Be a Better Farmer: Understand
the Living Soil
Part 1 - Introduction
Originally Printed in Country Guide, Spring 2001
Table of Contents
- Introduction
- The Living Soil
- The Nitrogen Cycle
- Biological Activity of Soil Life
- Soil Organisms
- Don't Squash These Beetles!
- Related Links
Introduction
Soil organisms can help or hurt commercial crops, but our knowledge of
these subterranean critters is limited. In these Living Soil articles,
OMAFRA soil management specialist Anne Verhallen throws some light on
their hidden world. This first installment, an overview, will lead into
a more detailed examination of specific good and bad organisms, and how
we can adapt farming practices to encourage or discourage them.
The Living Soil
Farmers walk or drive over their fields, apply inputs and harvest crops
with little thought to life beneath the soil surface and how they affect
it. But there is plenty going on underfoot, and learning more about it
can make us better farmers.
Without soil organisms, processes like decomposition, mineralization,
and degradation of potential pollutants would not occur. We need soil
organisms and should strive to encourage their survival.
The Nitrogen Cycle
The nitrogen cycle is a good example of the far-reaching effects of soil
life. The cycle begins when bacteria in the root nodules of leguminous
plants like alfalfa and soybean convert nitrogen gas from the air to ammonium
or nitrate ions in the nodules.
Different microbes then break down the nitrogen-rich plant tissue to
release ammonium into the soil. This step in the cycle is called mineralization.
Next, yet another set of soil microorganisms convert ammonium to nitrate,
the nitrogen source most readily taken up by plants.
Some of this ammonium and nitrate, however, is absorbed by other soil
organisms as part of their bodies. This makes the nitrogen temporarily
unavailable to plants. In effect, it's stored for later use. As the organisms
die, their nitrogen is released for uptake by other organisms or plants.
The final step in the cycle is denitrification, in which still one more
group of bacteria convert nitrate to nitrogen gas, releasing it back into
the air. This process takes place mostly under water-logged conditions.
Soil bacteria need nitrogen to decompose plant residues, mostly made
of carbon compounds, to release ammonium (mineralization). If residue
contains insufficient nitrogen relative to the amount of carbon, the bacteria
tie up (immobilize) soil nitrogen reserves, including what was applied
as fertilizer.
At a carbon:nitrogen ratio below 25:1, mineralization occurs. At ratios
above 25:1, you get immobilization. Alfalfa hay at 13:1, for example,
allows mineralization to proceed. But corn stalks (60:1) and sawdust (400:1)
result in immobilization of nitrogen - it's present in the soil but temporarily
unavailable to a crop.
Biological Activity of Soil Life
Biological activity of soil life helps to build and maintain soil structure
or tilth. Burrowing by ants, beetles and earthworms, for example, creates
channels (macropores) that allow water to move into and through the soil
rapidly.
Under no-till conditions, macropores remain largely undisturbed which
helps explain reports by farmers that fields drain better under no till.
Various types of soil life also improve soil structure by blending soil
particles and organic matter.
The level of biological activity determines in large part a soil's resilience,
or ability to recover from stresses like compaction, excess water, and
tillage. Think of the fields you're familiar with. A field with high soil
resilience is one that always seems able to put out a good crop, regardless
of conditions.
Soil Organisms
Soil organisms play a direct role in plant health, too. Some are crop
pests and others are predators on the pest organisms. A soil with a wide
diversity of organisms is more likely to be a healthy soil with fewer
plant diseases.
Soil is truly alive. It teems with plant and animal life, mostly invisible.
In these articles we'll be taking a closer look at the impact of those
organisms on our crops, and our impact on them. The accompanying item
on ground beetles is an example of what's to come.
Don't Squash These Beetles!
The poor ground beetle has been misunderstood for way too long. Often
mistaken for a pest, these insects actually benefit agriculture.
Quick facts: More than 2,500 ground beetle species have been identified
in North America. Also known as carabids, they're usually dark brown or
black but some are iridescent metallic blue, green purple or copper. Adults
are somewhat flattened, elongated and vary from 0.6 to 2.5 cm in length.
Most are ground dwellers and can be found under rocks, clumps of soil,
decomposing wood and crop debris, or in burrows within the soil. Very
few climb onto vegetation. When disturbed, these beetles run rapidly and
seldom fly. They tend to hide during the day and feed at night. One generation
per year is normal but adults can live up to 3 years.
Both adults and larvae feed on other insects including many agricultural
pests such as the Colorado potato beetle, European corn borer, corn rootworm,
armyworms, cabbage maggot, diamondback moth, tentiform leafminer and cutworms
to name just a few.
High ground beetle populations can help keep insect pests below their
economic thresholds. Studies in Iowa found that ground beetles can consume
up to 5 cutworm larvae per day or 13 cutworm eggs per week, and in high
enough populations can reduce injury from this pest by up to 98%.
In the field, high ground beetle populations indicate a healthy ecosystem.
We can help with cover crops and minimum tillage which leave surface residues
as habitat. Minimizing the use of insecticides also maintains populations
of this valuable insect.
On the downside, ground beetles can occasionally be found indoors, but
they're only a nuisance insect and do not harm humans or animals, or cause
structural damage or any sort. Don't squash them, though. They can emit
a foul odor as protection against enemies. Reduce the amount of plant
residue around outside walls, and caulk cracks and other entry points
to keep beetles out of your house.
A few species of ground beetles can become agricultural pests. A couple
of examples, seldom seen in Ontario, are the seedcorn beetle and the slender
seedcorn beetle. The seedcorn beetle is light brown with a black head
and 2 wide black stripes on its back. The slender seedcorn beetle is shiny
dark red with no stripes.
Both adult beetles can damage corn seed planted into cool wet soils or
in other conditions that slow germination. The larvae of these species,
however, are predators on harmful insects. Benefits from other ground
beetles far outweigh potential damage from seedcorn beetle adults in Ontario.
Use of seed insecticides on corn to prevent injury from seedcorn beetle
is not recommended or economical.
So next time you're walking you fields and look down to find a ground
beetle, smile instead of squash. This creature is a friend.
Related Links
