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Scientists warn farmers and
gardeners not to overemphasize results from one or two
steps, but to consider the test as a whole. For instance, "sandy soil is
pretty easy to work," says Mark Williams, Ph.D., assistant research
scientist of soil microbial ecology at the University of Georgia in
Athens, Georgia, "but the soil may not have other components of soil
quality."
You can do all 10 steps during the active growing weeks in spring. You
can test for soil structure and tilth, compaction and plant residue,
year-round. Check various locations in the garden for the broadest
picture possible. The more detail you have, the more accurate and
reliable the results.
Test 1. Soil structure & tilth
When the soil is neither too wet nor too dry, dig a hole 6 to 10 inches
deep. Separate an intact section about the size of a soup can and break
it apart with your fingers. Determine whether the soil is cloddy,
powdery, or granular. Ideally, your soil should be made up of different
sized crumbs that will hold their shape under slight pressure. Crumbs,
or aggregates, as soil scientists call them, that break apart only with
difficulty mean your soil is too hard.
Why it's important
"Soil rich in organic matter tends to form relatively round aggregates,
which leads to porosity," says Tom Thompson, Ph.D., professor of soil
science, also at the University of Arizona. Open, porous soils allow the
free movement of water and oxygen, he explains, so plants can develop
strong, healthy roots.
Test 2. Compaction
Plunge a wire flag vertically into the soil at different locations. Mark
the depth at which the wire bends. The sooner it bends, the more
compacted the soil. A foot or more of easily penetrable soil is ideal.
Why it's important
Compacted soil inhibits root growth and water availability, and keeps
earthworms and other vital soil fauna from circulating freely.
Test 3. Workability
You may have already learned about your soil's workability the last time
you got the garden ready for planting. If tilling or digging the soil
produces cloddy or platelike clumps, the workability is low. Farmers
measure workability by monitoring how much tractor fuel they use; you
can simply judge the effort necessary to prepare beds for planting.
Why it's important
Soil that's easy to work allows water to reach roots efficiently and is
less prone to compaction. Fail this step, and your garden will likely
show disappointing results for many of the other tests. "If the soil
isn't easily worked, other problems have already been going on for a
while," says Raymond Allmaras, soil specialist with the USDA's
Agricultural Research Service in St. Paul, Minnesota.
Test 4. Soil organisms
Measure the animal life in your soil by digging down at least 6 inches
and peering intently into the hole for four minutes. Tick off the number
and species of each organism observed, such as centipedes, ground
beetles, and spiders. Because most soil organisms spurn daylight, gently
probe the soil to unearth the more shy residents. If you count less than
10, your soil does not have enough active players in the food chain.
Why it's important
A thriving population of diverse fungi, bacteria, insects, and
invertebrates is one of the most visible signs of soil quality. The more
that creeps and crawls under your garden, the less opportunity there is
for pests and disease. Each level of soil life does its part to break
down plant residue and make more nutrients available for plant growth.
Test 5. Earthworms
When the soil is not too dry or wet, examine the soil surface for
earthworm casts and/or burrows. Then dig out 6 inches of soil and count
the number of earthworms squirming on the shovel. Three worms are good,
five are better. The absence of worms means the soil does not have
enough of the organic matter they feed on. An exception: If you live in
the Southwest, don't waste your time looking, even if the soil displays
other conditions of soil quality. "Earthworm activity is less likely in
the desert," says the University of Arizona's Dr. Walworth. "Worms don't
like hot soil."
Why it's important
Not only do earthworms aerate the soil, but their casts infuse the soil
with enzymes, bacteria, organic matter, and plant nutrients. They also
increase water infiltration and secrete compounds that bind soil
particles together for better tilth.
Test 6. Plant residue
If you've grown a cover crop, dig down 6 inches one month after turning
it into the soil and then look for plant matter. The range of organic
material is important to notice here. The presence of recognizable plant
parts as well as plant fibers and darkly colored humus indicates an
ideal rate of decomposition.
Why it's important
"The single most important component of healthy soil is organic matter,"
Dr. Thompson says. But plants and other organic materials decompose only
when soil organisms are there to do the work. Any sign of this process
is a good sign, but the speed of decomposition is important, too. Fast
decomposition is another indicator of soil quality. In poorly aerated
soil, plants break down slowly, a condition that gives off a faintly
sour scent.
Test 7. Plant vigor
Start this test during the active growing season and look for healthy
plant color and size that's relatively uniform. Overall health and
development must be judged for what's considered normal for your region.
One caveat: If you planted late or during a drought, or suffered a pest
infestation, results of this test may be unreliable.
Why it's important
Plant vigor indicates soil with good structure and tilth, a
well-regulated water supply, and a diverse population of organisms. It's
the best sign of effective soil management you'll have above ground.
Test 8. Root development
Use a shovel or hand trowel to dig gently around a selected plant,
preferably a weed you won't miss. Once you've reached root depth, pull
an annual plant up and check the extent of root development, searching
for fine strands with a white healthy appearance. Brown, mushy roots
indicate serious drainage problems—and a poor outlook for this year's
harvest. Stunted roots might also indicate disease or the presence of
root-gnawing pests. "When you look at the roots, you can really see
what's going on," Allmaras says.
Why it's important
Roots have the most immediate connection with and reliance on soil
quality. Without air, water, biological activity, and crumbly soil to
grow in, roots can't do their job.
Test 9. Water infiltration
Take an empty coffee can with the bottom removed and push it into the
soil until just 3 inches remain above the surface. Fill the can with
water, marking the water height, and then time how long it takes for the
water to be absorbed into the soil. Repeat this several times until the
rate of absorption slows and your times become consistent. Anything
slower than 1/2 to 1 inch per hour is an indication of compacted soil.
Why it's important
Good infiltration gets water to plants where they need it—at their
roots—prevents runoff and erosion, and lets air move more efficiently
into soil pores.
Test 10. Water availability
Wait for a soaking rain; then record how long until plants start to show
signs of thirst. Results will vary widely by region. The basic lesson is
that if plants require more frequent watering than typical for your
region, your soil is probably the culprit.
Why it's important
Porous soil can better resist evaporation and adequately supply plants
between waterings. "It could make all the difference in the world if
water were to go another inch deeper," Allmaras says.
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