Soil Searching: Amendments Are No Longer Recommended
Southwest Yard & Garden by Dr.
Marisa Thompson
partial reprint from October 2018
Frosty cottonwood leaves add
a nice layer on top of several inches of shredded woodchip mulch. Photo credit
M. Thompson.
Question: Should I add fertilizer or
amendments to the soil when planting landscape plants in my garden? -
Gardeners
All Over New Mexico
Answer:
The short answer is no. And the long-term answer is an even
stronger no. Soil amendments are materials that are worked into the soil with
the intention of improving soil physical qualities. The trouble is it’s just
not that simple. As City of Las Cruces Community Forester Jimmy Zabriskie said
when I asked if he ever recommends using soil amendments when planting a tree,
“Save your money for mulch.” Mulch is material that sits on top of the soil,
providing an insulation layer that helps control soil temperature, retain
moisture, and suppress weeds.
Shredded woodchip mulch helps soil hold moisture and insulates plant roots on a frosty November morning. Photo credit M. Thompson.
This
is assuming you’re planting ornamental plants that are recommended for your
area. The recommended species are the ones with roots that are well adapted to
our native soils, so they’re more likely to live long, sustainable lives. The
bottom line: Always put the same soil you dug up right back into the hole with
the new plant, and top dress with a thick layer of mulch.
Slow
and steady wins the race when it comes to perennial landscape plants. As
Albuquerque City Forester Joran Viers said last week at a tree planting demo
put on by Tree New Mexico and The Nature Conservancy, it helps to think of our
trees (and other perennials) as marathoners, not sprinters. When fertilizer is
applied to trees, they can grow too fast for their own good and are more likely
to develop structural problems and have other issues. Fertilizers are not
recommended at the time of planting perennial plants because we want the
plant’s energy to go into root establishment at a healthy rate to support the
aboveground growth. The growth hormones necessary for healthy root development
are found in the root tips and are made by the plant itself, so adding root
stimulator to living roots is not necessary, or even helpful. (Side note: Rooting
hormone is potentially useful when you’re trying to get roots to grow quickly
from cuttings where the roots have been removed completely.) Hold off on
fertilizing for at least a few years while your plants are setting down roots.
Research
on all sorts of landscape plants has shown that by adding amendments (like
organic matter or perlite) to your soil backfill, the initial, short-term
results are good. But the interface between the amended soil and the native
soil is bad news. Water doesn’t move well across the interface, and neither do
roots. With time, the roots may be inclined to stay in the original planting hole
and spiral around, so you end up with essentially a potted tree with reduced
growth rate, constant water and nutrient stress (because lateral roots never
fully developed), heightened vulnerability to pests and diseases, and even an
increased falling hazard (think root ball in socket).
Even
after landscape plants are established, the best practice is “test, don’t
guess.” Having your soil tested can help you decide which nutrients are
deficient, if any. The International Society of Arboriculture’s new Best
Management Practices publication for tree planting warns that even if soil
tests “reveal conditions that will limit tree performance, amending the fill
soil is rarely effective at correcting the long-term problem.” That’s the key—we’re
talking long-term plant health here. NMSU Extension Guide A-114, “Test Your
Garden Soil” (http://aces.nmsu.edu/pubs/_a/A114/welcome.html),
has great info on soil testing. I’ll include links to other soil and planting
resources on the blog version of this column (https://nmsudesertblooms.blogspot.com/).
For more gardening information, including
decades of archived Southwest Yard & Garden columns, visit the NMSU
Extension Horticulture page (http://desertblooms.nmsu.edu/), follow us
on social media (@NMDesertBlooms), or contact your County Extension office (https://aces.nmsu.edu/county).
Marisa Thompson, PhD, is the Extension
Horticulture Specialist for New Mexico State University and is based at the
Agricultural Science Center at Los Lunas.
Edible New Mexico Magazine article crosslink -- GROWING YOUR FUTURE FAVORITE TREE
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