Be a Better Farmer: Understand
the Living Soil
Part 1 - Introduction
Originally Printed in Country Guide, Spring 2001
Table of Contents
- The Living Soil
- The Nitrogen Cycle
- Biological Activity of Soil Life
- Soil Organisms
- Don't Squash These Beetles!
- Related Links
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.
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 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 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 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.
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.
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