sunfish

Linda Chalker-Scott, Ph.D.
Extension Horticulturist and Associate Professor,
Puyallup Research and Extension Center, Washington State University
The Myth of Soil Amendments:
"When transplanting trees or shrubs into landsca
pes, amend the backfill soil with organic matter."
Of all the fictions that abound in popular horticultu
ral, none is as deceptive as this one. It stems from
the old adage to "dig a five dollar hole for a fifty cen
t plant." Adding organic matter to a planting hole
appears to be a promising step towards achieving that fi
ve dollar hole. It seems logical that steer manure,
peat moss, compost, etc. would improve poor soils
by increasing aeration, nutritional value, and water
holding capacity. And it does - in the immediate vi
cinity of the planting hole. Eventually, amended
planting holes will have negative consequences to pl
ant health. To understand why, it's necessary to
examine plant physiology and soil water relations.
Let's say you have incorporated the recommended 25-50% organic matter to your backfill.
(Remember that an ideal soil contains 5% organic matter by volume.) The
initial
results are positive;
roots grow vigorously in this ideal environment as lo
ng as irrigation is provided. But what happens when
these roots encounter the interface between the plan
ting hole and the native soil? Native soil contains
fewer available nutrients, is more finely textured and is less aerated. Roots react much in the same way as
they do in containers: they circle the edge of th
e interface and grow back into that more hospitable
environment of the planting hole. The roots do not es
tablish in the native soil, eventually resulting in
reduced growth rates and hazard status as crown growth exceeds root ball diameter.
Soil water movement is problematic as well. Amen
ded backfill has markedly different characteristics
than surrounding native soil; it is more porous and wate
r will wick away to the finer-textured native soil.
In the summer, moisture within the
planting hole will be depleted by the plant but not replaced by water
held more tightly in the native soil. This results in
water stress to the plant unless the planting hole is kept
irrigated, a costly and often unrealistic practice.
During wet seasons water will move quickly through the
amended soil only to be held back by the more slow
ly draining native soil. The resulting bathtub effect,
wherein water accumulates in the planting hole, fl
oods the roots and eventually kills the plant.
Finally, all organic material eventually decomposes.
If you've incorporated one quarter or one half
organic matter by volume, within a few years you will
have a sunken garden in your landscape. This only
exacerbates the flooding problem during wet conditions.
No scientific studies to date show any measurable be
nefit of soil amendment except in containerized
plant production. Plants grown in native soil cons
istently showed better root establishment and more
vigorous growth. Only one study reported no negative
effects of amending soil with organic matter - but
there were no benefits, either. When you consider th
e cost of materials and labor needed to incorporate
soil amendments, it's difficult to justify the practice.
This outdated practice is still required in the specifi
cations of architects, landscapers, and other groups
associated with landscape installation. It is still r
ecommended by garden centers and gardening articles.
And there is a multi-million dollar soil amendment industry
that has little interest in debunking this myth.
As responsible green industry professionals, we need
to recognize and avoid non-sustainable management
practices.
Bottom line
:
•
Select
suitable
plant species for site conditions
•
Don't be an "enabler": use native soils for backfill
without
amendment
•
In extreme cases, replace the
entire
planting site with topsoil
•
Mulch
landscapes well with wood chips or another water-holding material
For more information, please visit Dr. Chalker-Scott’s web page at
http://www.theinformedgardener.com

RobG7aChattTN

The problem is that when you plant a container grown plant in most cases there is no way to get rid of the potting soil...so you will have solar results.