APLD Brochure: An APLD Guide to Sustainable Soils Available on the APLD website www.apld.org

This brochure is a really good basic summary of soil - and we all know its all about the soil!  

"The bottom line is that soils should be valued and preserved, because, literally, life depends upon it."

Recommendations for Designers include:

•  Design for your soils. Aim to restore the site's soil back to its native composition. In most cases, avoid overbuilding soils into something beyond what is natural for your area.
• Know the biology of your soils by sampling and testing the soil using a competent soil biology lab.
• Do not till or physically disturb a healthy, mature soil. Physical disruption severely damages or destroys the mature soil food web. 
• Physical soil compaction during construction has a long-term damaging effect on soils and is difficult to remedy after the fact. All efforts should be made to protect soils prior to and during any construction phase. 

The brochure also has a nice summary of the role soils play in sustainability:

• Soils are the foundation of the ecosystem.
• The living systems occurring above and below ground are determined by the properties of the soil. Soils store and cycle nutrients needed by these living systems, supporting life all the way from microbes to humans. A healthy, diverse ecosystem is critical for life, and it begins with the health of the soil.
• Soils store carbon
• Soils manage water.  Water enters the soil through the channels created by vegetation and the activities of organisms such as earthworms. It fills the empty pore spaces and is taken up by plants. Healthy soils have sufficient open pore space to absorb the water and allow it to infiltrate the soil, recharging the groundwater. Wetlands, also known as hydric soils, manage large quantities of water and also serve as buffers and filters in addition to supporting a vast array of wildlife.  

On the other hand, degraded soils exhibit erosion, which occurs when soils are not covered by vegetation, and rainfall both compacts the soils, forming a crust on the surface, and carries the top layer of sediment away. Compacted soil compresses the pore spaces so that air, water, and plant and animal life cannot penetrate.

• Soils filter, buffer, degrade and detoxify potentially harmful chemicals
• Soils influence climate.  Soils moderate temperature fluctuations, as soil heats more slowly than air and can absorb more heat on a hot day. Soils absorb heat during the day and radiate heat at night. Darker soils, which tend to have higher organic content, absorb the most heat. Soil temperature affects plant growth, which in turn affects climate.

Soils and Water

Our soils have changed.

When considering the loss of the eastern deciduous forest (75% gone) and the transition from native prairie / savanna plant systems to the mono-crops of industrial agriculture, we are looking at an incredible change within our soils. The depth and bulk of our current root systems no longer exist as they did two hundred years ago. Trees have extensive root systems in the top 24" of soil, and can reach five to eight feet or more in depth. The prairie existed on a root system depth between two and three feet, with some plants reaching four to six feet deep. The extensive root systems of our original "ground cover" opened up the soil, allowing for a deep penetration of precipitation and the slow exhale of moisture back up into the atmosphere through plant transpiration.

Not any more: corn, soybeans, wheat and other annual crops have temporary root systems in the 12 to 18 inch range. Perennial turfgrass, our American lawn, covering an area about the size of Wisconsin, has a root system of around six to twelve inches in the best of conditions. Precipitation run-off is now something we have to plan for after almost every rain.

Garth Conrad, APLD Garth Conrad Associates, LaPorte, IN

Soil Basics revisited

Understanding "the basics" of soil is no small matter.  One of the most basic problems with soil is compaction.  For many of my clients, the soil around their homes has been virtually ruined by what took place when their house was built or renovated.  Many (all?!) builders/contractors think of soil as dirt and make no effort to protect it or renovate it after they're finished.  Around here, most of the new development is infill development, so virtually the whole lot is disturbed - by tearing down the old house, by cutting down all the trees, by building an addition, by installing a pool.  Their machines churn across what might have been relatively undisturbed soil, often when the soil is saturated after a hard rain, creating ruts that are virtually impossible to correct.  They dig a new foundation and turn the existing soil-profile upside down in doing so.  What used to be subsoil is now on top.  When they're done, they flatten out the clay-ey subsoil, put back a couple inches of the "topsoil" that they allegedly scraped off at the beginning (is it full of weed seeds now?? has it been tested? what is its texture?  has compost been added?), throw down some "contractor's mix" el-cheapo grass seed and poof - there's your new lawn!  This is what we refer to as "urban soil".

Urban soils are typically inhospitable places for trees, other plants, and their oxygen breathing microorganisms.  Human activities, such as those described above, as well as grading and even foot traffic leave urban soils much more compacted than natural soils.  Typically 40-55% of the volume in a healthy forest soil consists of pore space.  This pore space consists of varying proportions of air and water depending on the weather.  With compaction, soil particles are pushed together and fill up pore spaces, so pore space in urban soils often goes down to 20-30%.

Soil compaction is generally estimated by measuring bulk density, which is the mass of dry soil divided by its volume, expressed in grams per cubic centimeter (gms/cc).  Compacted soil has less pore space and therefore higher bulk density.  Surface bulk density of most undisturbed soils ranges from 1.1 to 1.4 gms/cc, depending on the soil texture (clay, sand and silt fractions).  The bulk density of urban soils often ranges from 1.5 to 2.0 gms/cc, just slightly less than the bulk density of concrete (2.2 gms/cc)!  

The reduced porosity of compacted soils results in a lower water-holding capacity and reduced infiltration rate, so compacted soils produce much more stormwater runoff than undisturbed soils.  In many urban areas, pervious areas, like "lawns", produce almost as much runoff as impervious surfaces because they are so compacted. The reduced water holding capacity of compacted soils also renders plants more prone to drought and results in more extreme summer soil temperatures.

Compaction also drives oxygen out of these soils to suffocatingly low levels and oxygen-dependent soil microorganisms can no longer survive.  Without adequate soil organisms, urban soils generally have a lower organic matter content and lower nutrient retention than natural soils.  Soil compaction also limits root penetration and growth. Once soil bulk density exceeds 1.4-1.7 gms/cc (depending on soil texture), roots are no longer able to penetrate soil, and vegetation growth becomes limited.

That's one of the reasons that the "landscaping" your contractor installed before you bought your new house or moved into your renovated one has declined every year!