Design in a Time of Uncertainty
By Rives Taylor, Principal, Global Resilience Research Lead
Most of us take for granted the soil beneath our feet, so familiar that we may forget how truly miraculous it is. Indeed, it literally underlies everything we do. We recognize that soil is important when considering a construction project, relying upon an understanding of its mechanical properties to appropriately design foundations and earthworks. Yet we can often be oblivious to the amazing and diverse biological properties of healthy soil.
In most built environments the first thing to be done is to cut down and clear the existing vegetation, then efficiently strip and remove the topsoil. This is done because topsoil is unsuitable as a foundational material. After construction is completed the site soils must be rebuilt in order to support plant life for landscaping. All too often this essential restoration is neglected, leaving us frustrated when attempting to cultivate a vigorous landscape.
But it is actually much more that is lost when we fail to appreciate the importance of healthy soil. The organic matrix and organisms that dwell in the soil make up a complex network of ecological functions and synergies. The tiniest bacteria, fungi, and other living things that inhabit the soil are actually powerful forces responsible for carbon and nutrient cycling over the land masses of the Earth.
One gram of fertile soil can contain up to one billion bacteria. Fungi are among the most important soil organisms because of the ecological and economic functions they provide. Plants rely upon a symbiotic relationship with mycorrhizal fungi in the soil to process nutrients into a form that plants can use. There are many other organisms and animals that inhabit and interact with the soil. The Soil Food Web aptly describes these industrious residents of the soil and their relationships and functions. Healthy soils are rich in humus and other organic matter with a robust, diverse biological community of microorganisms, fungi, insects, earthworms, nematodes, and larger animals. This is what must be restored when it has been lost in conventional development practices.
Chemical fertilizers and pesticides can upset this delicate balance of life in soil. These quick fixes can ultimately prove detrimental if we do not respect the all-important biological aspects of soil. Organic landscaping practices focus on building soil structure and robust biological populations that facilitate healthy, vigorous plant life. Increasing the organic matter content of soil helps make it an attractive habitat for these essential organisms. This can be achieved with the use of quality compost and mulch, along with other organic soil amendments that promote plant growth without harming soil organisms.
Compost is not soil – it is a soil amendment, added to soil to improve characteristics desirable for plant growth, moisture holding, and resistance to erosion. It is essentially organic matter such as yard trimmings, manure, and food scraps that have been decomposed through a composting process. Compost can be produced by individual homeowners using material generated at their property, or at a larger-scale commercial composting facility that gathers these materials as feedstock. Compost is usually produced through an aerobic process (in the presence of air/oxygen), but can also be produced by the cultivation of earthworms, known as vermicomposting. These two processes are quite different, but both can yield a quality compost product. Various methods of composting can yield a variety of product characteristics, such as particle size and microbial community composition.
Mulch differs from compost in that it is less decomposed and would typically have larger particles. Mulch can include materials such as partially decomposed wood chips, bark, or straw. Mulch helps keep the soil cool and moist, suppresses weeds, resists erosion, and is an attractive landscaping material.
However, it may be difficult to select the right compost or mulch for a particular application. This is not made any easier by a lack of comprehensive standards for compost and mulch products. There have been instances of material sold as compost that is really not what it purports to be, sorely lacking in the diverse microbial life of a true compost. So be careful when selecting a compost to assure that it is rather light and soft, with obvious organic content and an ‘earthy’ aroma. Good compost should NOT have an unpleasant odor.
Beyond promoting healthy plant growth, the use of compost and mulch improves the texture of the soil so that plant roots can better develop vigorous structure. The organic matter and fungal structures in healthy compost are also highly effective at holding moisture. This means that irrigation can be reduced or even eliminated except for periods of drought. This moisture-holding effect helps retain rainwater, thus reducing stormwater runoff that contributes to flooding. Compost and mulch have also been shown to dramatically improve water quality by reducing erosion and capturing pollutants within the organic matrix, affording microbes the opportunity to metabolize certain substances such as nutrients and hydrocarbons. This results in less sediment and pollutants entering our surface water bodies.
Compost and mulch are essentially recycled organic products, made from feedstocks such as yard trimmings, food scraps, woody debris and other materials that would otherwise end up in a landfill. The composting process converts these feedstocks into beneficial materials. Nature does this automatically, as leaves and other biological material are consumed by bacteria, fungi, and other organisms to produce the rich, beautiful, healthy soil found beneath the detritus of a forest floor.
So, as we take to our yards in the annual ritual of springtime spruce-up, take the time to find out about the properties and uses of compost and mulch. Before long you will see the results and become yet another fan of these amazing natural materials. With a little help from us, the various life forms of the Soil Food Web will naturally populate the soil and help us maintain vigorous landscapes, conserve water, and keep pollutants out of our streams and rivers.