Soil Physical Conditions in Conservation Tillage Systems
Kent A. McVay
Kansas State University--Research and Extension
The key to successful production agriculture in Kansas is efficient use of precipitation.
Rainfall distribution across the state ranges from 44 to 102 cm y-1. Potential evaporation
can be three times the amount of precipitation in the western (drier) part of the state so
storing precipitation in the soil is a primary objective. Conservation tillage practices that
maintain residue at the soil surface are needed to maintain infiltration rates at high levels
so that when precipitation arrives, water is stored in the soil profile instead of resulting in
runoff.
During the 1930’s, Kansas along with many other states in the Great Plains experienced
many severe wind erosion events. Crop failures were common, and many producers
were forced out of farming. History has labeled this decade the Dust Bowl due to the
frequency of dust storms and large number of farm failures. Long-term rainfall data,
coupled with knowledge of production practices at the time gives insight into why these
kinds of events occur infrequently today. Crop production in the 1930’s consisted of
tillage practices that left little to no residue on the soil surface. The majority of the land
was in a wheat-fallow system, with summer crops grown on wide row spacing, low
populations, and varieties that had limited yield potential. In addition, soil cultivation
several times over the growing season was a common practice. We no longer see dust
storms of the magnitude of the 1930’s. The difference is due primarily to changes in soil
conservation practices and cropping systems rather than changes in rainfall patterns
[Fgure 1]. Even during the dust bowl period, the rainfall distribution pattern in Kansas
favors the production of summer crops [Figure 2]. The majority of precipitation arrives
during the summer season. Using the moisture when it arrives, rather than attempting to
store it for subsequent winter crops is more efficient for crop production.
Residue serves to conserve rainfall by absorbing the kinetic energy of the raindrop, and
maintaining infiltration rates at a high level. Many studies [Figure 3] have shown that
when the soil surface is covered with crop residue, soil infiltration rates remain high over
the duration of a rainfall event. Other studies in the region have shown the yield
advantage of not removing residue from fields. A long-term tillage by nitrogen rate, by
cover crop study at Lincoln, Nebraska (Power et al. 1998), placed on top of a previous
residue study documented the benefit of maintaining residue in the production system.
Corn yields did not respond to tillage and cover crop treatments, but did respond to
nitrogen additions [Table 1]. The most interesting result of this study was not the
response to treatments but the response to differential residue additions that had been the
treatments of the study conducted 10 years earlier. Even after 10 years, corn yields were
positively influenced by the greater amounts of residue that had been added to plots
[Figure 4]. They attributed yield responses primarily to higher levels of organic matter
and the potential mineralization of nitrogen from a larger organic matter pool. Other
differences such as improved soil physical properties and enhanced microbial activity
contributed to these yield differences. These results indicate how important maintaining
crop residue can be in the long-term productivity of soils.
Soil physical properties can slowly change with tillage intensity, and documenting those
changes is a difficult task. When comparing no-tillage to conventional tillage systems,
changes in aggregate stability can occur. Beare et al. (1994) showed that the larger size
aggregate fractions in no-tillage soils were more stable than those from a disk tillage
system [Figure 5]. In incubation studies, more carbon was released when these
aggregates were crushed prior to incubation [Figure 6]. This difference in CO2 evolution
was greater in the no-tillage soils than in the disked soils, indicating that a certain fraction
of carbon was being protected, or was less available to microbial degredation in the notillage soils. Greater total evolution of CO2 in the no-tillage soils indicated a greater
amount of carbon was retained as tillage intensity was reduced. This protected fraction
of carbon likely improved the stability of the aggregates.
Another change that occurs as tillage is removed from the crop production system is the
development and retention of macropores. One fundamental concept must be kept in
mind when considering macropores. Poiseulle’s law describes flow in a cylindrical tube,
and is applicable to macropores that are continuous to the surface. The amount of flow is
a function, among other variables, to the radius raised to the 4th power. Therefore, a
doubling of the radius results in a 16 fold increase in the volume of water that a pore can
conduct. In a no-tillage environment, a large source of macropores is last year’s root
channels. As they decompose, and are left intact and connected to the soil surface, they
become conduits for transporting water into the soil.
Excess tillage can lead to lower levels of organic matter, poorer soil structure, and
potential compaction of surface soils. Compaction does not occur under natural
conditions, but is caused by excess mechanical force. In Kansas, the majority of
compaction occurs due to excess axle loads in harvesting equipment and grain carts. A
secondary cause of compaction is tillage practices such as blading and disking that never
vary in depth, which over time creates a tillage pan at the depth of tillage. Amelioration
of compaction can be accomplished by mechanical means, such as chisels or plows.
There are basically two types. One has a set of disks that work ahead and behind the
chisel points. Residue on the surface is typically reduced by 50% in one pass of this
operation. Another option is a modified plow that lifts and drops the soil, shattering the
compacted layer. Both types of implements are effective in removing the compacted
layer, but the modified plow does not turn the soil, so surface residue amounts are not
reduced. A second option in addressing compacted soil is to rotate cropping systems to
include a taproot type crop. In Kansas, rotating to sunflowers or soybeans can be as
effective as using a chisel or plow to reduce soil compaction. The long term benefits of
using a crop that has a taproot in the cropping sequence is the building of soil structure,
establishing large continuous root channels that are connected to the soil surface.
In the semi-arid environment of Kansas, water conservation is the key to productivity.
Making efficient use of the rainfall received is critical. The fallow system of crop
production was not efficient for water use. A more efficient system is to use the
precipitation as it is received, therefore acreage of summer crops such as corn, grain
sorghum, soybeans, and sunflowers are increasing in Kansas. A critical component of
growing these crops is conserving and storing the precipitation received. Soil
management such as reduced tillage, and no-tillage results in greater amounts of residue
at the soil surface. Residue is needed to maintain high infiltration rates, reduce
evaporation potential, and build soil structure. Intensifying production by increasing
summer crops in the rotation in a no-tillage system is helping Kansas producers make the
most efficient use of the limited precipitation the region receives.