but was the mulching qualities as good as bark? To find out, a cubic yard of each was placed in an open field about 15 feet apart and spread to a layer four inches thick. A water sprinkler was placed between them and both were watered well. The test began on June 3, 1989. On June 13, it rained one and a half inches, then we entered a drought period, and that was the last moisture the two mulches got until a rain 108 days later. Weekly I poked a moisture meter through each mulched area at least one inch into the soil. For the first 60 days, the meter needle moved as far as it could to the wet side of the dial. Not until the 65th day did the needle start moving back from wet to moist. The last reading was taken on the 105th day, and the needle still read in the moist area, although on the low side. The same instrument was used to check both mulched areas, and readings were always taken near the center. The readings were always exactly the same in both the pine bark and native mulch. The bare soil between the two mulches read completely dry on the meter at the same depth by the 15th day after the June 13 rain.

One day in August the temperature got up to 104'F. Curious about the temperature under the mulch, I used a thermometer with a probe and checked both mulched areas. The soil under the pine bark was 85'; under the native mulch it was one-half degree cooler, probably because of the slightly lighter color of the mulch. Then I checked the bare soil between the two piles and at the same depth, the soil temperature was 120', 35 degrees difference between mulched and un-mulched soil. On an average summer day, the temperature under the two mulches was always between 80' and 83' during the hottest part of the evening.

The pine, a flake or nugget type of mulch, was more easily blown or washed out of place. The native mulch seemed to bind together and stay in place better.

After using this native mulch around my home and orchard for a while, I noted the plants seemed to be growing much better than those I had mulched with pine bark. I thought perhaps this was just my imagination or wishful thinking, but I found out that I wasn't alone. Landscapers began buying native mulch instead of pine bark. They said they were noticing that the native mulch acted as a fertilizer and made the plants grow instead of holding them back like bark did.

I discussed the fertilizing effect of the native mulch with a retired county extension agent who works for me as a consultant. He told me about a study Texas A&M did on the feed value of small twigs and branches for animals. The study showed that these small parts of the tree are rich in protein and that is why goats and mules and other farm animals browse on them.

The tree trimmers that produce native mulch are cutting mostly from the small branches that stick up in the phone and electric lines. These branches have the live cambium bark, live buds, and green leaves that are full of protein. When these materials are used as mulch, the decomposing microbes working at the soil level are breaking up the proteins and releasing nitrogen which the plant can readily use. When bark is used as a mulch, the microbes working at the soil level don't find enough nitrogen in the high-carbon dead bark even for their own use, so they rob it out of the soil and take it away from the mulched plants.

With all the emphasis on water conservation, rising fuel prices, the pollution trucks contribute to the environment, combined with the lower price, native mulch looks better all the time!

 

The Garden-Ville Method - Lessons in Nature