The physical properties of soil - including soil composition, drainage properties, pH and fertility - all affect your ability to grow plants. So it’s important to learn about them early in the process of planning your garden.
Last month (October, 2012) we
discussed several simple soil tests for determining the physical
characteristics of your soil. If you’ve
done these tests, you now know whether your soil drains quickly, slowly or
somewhere in between. But how can you
use this knowledge to improve your gardening success?
In the past, California gardeners
were encouraged to amend their soils prior to planting. If your soil was slightly basic/alkali (pH 7.5-8.0
– common in our area) and you wanted to plant an acid-loving plant like an
Azalea, you were advised to amend the soil with peat moss and use an acid
fertilizer. If your soil was sandy, you
were encouraged to add organic amendments like steer manure and compost to
‘improve’ your soil. And so on and so
forth.
The Colorado State University
Extension (see suggested references, below) defines soil amendments as follows:
A soil amendment is any material added to
a soil to improve its physical properties, such as water retention, permeability,
water infiltration, drainage, aeration and structure. The goal is to provide a
better environment for roots.
To do
its work, an amendment must be thoroughly mixed into the soil. If it is merely
buried, its effectiveness is reduced, and it will interfere with water and air
movement and root growth.
A visit to the local gardening center can be a
confusing experience; there are so many different soil amendment products
available! All of them cost money and require time and effort to use. So,
what’s a sensible approach to soil amendments and fertilizers?
Here at Mother Nature’s Backyard, we like to garden
in concert with Mother Nature. We suggest
working with what you have rather
than trying to change it. Consider your
soil’s characteristics as assets,
rather than properties that automatically need improvement. And tailor your plant choices and irrigation practices
to your un-amended soil.
This approach runs counter to the advice of many
traditional gardeners, but we think it has merit. For one thing, you’ll save money and time. You won’t be overusing products that can
contaminate the soil and ground water.
You’ll also learn about a whole new set of plants – California natives
for the most part – that may have grown in your soil for thousands of
years. And you’ll have the satisfaction
of seeing your garden grow into a small functioning ecosystem, above ground and
below.
Do
I have to use soil amendments?
The simple answer is no. The plants that do best in a garden are
usually those suited to the natural soil conditions. As it turns out, it’s actually
difficult and expensive to change some of the basic soil properties, like pH, using
soil amendments. In the long run, it’s usually
easier to plant the right plants for the soil conditions rather than the other
way around.
Many gardeners don’t realize that a
large number of California native plants thrive in local sandy and clay soils. These include everything from annual
wildflowers and native grasses to groundcovers, shrubs and trees. For lists of plants particularly suited to sandy
and clay soils in our area see:
·
Sandy
Soils List: http://www.nbs.csudh.edu/biology/projectsound/habitat/documents/Sandy_Soils-3-11.pdf
and a talk on gardening in sandy soils:
·
Clay
Soils List: http://www.nbs.csudh.edu/biology/projectsound/habitat/documents/Clay_Soils-3-11.pdf
and a talk about gardening in clay soils: http://www.slideshare.net/cvadheim/clay-hummingbirds-2009
Can
I amend just part of my garden?
Yes – in fact that’s often a good
idea. With a little planning, you can
amend only areas that really need it and plant better suited plants in the rest
of the garden. By grouping plants (like vegetables) in one
area you can save effort while still giving each plant the soil conditions it
needs.
What
if I want to grow a plant with very different needs than my soil provides?
If possible, consider growing the
plant in a large container. It’s easy to
give plants the specialized soils they need in containers. Another possibility is to grow the plant in
a planter or in raised beds. Once
again, the idea is to group plants with similar needs so you amend only
a portion of your total garden.
Should
I amend my vegetable garden's soil?
Yes. The vegetable garden has soil
requirements that are often different from the rest of the garden. Vegetables need a well-drained soil with relatively
high nutrient levels. The best way to
provide this is to amend vegetable garden soils with compost and supplemental
fertilizer. If you have a vegetable garden you will likely want to amend it
yearly.
Consider growing vegetable in raised
beds or containers which can be amended more easily. For
some great resources on vegetable gardening see: http://celosangeles.ucdavis.edu/Common_Ground_Garden_Program/Gardening_Articles/
Should
I amend soils in containers?
The environment in containers
(pots/planters) is quite different from that in the ground. Drainage and soil nutrients are particular issues. We suggest using a good potting mix (rather
than garden soil) in containers. Garden
soil usually doesn’t drain as well as potting mix and may contain undesirable
soil organisms. You can amend the
potting mix to suit the needs to the plant: more sandy, more acidic, higher nutrient
levels, etc.
Pots, planters and containers
usually benefit from occasional to yearly addition of soil amendments or even
an entire change of soil. We’ll discuss
container gardening in more detail in another post.
My
soil drains slowly. Shouldn’t I amend it
to improve drainage?
Not necessarily. Clay and clay-loam soils do drain more slowly. But they also retain soil moisture, so you
don’t need to water as often in the summer and fall. We know from experience in
Mother Nature’s Backyard!
There are plenty of plants that like
clay soils, so you can tailor your plant choice to your soil drainage
characteristics. And there are other
things you can do to improve soil drainage.
We’ll discuss these in some detail next month (December, 2012 –
Improving Soil Drainage).
How
do soil amendments differ from mulches?
Soil amendments are incorporated
into the soil before planting, while mulches are spread on top of it (see July,
2012 for a discussion of Mulches).
Organic mulches do decompose and change a soil’s characteristics over
time. But they are not soil amendments.
Should
I amend soil in the potting hole before I plant a tree/shrub?
Not
a good idea – for native and non-native plants alike. Research has shown that this old practice actually
inhibits the development of a good root system.
The following site debunks this and other common horticultural myths (and is fun, informational reading): http://www.puyallup.wsu.edu/~Linda%20Chalker-Scott/Horticultural%20Myths_files/index.html
What
are common soil amendments?
Like mulches, soil amendments are inorganic
(either mined or man-made) or organic (from something living). The
table below summarizes characteristics of the most common soil amendments
used in S. California gardens.
Amendment
|
Uses and Cautions
|
Sand
|
Why
used: once
believed to improve drainage – usually does not
Best
uses: for cacti grown in containers (mixed
with potting soil to create a ‘cactus mix’)
Cautions:
never use in clay
soils; will create ‘concrete’
|
Perlite
|
Why
used: to improve drainage
Best
uses: In containers; in potting mix for
raising plants from cuttings or seedlings (need a well-drained potting mix)
Cautions: costly
|
Vermiculite
|
Why
used: to improve water retention
Best
uses: In containers; in potting mix for
raising plants from cuttings or seedlings (need moist soil)
Cautions: costly; becoming hard to find
|
Soil acidifiers
(Sulphur)
|
Why
used: to decrease soil pH
Best
uses: For acid-loving plants grown in
containers;
Cautions:
difficult to
decrease soil pH; need to continue treatment over time
|
Wood products
(bark, wood chips, hulls, etc.)
|
Why
used: to
improve drainage and soil fertility
Best
uses: as
mulch (rather than soil amendment); in compost (where it adds nutrients)
Cautions:
the breakdown of wood products requires high levels of soil Nitrogen –
need to add N fertilizer for 3-4 years after use; breakdown may release natural chemicals (tannins)
which inhibit seed germination.
|
Peat/Sphagnum Moss
|
Why
used: to
decrease soil pH and improve water retention
Best
uses: in
containers or planters where acid-loving plants are grown
Cautions: decreases soil fertility and
drainage over time; environmental concerns over mining of peat bogs
|
Grass clippings/
chopped leaves
|
Why
used: to improve soil fertility
Best
uses: to
produce compost; chopped leaves for mulch
Cautions: may contain weed seeds
|
Manure (aged)
|
Why
used: to
improve soil drainage or water retention; to improve soil fertility.
Best
uses:
vegetable garden
Cautions: increases soil salts - can make
soils toxic to some plants & soil organisms; will ‘burn’ plants if not
well aged
|
Compost (home
composted)
|
Why
used: to
improve drainage and soil fertility
Best
uses:
vegetable garden; rose garden; areas with non-native plants (for example, a
traditional flower bed)
Cautions:
may increase soil nutrients too much for local native plants.
|
Commercial Compost/
Amendment Blends
|
Why
used: to
improve drainage and soil fertility
Best
uses: containers;
houseplants
Cautions: often expensive; not needed/can
be too ‘rich’ for many native plants
|
Worm Castings
|
Why
used: improve
soil fertility and raise pH
Best
uses: containers; houseplants
Cautions: expensive if purchased; you can
raise earthworms & create your own, but that takes space, effort.
|
Fertilizers
(‘chemical’ or ‘natural’)
|
Why
used: to
improve soil fertility
Best
uses:
vegetable garden; containers; rose garden; flower garden (non-native plants);
native plants from wetlands or ponds/stream banks (1/4 recommended dose)
Cautions: most native plants need none (or
low doses); for all plants, over-use of fertilizer can lead to excessive
growth & decreased plant health/increased pests; long-term use of
fertilizers can actually make soils toxic to plants and beneficial soil
organisms. Use only as needed.
|
Learn more about soil
amendments at:
· http://celosangeles.ucanr.edu/Common_Ground_Garden_Program/Gardening_Articles/ (Los Angeles Co. Master Gardener
publications)
·
http://www.commongroundinpaloalto.org/fertilizers.htm#boxedandbaggedfertilizers
(vegetable garden)