'Will work for carbon' is the sign hanging in the hidden world of soil organisms.
November 15, 2007 By Top Crop Manager
Billions and billions of hardworking microbes are willing and able to release
valuable nutrients for crops by breaking down carbon-based crop and herbicide
residues, but only if the soil is healthy enough to permit the process.
A key indicator of soil health is its organic matter content. Even though organic
matter content of cultivated prairie soils is a small fraction (between one
and eight percent), the impact is huge – nearly all soil nitrogen, up to
60 percent of phosphorus and up to 80 percent of soil sulphur is found in organic
matter. In addition, healthy levels of organic matter in the soil tend to improve
tilth, reduce wind and water erosion, facilitate drainage and break down any
residual compounds in the soil.
"Soils with higher levels of organic matter are rich in the lignin, cellulose
and other carbon sources that are associated with increased soil micro-organism
activity," says Marcia Monreal, research scientist at the Agriculture and
Agri-Food Canada (AAFC) Brandon research station. "The micro-organisms'
work releases nutrients in plant-available form somewhat analogous to tillage,
but with a key difference; the release is slower and at rates dependent on climatic
conditions, such as temperature and moisture."
Examples of micro-organisms at work are mycorrhizae, which are soil fungi that
live in a symbiotic relationship with most plants, except for those in the mustard
and lamb's quarters families. Mycorrhizae receive carbon from plants and, in
return, plants receive mineral nutrients from mycorrhizae, particularly phosphate.
Mycorrhizal hyphae (small root-like structures) explore the soil around the
host plant and are very good at extracting nutrients from the soil and transferring
it to the host.
For crops, healthy levels of microbial activity mean a steady supply of nitrogen,
phosphorus, potassium and sulphur as well as other nutrients, throughout the
growing season. By contrast, tillage promotes a more rapid and short-lived burst
of released nutrients that leaves a pool of mobile nutrients such as nitrates
susceptible to leaching or other loss. To a certain extent, additional tillage
ends releasing nutrients that growers have paid to put into the soil.
Soils with higher levels of organic matter also encourage higher levels of
microbial activity that work to break down herbicide residues applied in-crop.
To some soil microbes, residues can be one of the many forms of carbon-based
material that they feed on.
"What we have found in the field is that a healthy soil is necessary to
sustain and support biological activity, which in turn is necessary for plant
health, and the breakdown of various residual compounds. To maximize biological
activity, a 'healthy' soil should have an adequate supply of inorganic nutrients,
sufficient moisture, a balance of water and air-filled pores, a soil pH near
neutral, and a soil temperature between 15 degrees C and 30 degrees C,"
says Brian Schilling, technical services representative, with Arvesta Canada.
Seeing the difference
Monreal adds that the small size and extreme diversity of soil micro- organisms
mean that measuring populations directly is impractical when trying to determine
the overall microbial activity, including fungi and bacteria. Instead, researchers
measure the levels of carbon dioxide respiration that mark soil biological activity.
This slow respiration of carbon dioxide is another contrast with tillage operations,
which generally is associated with a more sudden release of larger amounts of
CO2 and a larger reduction of soil organic matter.
In the field, that difference is indicated by the colour of soils. The black,
biologically active soil exposed by tilling sod or a forage stand clearly contrasts
with the lighter coloured, less biologically active soils on intensively tilled
soils with low organic matter. The difference can be particularly notable on
knoll tops, where drier soils and thinner crops mean less crop residue, more
erosion, lower organic matter and lower soil biological activity. In these areas,
risk of herbicide residues in the soil can also be expected to increase.
What can be done?
To keep soils healthy, soil organic matter and microbial activity can be retained
or increased with choices in tillage and crop diversity.
Long-term measurements show that soil organic matter has generally decreased
over the years since prairie soils were first farmed. The most rapid declines
occur where a fallow year without cover crops is part of the rotation, and where
low fertility decreases the amount of crop cover. Where moisture conditions
permit, practices including reduced tillage, continuous cropping, manuring and
correct application of fertilizer can gradually increase or even rebuild levels
of soil organic matter.
For a healthy, biologically active soil, Monreal explains that crop diversity
is also key. "In fertile, biologically active soils, there is a vast range
of soil micro-organisms with a very complex system of interaction," she
says. "For example, even after they themselves die, the decaying bodies
of soil micro-organisms release enzymes that continue the breakdown of diverse
Monreal sums up by saying, "A wide diversity of cropping choices promote
a wide diversity of soil micro-organisms, which in turn enables the complexity
of interaction that is associated with healthy soils." -30-