Last
month (December 2012) we discussed methods for modifying soil drainage. We end our soil series by considering the
soil as a functioning ecosystem, with implications
for the home gardener.
Many
gardeners focus on inorganic soil particles. We ourselves spent most of a blog post
(October, 2012) talking about soil particles and their effect on soil drainage. But the inorganic (mineral) particles are
just part of the story. In fact, soils
have four equally important components:
·
the
mineral particles (sand, silt and clay)
·
air
·
water
·
organic
material (living organisms and decomposing matter)
A
soil in good condition for growing crops, garden vegetables and other plants is
said to have ‘good tilth’. It has the
proper soil structure and nutrients to support healthy plant growth. Good farmers and gardeners aspire to have soil
with good tilth.
A
soil with good tilth has a proper balance of mineral particles, air, water and
organic material. It smells good – not
sour or rotten. Often it will crumble
easily in the hand. It contains
decomposing matter (humus) derived from mulch, dead leaves/branches and old
roots. And it supports a wide range of life from gophers and earthworms to
microscopic organisms – the soil biota.
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| Credit: USDA-NRCS |
The
soil is, in fact, a living ecosystem, complete with carnivores (meat eaters),
plant eaters (herbivores), decomposers and parasites. What many people assume is ‘just mineral
soil’ is a whole world of living creatures.
The air, water and mineral components of soil provide food, living sites
and other requirements for life. Plants
are just one part – albeit and important one - of this ecosystem.
In
nature, soil organisms perform many important tasks. For example, earthworms
burrow into the soil, loosening it and making it more porous. This helps water to infiltrate and makes it
easier for plant roots to grow. When we ‘loosen’ the soil before planting, we
do what earthworms do in natural soil ecosystems. If we ‘double dig’ a vegetable garden, we
mimic the effects of energetic gophers at work in our garden. How
much easier – and more in tune with natural cycles – to encourage soil
organisms to do the tasks they were meant to do.
Soil organisms perform a number of services,
including some we cannot do for ourselves.
These services include:
- loosening
the soil, decreasing soil compaction
- building
complex soil structure that both drains and holds water
- increasing
soil nutrient levels in the root zone of plants
- producing
root-stimulating hormones
- ‘fixing’
nitrogen into a form that can be used by plants (and humans)
- competing
with – and combating – destructive soil bacteria and fungi
- decomposing
dead organic matter into humus
It’s time we take
seriously the health of our garden soil biota and its effect on garden plants. Fortunately, recent research suggests several
things you can do to promote soil health.
Soil
tilth is affected by many factors including soil compaction, how much we dig/turn
the soil, the fertilizers and other products we apply and physical factors like
rainfall and temperature. But perhaps
the most important factor is the organic part: the humus and the living organisms. According to soil science research [1] ‘An acre of living soil can contain 900 pounds
of earthworms, 2400 pounds of fungi, 1500 pounds of bacteria, 133 pounds of
protozoa, 890 pounds of arthropods and algae, and even small mammals in some
cases.’ That’s a whole lot of living going
on beneath the ground!
How
do you know if your soil supports life?
The best way is to look. Dig up
a scoop of soil, spread it out on a sheet of paper and look at it with a
magnifying glass. Here are some of the
organisms you might find in a healthy soil:
Earthworms: 5-30 in a cubic foot of garden
soil; expect at least 1 in your sample
Soil Arthropods (insects, mites,
spiders, springtails, & millipedes) : 100-300 per cubic ft – you should see
some small Arthropods in your sample.
Fungi (evidence of fungal strands or
hyphae): may be present
Nematodes (non-segmented round worms): may be
present
Several
types of soil organisms will be too small for you to see. Individual
species of many of these microscopic soil organisms have not yet even been
described and named. Microscopic soil organisms
include:
Bacteria
Protozoa
Viruses
Others
We’ve
included several good references at the end of this post. They provide more details on soil organisms
and their roles. We hope you’ll want to
learn more!
If
your garden is new, you may not have seen many organisms in your scoop of
soil. This should not alarm you – the numbers
and types of soil organisms tend to increase as a garden matures. Soil building
takes time. But there are several good,
scientifically-based management practices that can make your soil more hospitable
to soil organisms. We suggest you follow these practices to
increase your soil tilth.
- Add
organic matter (compost) to the vegetable garden. This provides food for many soil
decomposers. The humus produced
will increase soil structure and fertility. If gardening with California native plants,
be sure that your plants do not
require a ‘nutrient-poor soil’ before adding organic material. See our November 2012 post on ‘Soil
Amendments’.
- Use
mulch (if appropriate). Mulches help keep soil moisture and temperature
even, providing better habitat.
Organic mulches break down, supplying organic food for decomposers.
Be sure to use the correct type of
mulch, particularly if gardening with California native and other
water-wise plants (see our posting on Mulches – July 2012).
- Water
appropriately. The lives of native soil organisms are tied
to the moisture cycles that occur in nature. In our local gardens, soil organisms ‘expect’
adequate soil moisture from late fall through spring. But they are adapted to periods of
drought in summer and fall. Plant native plants – and water them
appropriately – and you’ll supply the water regimen local soil organisms
need. We’ll provide more advice about
soil moisture and irrigation this spring.
- Plant
native plants.
Plants and soil organisms have complex relationships that are
not entirely understood. The more
native plants you include in your garden, the better the habitat for native
soil organisms. Choose plants that are native to your area and reap the
benefits in healthy soil.
- Avoid
unnecessary digging, turning or tilling of soil. All of these effect soil structure and
adversely affect soil habitat. The
best soil for soil organisms is one that’s simply left alone.
- Prevent
soil compaction. Soil compaction decreases soil pores and
the water and air they contain.
Provide walkways in the garden to limit compaction. Avoid walking on clay soils when wet.
- Limit
use of pesticides. Many kill beneficial organisms along
with the pests. And beneficial soil organisms often eat pest
organisms for food.
- Limit
use of fertilizers. These amendments change the chemical environment
of the soil. Research has shown
that over-use of fertilizers actually decreases soil biota in many soils. Southern California gardeners in
particular should use chemical fertilizers with care.
- Plant
a variety of plants (diversify); rotate plants in the vegetable garden. Planting a variety of plants insures
that soil organisms obtain the nutrients and other materials needed for
life. It also provides insurance
against the effects of soil pests and diseases. Rotating vegetable crops helps prevent
the buildup of unwanted soil organisms.
- Respect your soil as a complex ecosystem. Remember that your soil supports a
living system every bit as complex as above-ground ecosystems.
Building
up a range of soil organisms takes time, patience and working with Mother Nature. But the benefits to your garden include increased
plant health, less time spent on gardening tasks and the conservation of important
soil organisms. We hope you’ll choose to support life –
including life that you cannot see - in your garden.
__________________________________________________________
Learn
more about soil tilth and soil organisms at:
References
1. Pimentel,
D. et al. 1995. Environmental and economic costs of soil erosion and
conservation benefits. Science. Vol. 267, No. 24. p. 1117-1122.
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