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:
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