Getting to Know Your Garden’s Soil: Urban Soils

Soil is truly a garden’s foundation.  Plants obtain water and many nutrients from the soil – and interact with soil organisms.  Soil conditions may also limit the types of plants you can grow successfully.  The more you know about the characteristics of your soil, the better you can match plants and watering methods to your unique soil conditions.

A well-developed soil is a complex ecosystem.  When people talk about soils, they’re usually referring to the inorganic part of the soils – the sand, silt and clay particles.  But a good soil contains much more.  It has pores which hold vital air and water.  It contains decomposing organic matter (old roots; decayed leaves; etc.) that provide plant nutrients and help retain soil moisture.  And it’s home to a whole world of soil organisms, ranging in size from gophers and earthworms to microscopic bacteria and fungi.  In fact, a good soil is literally teeming with life!

Soils are characterized by the parent material (rock) from which they were formed and the conditions of their formation. In western Los Angeles County, most native soils were deposited by the L.A. River and its tributaries - or by ancient oceans.  Local soils range from almost pure sand near the beach (remains of ancient sand dunes) to denser, river/lake deposited clays and clay loams further inland.  Soils on the Palos Verdes Peninsula were formed from complex, highly weathered marine deposits.  The most common native soil type on the Peninsula is clay loam.   If you’d like to learn more about the original (native) soils for your local area we suggest the following resources:

 

·     Original L.A. County Soil Survey (1903) - http://soils.usda.gov/survey/online_surveys/california/losangelesCA1903/losangelesCA1903.pdf

 

·    Other California soil surveys (some on-line) see - http://soils.usda.gov/survey/printed_surveys/state.asp?state=California&abbr=CA

 

Urban soils – including those in our gardens - are more complex than native soils. In fact, urban soils are among the most difficult to classify – and work with – of any soils world-wide.  Like all soils, urban soils are composed of a parent rock material that has weathered over time to produce soil.  But it’s the human effects that make urban soils so ‘difficult’.  Soil scientists are just beginning to get a true understanding of the differences between urban and native soils – and their effects on plant growth.   Some of these differences are discussed below.

Depending on where you live, your garden may once have been a farm or ranch – or even had a factory on it!  Human usage leaves an indelible footprint on the soil. And the effects of past land use can influence your gardening success.  For example, farm soils have usually been fertilized and their structure modified by plowing. Farming practices almost always modify the nutrient content and drainage properties of soils.  Recently active farms may also have used herbicidal chemicals to control weeds.  

Plants themselves can affect soil nutrient content and soil organisms. Some plants are ‘heavy feeders’, depleting soils of major nutrients, while others leave plant chemicals in the soil.  For example, it may be difficult to grow plants in soil where a  Walnut tree recently grew. Even weeds, which are often ‘nutrient hogs’,  can deplete soils of important nutrients.  The potential effect of previous vegetation is important whether the plants were agricultural crops, weeds or garden plants planted by a previous home-owner.   We’ll talk more about basic soil nutrient testing next month (October, 2012). 

Your soil may have been compacted by heavy equipment during house or road construction.  Compaction is routinely done to provide a stable base for house foundations, roads, walkways and patios.  In addition, construction equipment itself compresses the soil.   Soil compaction changes soil drainage; compacted soils drain more slowly. Compaction decreases the soil air/water pockets, making it more difficult for roots to grow.  Compaction problems are most acute in the first 20 years after construction.  After that, natural processes, including plant growth and decomposition, gradually return the soil to a more normal composition.  We’ll talk more about testing – and improving - your soil drainage next month (October, 2012).

Your garden soil may be composed of ‘fill dirt’ brought in from somewhere else.  Fill dirt can have a very different composition than the original soil on a site. And fill dirt doesn’t have the complex soil structure that good soils develop over time.  Studies in other areas suggest that fill soils in general have higher levels of clay and less organic matter than native soils, making them drain more slowly.  A recent study suggests that sites with fill/disturbed soils on the Palos Verdes Peninsula also have more clay – and less organic material (humus) – than native soils in nearby areas (see www.csulb.edu/depts/geography/gdep/posters09/zamora.ppt. for more).  The good news is that well-chosen garden plants modify fill soils, converting them from ‘dirt’ to mature, functional soils over time. 

Your garden soil may even include ‘buried treasure’.  For example, the soil in Mother Nature’s Backyard has old bricks, asphalt, glass, chunks of cement and other debris dumped since the 1940’s (see top picture for some of our more interesting ‘finds’).  In Mother Nature’s Backyard, we chose to leave the majority of the debris in place. The choice was both a practical and conscious one.  We plan to study the effects of native plants on the garden's soil over time. We also want to see whether our ‘junk filled’ soil has adverse effects on the plants. We hypothesize that native plants are far hardier – and more effective soil-changing agents – than anyone suspects.  We’ll keep you posted of our results over the next few years.  

Old industrial sites are of particular concern for gardeners.  They may contain chemical and other debris harmful to humans.  If you live on a former industrial site – and particularly if want to grow edibles – we suggest the following:

 

·    Trace Element and Urban Gardens -http://celosangeles.ucdavis.edu/Environmental_Horticulture/Trace_Elements_and_Urban_Gardens_568/

·    Urban Gardens and Soil Contaminants - http://www.misa.umn.edu/prod/groups/cfans/@pub/@cfans/@misa/documents/asset/cfans_asset_287228.pdf

 

Urban soils are challenging and interesting.  There’s still much we don’t understand about them.  But it’s becoming clear that certain aspects of urban soils predict gardening success.  Next month (October, 2012) we’ll discuss some simple soil tests that you can do to learn more about your soil’s characteristics. In the following months we’ll discuss things you can do to promote healthy, productive garden soils.   In the meantime, if you’d like to learn more about urban soils we suggest the following:

 

·    Urban Soil Primer (USDA Natural Resources Conservation Service publication)  - http://soils.usda.gov/use/urban/downloads/primer(screen).pdf

 

·    Urban Soils - by Phillip J. Craul, Professor of Soil Science, SUNY-College of Environmental Science & Forestry  - http://www.ces.ncsu.edu/fletcher/programs/nursery/metria/metria05/m57.pdf

 

 

As always, you can e-mail your Garden Soil questions to: mothernaturesbackyard10@gmail.com .