croP resIdues: fueL vs. soIL orGAnIc MAtter
There is currently a huge effort under way to more efficiently
convert structural plant material (cellulose) into fuel. As we write
this, it is not commercial y feasible yet—but this may change in
the future. One of the dangers for soil health is that if the conver-
sion of plant structural material (not grain) to ethanol becomes
commercial y viable, there may be a temptation to use crop
residues as an energy source, thus depriving the soil of needed or-
ganic inputs. For example, most aboveground corn residue needs
to return to the soil to maintain the soil’s quality. It is estimated
Partial removal of corn stover after harvest for use as biofuel.
that between 2 and 5 tons of corn residue are needed to maintain
a soil’s favorable properties. A long-term study in New York
indicated that, at least for that particular soil, modest removal of cornstalks did not cause a deterioration of soil. However, we must be very cautious when considering removing crop residue as a routine practice. As the legendary soil scientist Hans Jenny put it in 1980, “I am arguing against indiscriminate conversion of biomass and organic wastes to fuels. The humus capital, which is substantial, deserves being maintained because good soils are a national asset.”
If a perennial crop such as switchgrass is harvested to burn as an energy source or to convert into liquid fuel, at least soil organic matter may continue to increase because of the contributions of extensive root systems and the lack of tillage. On the other hand, large amounts of nitrogen fertilizer plus other energy-consuming inputs will reduce the conversion efficiency of switchgrass into liquid fuel.
occurs routinely in some reduced-tillage systems when
• weed control
high-residue-yielding crops are grown or when killed
• less extreme changes in soil temperature
cover crops remain on the surface. In some small-scale
• reduced splashing of soil onto leaves and fruits
vegetable and berry farming, mulching is done by apply-
and vegetables (making them look better as well as
ing straw from off site. Strawberries grown in the colder,
reducing diseases)
northern parts of the country are routinely mulched
• reduced infestations of certain pests (Colorado po-
with straw for protection from winter heaving. The straw
tato beetles on potatoes and tomatoes are less severe
is blown on in late fall and is then moved into the inter-
when these crops are grown in a mulch system)
rows in the spring, providing a surface mulch during the
On the other hand, residue mulches in cold climates
growing season.
can delay soil warming in the spring, reduce early-season
Mulching has numerous benefits, including:
growth, and increase problems with slugs during wet
• enhanced water availability to crops due to better infil-
periods. When it is important to get a rotation crop in
tration into the soil and less evaporation from the soil
early, you might consider using a low-residue crop like
(approximately 1/3 of water loss in dryland irrigated
soybeans the previous year. Of course, one of the reasons
agriculture is from evaporation from the soil, which
for the use of plastic mulches (clear and black) for crops
can be greatly reduced by using a surface mulch)
like tomatoes and melons is to help warm the soil.
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Building SoilS for Better CropS: SuStainaBle Soil ManageMent