Perfecting Soil
Amend the planting hole? Can you add too much of a good
thing? Here are answers.
By Marie Hofer
HGTV Ideas magazine
In a perfect world, fall is the perfect time to take steps
to improve your soil. Why? There's time to correct soil imbalances, time to let
amendments work their magic, time to prepare for the freezing and thawing that will loosen
well-worked clay soils. And there's plenty of free organic matter at hand.
But chances are you'll be doing a lot of amending this
spring. Whether you're planting perennials and trees, opening up a new landscape bed or
expanding the vegetable garden, your garden will benefit from your toils in the soil. Here's
some advice from the experts.
Test your soil
Depending on what type of soil you have, adjusting its pH
usually takes months. Next to its structure (size and arrangement of particles), no other
factor of your soil so affects the availability of nutrients and the vigor of your plants.
You can lob on the fertilizer, but none of its ingredients will be available to your plants
unless the pH is correct.
Add organic matter
Organic matter improves tilth (fluffiness) and opens the
pores of the soil, allowing easier penetration of roots, air and water. It also sustains
millions of microbes--beneficial bacteria, protozoa, nematodes and fungi--that help
contribute to a healthy ecosystem. In sandy soils, it encourages the retention of water and
nutrients. In clay soils, itdecreases the hard lumpiness, makes it easier to work and
improves drainage.
Can you add too much? Yes, especially if it's the wrong
kind. High-carbon material such as bark and sawdust can create nitrogen
deficiencies; add either a nitrogen fertilizer or manure to speed decomposition and restore a healthy nitrogen
balance. Even adding finished compost temporarily sets off another
imbalance, says Roland Meyer, extension soils specialist with University of California
at Davis.
"You're presenting a whole new food basket for the organisms. They have
the first crack at the nutrients, so if you plant, the plant is shortchanged.
That's going to happen to a certain extent no matter what organic material you
use, even really green grass clippings that have high nitrogen. " The lag
time can last anywhere from a few weeks to a few months-another good reason for
adding organic material in the fall, not in the spring.
Add wood ash in moderation, unless your soil is extremely
acid. A valuable source of nutrients, wood ash--particularly that of hardwoods like
oak--is a potent source of lime and will drive up the pH of your soil. If your soil is
neutral or alkaline, don't use wood ash at all. Also, don't apply ash when seeding; it's too
salty for seedlings.
Holes: To amend or
not to amend
For decades, the advice of the day was to add liberal
quantities of bagged topsoil, bark chips and other goodies to the holes in which shrubs and
trees were being planted. Then university research in the '80s found that roots could
actually go into a kind of shock when they first left the cozy surroundings of the amended
hole and encountered the real thing.
"Our philosophy is to get the plant established in the
native soil in which it 's going to spend the rest of its life," says Gary Alan, host of
The Designer's Landscape. "If you're adding amendments, you're just creating a pot in the
ground, and you're not encouraging the roots to branch out as far as they
can." But there are times when you might want to bend--for
example, when sandy soil needs
organic matter in order to give young plants or trees a good
start, says Alan.
Don't work wet soil
Refrain from working your soil when it's wet--no matter how
wonderful your tiller or other tool is. Manipulating wet soil causes
compacting--especially in clay soils but even in sandy loams--forcing layers to collapse together
where they harden into iron fists. To determine whether your clay soil is dry enough for
tilling, squeeze a handful of soil into a ball; if the ball crumbles easily, it's ready to
till.
To avoid compaction of the subsoil beneath the six inches or
so that are being cultivated, consider reducing the number of passes you make with your
tiller--or even not tilling at all after the garden's initial preparation.
Leave some organic
matter on the surface
New research by the USDA's Agricultural Research Service in Saint Paul, Minnesota, suggests that leaving plant matter on the surface helps
encourage earthworms to dig vertical tunnels to reach their food source. The vertical
tunnels--unlike the usual horizontal patterning--help channel water into the ground.
In his backyard, Randy Southard, soils professor at the University of California
at Davis, practices no-till: cutting cornstalks and leaving all garden
refuse on the surface. "It's hot and dry here, and organic matter decomposition rates are
fairly high. One of the things I've noticed is that if I mix the stuff in and
get the microbes working on it, there's less organic matter on the surface and
much less water filters into the ground."
No matter whether you're reducing soil water loss or you're
simply lazy, you'll rest easy knowing that the organic matter will some day give your soil
the color and texture of homemade chocolate cake.