CONSERVATION TILLAGE
HANDBOOK SERIES
Chapter 1 - Erosion Impacts, No. 9, Spring 1988
Much of the Northwest
cropland has sustained substantial topsoil loss by water and wind erosion,
and the mechanical movement of soil downslope by tillage. On some Palouse
hilltops, it has been estimated that up to 4 feet or more of topsoil and
underlying subsoil have been lost over the last 100 years of farming.
Considering that it has taken hundreds to thousands of years to form each
inch of productive topsoil, allowing this high rate of erosion to continue
jeopardizes a productive sustainable agriculture for future generations.
There is very little documentation of original topsoil depth and other
soil properties when the land was first plowed near the turn of the century.
Consequently, it is often difficult to accurately determine the decline
in topsoil depth and degradation of other soil properties affecting productivity.
Few remnants of virgin land, typical of the surrounding cropland, exist
for comparison. Where they can be found, these tracts of land offer an
opportunity for potential comparisons with nearby cropland to evaluate
the impacts of farming practices.
One such virgin site of native range near Colton in southeastern Washington
was used for this type of comparative study with a nearby field cultivated
for at least 60 years. The cultivated field had been conventionally farmed,
with the exception of uphill plowing for the past 10 years. The research
was conducted by Ann Rodman, a graduate research assistant in soils at
Washington State University in Pullman, in cooperation with WSU soil scientists
Bruce Frazier and Alan Busacca. It was conducted in 1986 and 1987 as part
of the STEEP conservation research program in the Northwest.
Sampling and Analysis
Soils were described and sampled on transects which crossed a ridge
with north- and south-facing slopes on the native and nearby cultivated
sites. Sample locations were grouped into specific slope positions on
the landscape for comparison (Fig. 1). A 30 percent slope was measured
on the most steeply sloping portions of the ridge on both slope aspects
of the cultivated site and the south aspect of the native site. The north
aspect of the native site had a 60 percent slope, the main reason it was
not under cultivation,
Although a number of soil properties were described and analyzed, data
on two properties will be presented here. These include: (1) percent organic
carbon content of the surface 6 inches and (2) depth to 1 percent organic
carbon. Percent organic matter may be more familiar to most people, however,
percent organic carbon is a more exact measurement for scientific purposes.
Fig. 1. Schematic Illustration of the south-facing slope and slope
positions of the study area on fields D1 and D2 near Dayton, WA (Rodman,
Frazier and Busacca, WSU).
For an approximate conversion of percent organic carbon to percent
organic matter, multiply the organic carbon percentage by 1.7, The depth
to 1 percent organic carbon is an approximation of the thickness of the
organic matter-rich surface layer of soil, commonly referred to as A horizon
(including transition AB horizons). This method removes some of the subjective
variation in visual descriptions of soil horizon boundaries.
Organic Carbon Content
Organic carbon content of the soil, in the form of soil organic matter,
is an important factor in fertility, plant available water-holding capacity,
aeration, aggregation and tilth and other soil properties affecting productivity.
A comparison of the percent organic carbon content of the surface 6 inches
of soil for specific landscape positions on north- and south- facing slopes
on the native and cultivated sites is shown in Fig, 2. No data are available
for the footslope and lower backslope positions on the south facing slope
of the native site because these slope positions were under cultivation.
Percent organic carbon was substantially higher on the native site than
the cultivated site on all comparative slope positions sampled. On the
summit and shoulder positions of the south-facing slope, organic carbon
content averaged 2,6 percent on the native site compared to 0.7 percent
on the cultivated site. On the north-facing slope, comparative figures
were about 3.4 and 1.9 percent on the native and cultivated sites, respectively.
The researchers point out that this reduction in organic carbon content
is due largely to the erosion of organic matter-rich top soil from the
cultivated site. As topsoil depth is reduced, organic carbon content is
further reduced by mixing of underlying B horizon (low organic carbon
content) with the surface soil in the tillage zone. Past crop rotations,
particularly the use of conventional summer fallow, and other farming
practices also probably influenced organic carbon content over time.
Fig. 2. Comparison of percent organic carbon content of the surface
6 inches of soil at specific slope positions on both south- and north-facing
slopes of native and cultivated sites near Colton, WA (Rodman, Frazier
and Busacca, WSU).
Depth to 1% Organic Carbon
A comparison of the depth to 1 percent organic carbon as a measure
of the thickness of the organic matter-rich soil layer (the approximate
thickness of the A horizon) is shown in Fig. 3. Summit, shoulder and upper
backslope positions on the south-facing slope of the cultivated site had
lost all the A horizon. In some cases, the upper portion of the underlying
B horizon was also missing. The percent organic carbon content of the
surface soils at these positions was less than 1 percent, consequently
negative values are shown in Fig. 3.
Fig. 3. Comparison of the depth to 1 percent soil organic carbon content
(approximate thickness of the A horizon) at specific slope positions on
both south- and north-facing slopes of native and cultivated sites near
Colton, WA (Rodman, Frazier and Busacca, WSU).
Based on the depth to 1 percent organic carbon, the summit of the
south-facing slope on the cultivated site has lost an average of 25 inches
of topsoil compared to the native site. This estimate, however, is based
on an average depth to 1 percent organic carbon on the summit of the native
site, which ranged from 11 to 42 inches. The summit and upper slopes of
the cultivated north-facing slope appear to have lost about 11 to 14 inches
of topsoil (about 50%) compared to the native site. On the cultivated
site, the depth to 1 percent organic carbon content is greater on southfacing
lower-backslopes and footslopes, and north-facing footslopes than on upper
slope positions.
The researchers explain that this is a result of soil material which was
eroded from upper slope positions and redeposited on lower slope positions.
This may be more an effect of tillage erosion, since water-eroded material
is often carried to flat bottomland areas and into the drainageways. They
also point out that where subsoils are exposed on the summit and upper
slopes, this downslope transportation process carries soil material which
can cause a significant reduction in production potential on the formerly
highly-productive footslope and lower-backslope positions. This is because
the subsoil material typically provides a less desirable seedbed and plant
growth-medium. It often has a higher clay or calcium carbonate (lime)
content, lower organic matter content, lower fertility level, slower water
infiltration rates, poorer structure and aggregation and other qualities
resulting in lower production potential when compared to the surface A
horizon.
On the cultivated north-facing summit, shoulder and backslope positions,
an abrupt boundary existed between the A horizon and underlying B horizon.
Soils on the native site, in contrast, had a diffuse boundary between
the A and B horizons on these slope positions. The researchers concluded
that the abrupt horizon boundary indicates that sufficient A horizon has
been eroded to allow mixing of the subsoil B horizon in the plow layer.
The upper layer of the B horizon could also have been eroded as the surface
mixture of the A and B horizons was removed.
Conclusion
The 60-plus years of conventional farming has sharply reduced the
organic carbon content of the surface 6 inches of topsoil and topsoil
thickness compared to the native range site. Organic carbon content declined
about 1 to 2 percent on all slope positions. South-facing summits and
upper slopes had lost all of the A horizon topsoil, estimated to be 11
to 42 inches in thickness, and probably some of the underlying B horizon.
Upper north-facing slope positions had lost an estimated 11 to 14 inches
of topsoil. Abrupt boundaries between the A and B horizons indicate that
sufficient A horizon had been lost to allow incorporation of B horizon
material in the tillage layer. Part of the soil lost from upper slope
positions was deposited on lower slopes, as evidenced by thicker A horizons
and higher organic carbon contents.
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