PNW CONSERVATION
TILLAGE HANDBOOK SERIES
Chapter l -Erosion Impacts, No.6, Spring 1987
Erosion-Productivity
Relationships
Roger
Veseth
Loss of topsoil from
cropland alters nutrient supply and distribution, water storage potential,
physical properties of the seedbed and root environment. Growers are well
aware of the reduced crop yields where all of the topsoil has been eroded,
exposing less productive clayey or calcareous subsoils. Yields from these
eroded areas are often less than one-half or one-third of the yields where
most of the original topsoil still remains. The "clay knobs"
of the Palouse region and the' 'white soils" of southern Idaho or
eastern Oregon and Washington are stark reminders of lost soil productivity
due to erosion.
The gradual year-to-year impact of erosion on yield potential often goes
unnoticed because of the annual variations in weather patterns, crop diseases
and other factors. The yield impact of the loss of 1 inch of topsoil is
also relatively small early in the erosion process, but the impact increases
with decreasing topsoil depth. After 30 years or more of farming, growers
are faced with major reductions in yield potential from topsoil loss,
often measured in feet instead of inches.
Research to Quantify Yield Losses
To help quantify the impact of topsoil loss on wheat yield, Maynard
Fosberg, University of Idaho soil scientist and STEEP researcher, initiated
an intensive field study in 1982. The research was accomplished with assistance
from research associates Mary Bramble-Brodahl and Anita Falen, and research
technician Dave Mital.
Field studies were conducted in the northern Idaho Palouse region and
in southeastern Idaho. Study sites were located in fields of cooperating
farmers who used similar, conventional management practices, Several different
soil series were sampled on each farm. Within a series, 10 to 15 sites
were sampled where topsoil depth evenly spanned the range of depths expected
for each series, ranging from severely eroded to uneroded. Soil profiles
were described and sampled for laboratory analysis. Wheat yields were
determined from three hand-harvested plots each 1 foot square at each
site. From the soil and yield data, mathematical models were developed
to estimate yield loss with declining topsoil depth.
Palouse Results
The Palouse study sites were located in Latah County, south and
east of Moscow, in a 22-inch annual precipitation area. The fields had
been in a crop rotation of winter wheat-spring peas.
The researchers found that the presence of a subsoil horizon, that had
physical or chemical properties which sharply contrasted with and were
less productive than the overlying surface soils, had a significant influence
on topsoil depth-yield relationships. This was true both for soils in
the Palouse as well as in southeastern Idaho. Research results from Naff
and Palouse soil series in northern Idaho help show the importance of
subsoil properties. The two soils were extensively sampled for 4 years,
from 1982 through 1985.
The Naff soil typically has a relatively shallow, silt loam topsoil over
a heavier, silty clay loam subsoil (argillic horizon). Compared to the
topsoil, the clayey subsoil reduces water infiltration, slows root growth,
and generally creates a less desirable seedbed. In contrast, the Palouse
soil does not have a significant textural change between the topsoil and
subsoil. The subsoil of the Palouse series has acceptable physical properties
for seedbed preparation and root growth.
The average topsoil depth beyond which yields did not increase was called
the "influential soil depth, " From 1982 through 1985, it averaged
27 inches for the Naff soil, but only 18 inches for the Palouse. Calculated
average change in yield per inch of topsoil in this' 'influential soil
depth" was 2 bushels/acre for the Naff soil and 1.6 bushels/acre
for the Palouse soil. Average total yield change over the "influential
soil depth" was 46 and 27 bushels/acre for the Naff and Palouse soils,
respectively, This indicates that, with similar levels of topsoil erosion,
a higher percent yield reduction would occur on Naff soils than on Palouse
soils.
Soil properties most strongly correlated with yield reductions and declining
topsoil depth were organic matter content and bulk density at the 12-inch
depth. Soil water content was also an important factor in the yield-topsoil
depth relationship in the dry 1985 crop year. Eroded soils typically had
a lower water content than soils with thicker topsoil depths.
Southeastern Idaho
Research on wheat yield-topsoil depth relationships in southeastern
Idaho began in 1985. The study was conducted in a 14 inch annual precipitation
area in southern Bannock County about 40 miles southeast of Pocatello
near Downey. Winter wheat-fallow is the typical crop rotation in this
area. Three soil series were studied: Rexburg, Ririe and Watercanyon.
All three silt loam soils have calcic horizons (layer of calcium carbonate
accumulation/ cementation) in the subsoil and they differ primarily in
topsoil thickness. In 1985, average yields typically increased with increasing
topsoil depth (Table 1).
The yield-topsoil depth model estimated a 1.1 bushels/acre average yield
loss for each inch loss of topsoil when Watercanyon soils were excluded.
No soil property consistently correlated with wheat yields. However, the
level of organic matter, and concentration of phosphorus and several micronutrients
typically decreased with decreasing topsoil depth. In contrast, soil pH
and calcium carbonate (free lime) content increased with decreasing topsoil
depth.
Table 1. Mean and range values for topsoil depth and yield of hard
red winter wheat In 1985 at Downey, ID (Foaberg, Ul).
| Soil series | Topsoil
Depth1 Mean -Range |
Wheat
Yield Mean -Range |
||
| Rexburg | 21 |
12 - 32 |
57 |
44-74 |
| Ririe | 10 |
0-16 |
42 |
31-49 |
| Watercanyon | 0 |
02 |
37 |
19-55 |
1 Measured by depth to effervescence with 1N HCl (hydrocloric acid)
2 All
sample sites effervesced (had free lime) at the surface: no topsoil remained.
Current and Future
Research
Wheat yield and soil data were collected in 1986 at 146 sites on 10
farms in the Palouse area of northern Idaho. The data collected are currently
being analyzed. Sampling was conducted on the Naff, Palouse, Thatuna,
Tilma, Santa, Taney, Driscoll and Larkin soil series. Five years of data
have been collected on Naff and Palouse series, and 4 years of data on
the Thatuna series. The latter five soil series now have 1 to 3 years
of data in the study. In southern Idaho in 1986, wheat yield and soil
data were collected on 28 sites of the Rexburg and Ririe soil series in
the second year of research there.
In 1987, additional data will be collected to refine and verify the wheat
yield-topsoil depth relationships for all the previously sampled soil
series. The next step will be to analyze and compare the cost of erosion
between the soil series.
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