The Rural Voice, 2000-06, Page 18a
A *tJPUa.
protect base flow during the summer
and fall periods.
The key challenge is to design
farms so that water storage and
movement are not unduly
compromised by the combination of
extreme heat, drought and variable
rainfall patterns. Many farms have
significant periods of the year with
minimal soil cover, inadequate soil
organic matter levels and
compaction, and drainage systems
that are more likely to take away
excess water (e.g., tile drains) rather
than conserve it. All these conditions
are likely to lead to reduced capacity
to adapt to climate variability.
There are a spectrum of
cropping system strategies
farmers can employ to increase
their resilience and flexibility in the
fact. of climate variability. At the
more resilient end of the spectrum
are complex crop rotations
characterized by significant use of
green manures, intercrops and
legumes and reduced tillage, deep
and extensive root masses, and high
soil organic matter levels, and good
soil tilth.
Studies from the U.S. mid -west,
examining corn, soybean, wheat
systems reveal that longer rotations
involving legumes leave farms better
14 THE RURAL VOICE
able to withstand drought. One
series of studies from the University
of Nebraska showed that the longer
rotations reduced the risks of
suffering through a bad year, and less
variable net returns. These longer
rotation systems have performed
consistently as well or better than
short corn -soybean rotations.
What accounts for this ability of
longer rotations to withstand uneven
weather? It appears to be some
combination of root development and
soil tilth. Soil organic matter content
is a critical feature. It is not
necessarily so that organic matter has
a direct impact on moisture retention,
but rather that it changes soil tilth.
Organic matter, especially in
loamy soils, can improve soil
aggregation. Aggregation creates
more pore space for root movement.
Although the traditional view is that
the kind of organic matter is less.
significant than the quantity, it is
actually the kind that really matters.
The more digested organic matter
fractions appear to be the most
significant for soil tilth and moisture
retention — microbial gums and
mucilages, low molecular weight
fulvic acid molecules, and fats and
waxes, according to R. J. MacRae
and G. R. Mehuys in their 1985
article in "Advances in Soil Science".
Drinkwater et al. in their study
contrasting conventional and
alternative corn - soybean cropping
systems in Pennsylvania, found that
longer rotations involving
leguminous plants did not necessarily
add more total organic matter to the
soil, but because of the lower carbon
to nitrogen ratio additions resulted in
greater organic carbon sequestration
and improved soil physical
properties.
Jt appears that certain kinds of
crop rotations will make greater
contributions to these forms of
organic matter than others. Systems
that leave a lot of high
carbon:nitrogen ratio material (e.g.,
post harvest crop residue) are less
likely to provide the right kind of
organic material for building soil
aggregates than lower C:N ratio (less
mature) material.
Tillage: To be more resilient in the
face of climate variability, tillage
operations must strike a balance
between organic matter
decomposition, weed control and
moisture retention. With steady and
significant additions of organic
matter, tillage is important at key
times for accelerating the creation of
organic matter fractions that promote