Soil Building - The Arboretum
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To understand how a garden thrives, we must turn our attention to the soil.  Every conversation at the Crescent includes an evaluation of soil health.  We work to incorporate the latent power of regeneration by recognizing the importance of ‘feeding’ the beneficial fungi and microbes at work below ground.  It’s not a complicated practice but requires lots of MULCH.  Read below for an expert description from our favorite soil scientist.

Soil Power: Enhancing Drought Resilience in a Changing Climate

By Lynn Fang, MS
October 2016

That vast abyss of darkness beneath the lush green plants and colorful blossoms has remained mostly a mystery for many. Soil has been called the final frontier of science, as we know more about the oceans and of the cosmos than we do of the ground beneath our feet. Often overlooked as a simple thing with little to understand, soil is gaining attention as one of the major keys to mitigating climate change, as well as to growing opulent, verdant gardens during our historic drought.

soil_breakdown pie chartSoil is not dirt, is not dust, and is not simply a mix of minerals and chemicals. It is a dynamic matrix of life, breathing, reproducing, and transforming organic compounds into plant available nutrients. A little less than half of the soil consists of mineral particles, a quarter is air, a quarter is water, and the remaining 5-10% is organic matter, which describes any plant or animal matter in decay, including plant roots, biological organisms, and humus, the end product of decomposition. It is this little slice of organic matter that gives the soil all of its life, structure, and vitality, its ability to act as a carbon sink (sequestering carbon from the atmosphere), as well as its ability to soak in and retain water. When we begin to understand the biological properties of the soil, we can nurture these processes to work for us, so we can create resilient soils.

At the Crescent Farm, experimental soil building processes such as lasagna mulching (layering cardboard, green waste, and wood chips) and hugelkultur (a mound of logs, green waste, mulch, and soil) are used to conserve water, creating ecological fertility for healthy blossoms and bountiful vegetables without the use of any pesticides or herbicides. Lasagna mulching produced giant stalks of heirloom corn, with cobs over a foot long, and oftentimes up to 6-7 ears of corn per stalk, virtually unheard of for heirloom corn.

Despite the success of these popular methods, there has been little documentation on the actual effects on overall soil health. We sent soil samples of the hugelkultur, bark mulch, lasagna mulch, and a control comparison of unamended soil to the Cornell Soil Health Analysis Lab, which specializes in assessing biological indicators of soil health, including available water capacity, organic matter content, and aggregate stability (the strength of the soil structure), among several others.

Overall, all three soil building processes vastly improved all soil health indicators compared to the unamended soil. Essentially, adding organic matter to your soil will improve your soil’s ability to retain water and provide plant available nutrients. Returning organic matter to the soil is absolutely key to maintaining a thriving garden in a changing climate.

Color Aggregate Stability (%)

Aggregate Stability is a great integrative indicator of available water capacity, organic matter, and the strength of the overall soil structure. It measures the extent to which soil aggregates resist falling apart when wetted by heavy rainfall. Strong soils have strong aggregates that remain in tact upon impact by water, making them more resistant to erosion. Our test showed that the bark mulch displayed the strongest aggregate stability, indicating its unique properties that vastly improve overall soil structure to hold more water and be more resistant to erosion.

Not all organic matter is created equal. Mulch and compost mixes can likely be improved with the addition of bark. There is something special about tree bark – it is the living skin of the tree, protecting the tree from pathogens and other invaders. It is rich in microbial diversity, especially fungi, which is most likely why the soil beneath the bark mulch is so much stronger and drought resilient.

For a more detailed report on the results of this study, and to learn about the specific soil health indicators, click here.

Lynn Fang, MS, defended her masters on compost use in plant disease suppression at the University of Vermont. She is an urban agroecologist who specializes in crafting soil and compost of the highest quality. She consults and teaches in the greater Los Angeles area.

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© 2019 Los Angeles County Arboretum and Botanic Gardens

Phone: 626.821.3222

301 N. Baldwin Ave, Arcadia, CA, 91007