Geology Home Page
Roger Weller, geology instructor
by Nurmi Ingram
Soils on Three Farms in Northwest Iowa
I discuss the soils on three farms in northwest Iowa. One is recently glaciated
and has variable topography and soil types. The other is not recently glaciated, is
level, with uniform soil. The soils have been thoroughly studied and data is available.
The CSR number is a way to judge soil types for crop production and valuation.
Some simple ways to evaluate soil are discussed.
My family owns three parcels of crop land in northwest Iowa owned by the
family. Two are in north Buena Vista County the other is over the county line in Clay
County. They are several miles apart and separated by the Little Sioux River flowing
The BV county farms have rolling surfaces with lots of small hills, slopes, swales
and a great variety of soil types. The topsoil is dark but a little digging reveals lighter
colored material consisting of clay, gravel and rock or some combination. This
topography is said to be the result of glaciation.
In Clay County, however, the land is level with no rocks, sand or gravel, and
more uniform soils. The difference is the Wisconsin Glacier which receded about
10,000 years ago. What is known as the “Des Moines Lobe”, of the glacier, in figure 1,
extended south into the state. Our property is located in the middle of the northwest
quadrant of Iowa, just to the right of the words "Iowa Plains". One farm is in the
un-glaciated Northwest Iowa Plains and the others in what is called the Des Moines
Lobe of the Wisconsin Glacier.
Of geological significance is the fact that during the Wisconsin glaciation the
flow of the Little Sioux was changed from southeast toward the Mississippi River
to Southwest toward the Missouri River following for a few miles the county line.
Soil. Soil is finely
broken down rock mineral on the Earth’s surface consisting of
sand, silt and clay. It is capable of supporting plants and has properties resulting
from the integrated effect of climate and living matter acting on the parent
Horizons. The “A” horizon, the
zone of leaching, is the top 12 inches or so of soil
and contains soil and humus. The “B” horizon is the zone of accumulation
containing clay, sand, and minerals such as iron oxides, and extends down to 30
inches or more, figure 2. The “C” horizon, regolith, is less decomposed material
such as boulders, cobbles, pebbles and sand. It is deeper but in the glaciated
areas may be at any depth.
Each particle size in soil plays a significant role. The largest particle,
sand, provides drainage and aeration. The tiny, submicroscopic, clay particles
are chemically active, binding with plant nutrients and water. Clay particles are
less than .002mm (80 micro inches) in size. Silt is between sand and clay in size at
0.002mm to .05mm (.002 in.). Sand is .05 mm to 2.0 mm (.08 in.) and consists
mainly of quartz. The ratio of sizes determines soil type, which is, loam, sandy
loam, sandy clay loam and so on. Loam in this context is characterized as a
mixture of 20% clay, 40% silt and 40% sand.
Figure 3. Below is an example of a soil survey map. The numbers
represent soil types on an 80-acre farm. Each of these has specific properties.
I have a hard copy of the Buena Vista County, Iowa soil survey that was
published in 1977. It covers all the soil in the county. It has over 90 pages
describing soil types and 80 pages of maps that look like the one in figure 3/f
igure2 below. Map 5 of the BV County survey contains an 80-acre parcel and
map 12 is a 337-acre parcel. These are the two BV farms mentioned earlier.
Making Soil Surveys. In the past, a soil scientist would take aerial photographs,
topographical maps and other aids into the field to map the soil types directly over
the photographs. More recently they also employ LiDAR, Light Detection and
Ranging, and GPS, Geographical Positioning Satellites, to gather distance and
other information. The scientists observe steepness, length, and shape of slopes;
the kinds of crops and native plants; and the kinds of bedrock. They dig many
holes and take core samples to study the soil profile, the sequence of natural
layers down through the several horizons. All this information is taken to a lab
and studied for soil properties. Some of these properties are particle size, soil
texture, plasticity, permeability in inches per hour, water capacity, reaction in
pH, and so on.
CSR. Given the
information in the soil survey, the corn suitability rating,
can be determined. CSR is an index procedure in Iowa developed to rate each
different kind of soil for its potential row-crop productivity. It is based on corn
because corn is a dominant crop in the state. Soil profile properties and climate
are the main factors that affect productivity. Slope gradient and slope length
affect potential erosion rates, water infiltration, and ease and efficiency of
machine operation. These all effect the CSR rating. CSRs can be used with soil
maps to calculate the weighted average CSR value for any sized parcel.
As one can see, the mapping process takes a fairly complex situation
and reduces it to an understandable number, CSR. This becomes the key factor
in appraising land for buying and selling, for property tax and for securing
financing. Land auction ads list the size of the parcels in acres, the locations, the
percentage that is tillable and the CSR. When a parcel is sold, price per acre and
price in dollars per CSR point become public knowledge, a good tool for following
Figure 4. Below is another soil map. The presentation here was provided by
Surety Customized Online Mapping of AgriData Inc. www.AgriDataInc.com. It is
based on the 1977 soil survey which established the Fsa soil type borders provided
by the Farm Service Agency. Below the map is a tabulation of soil data also provided
by AgriData based on the data provided by the NRCS, Natural Resources
The field in the map is 66 acres of an 80-acre
farm, about ¼ mile by ½ mile.
The numbers on the left designates different soil types. The first one, 138C2, Clarion
loam, 5 to 9 % slope, moderately eroded, is 12.9 acres. It’s the orange color on the
map and is 19.0% of the field. It has a CSR of 60 and a predicted crop yield in corn of
166 bushels per acre. The next one is Nicollet silty clay loam, 1-3% slope. This has a
very high CSR of 84 and a predicted yield in corn of 198 bushels per acre. The chart
continues for fifteen soil types including one intriguing one called “Blue Earth mucky
silt loam”. The aggregate CRS and yield for this parcel is 60.4 and 166.4 bushels per
The variability of the soils is a result of the glaciation. Much of this
not suitable for row crop, because of the presence of the Wadena, Storden and Blue
Earth and ten acres or so of wetland, the uncolored area. This is relatively low quality
property compared to others in the neighborhood and would appraise at around
$3,000 per acre. Some personal experience on this property witnessing the crop
performance of these soils confirms what the soil survey indicates. When the term
“Blue Earth” soil is mentioned I think of very difficult to drain soil. The Storden occurs
on steep slopes and has thin top soil. The Wadena occurs over sand and gravel and
has poor water retention. Corn plants there wither in dry years. They all have relatively
low CSR’s. They total 38.2% of the property. No one in the neighborhood is eager to
purchase this farm.
Farm land in the glaciated area need not necessarily be poor. Just south across
is the 337-acre farm that is the result of the same glacial action. It has an aggregate
CSR of 70 and is almost all tillable. Its value is about $5,000/acre.
Figure 5 above is 160 acres of the 172-acre (1/2 mile by ½ mile) farm in Clay
County. All but 2.5 acres are relatively level, productive soils of three types. They
are 1091 McCreath silty clay loam, 1092 Gillett Grove silty clay loam, and Ransom
silty clay loam. The aggregate CSR on this farm is 80.5 and the aggregate predicted
yield is 193.3 bushels per acre. There is no rock or gravel evident. This is a very high
quality, mostly tillable farm, and would appraise at close to $7,000 per acre.
Below are three photos of a soil samples taken 12 inches deep with
a ½ inch diameter soil sampler. Figure 6 is the Gillett Grove soil from the Clay County
farm very similar to the McCreath. It is described in the Clay County soil survey as
having “formed in silty clay loam loess over glacial till”. In its very dry state it is not
very friable. The middle image is the results of a “thumb crush test”, the writer’s
informal, somewhat subjective test. Some of the sample breaks up but there are still
some larger pieces. However, the addition of a little moisture results in a dark paste,
far right, that is very lubricious with little or no sand present. This soil is the loess, or
wind-blown silt and clay that make up much of the un-glaciated Clay County farm.
A simple test for clay is to roll a ½ inch diameter “rope” in a
dampened sample of the soil as potters would. I tried this with the McCreath without
much success. It acted like it was clay rope until when bent and it cracked and fell
apart. When wet it looks like black mud. This is the most clay-like soil of the five
samples that I have. The McCreath has a little too much silt for pot making but it’s
good for farming.
Nicollet. Figure 7 is Nicollet
soil. It is described in the BV County soil survey as having
“formed in loamy glacial till”. In its dry state it appears quite friable per the thumb
test. This is probably due to larger average particle size, that is, a larger percentage
of silt and sand present. It accepts moisture readily and the smear test confirms the
presence of sand. The lighter color is probably due to there being less organic material
present than in the McCreath.
Okoboji. The last sample,
Figure 8, is Okoboji from the 337-acre BV County farm. It is
described as “formed in silty, moderately fine textured, local alluvium under a native
vegetation of swamp grass and sedges”. The sample area was from what is called a
“land-locked depression” or as we would say, a marsh, of about two acres. It requires
special drainage but with that accomplished, Okoboji is very productive in row crops,
especially in a dry year. It is dark and not very friable when dry, perhaps because of
the high amount of organic matter present. When watered a little, it acts like the
Nicollet absorbing water readily indicating the presence of sand. This is no surprise
as both Nicollet and Okoboji are present in the same field.
Simple Soil Tests.
Some simple soil tests are detailed in Appendix 13 of Guidelines for
the Resort Areas of Kosciuszko National Park. See references. The procedure starts
with a damp sample of soil. If it sticks together it is “sandy loam”. If it doesn’t, it is
sand. If the sample rolls into a ribbon, it is one of several versions of clay, sandy-clay,
clay, clay-loam or sandy-clay-loam. If it doesn’t roll into a ribbon it is loam or
sandy-loam. In all, there are eight classes of soils, all the way from sand to clay. One
must view the site to get all of the details.
Figure 6 Figure 7 Figure 8
List of Figures:
Figure 1. http://www.igsb.uiowa.edu/Pubs/Ragbrai2010/Ragbrai2010.htm
Figure 2. http://extension.agron.iastate.edu/soils/SSDS_maps.html
Figure 3. Soil Survey of Buena Vista County, Iowa, 1977.
Figures 4. and 5. Surety Customized Online Mapping of AgriData Inc. www.AgriDataInc.com
Figures 6, 7, and 8. Soil samples acquired in Buena Vista and Clay County Iowa,
2010. Photographs by writer, ASA 400 in “north light” with close-up lens.
Soil Survey of Buena Vista County, Iowa, 1977. USDA, Soil Conservation Service.
Soil Survey of Clay County, Iowa, 2005, USDA, Soil Conservation Service.
Rehabilitation Guidelines for the Resort Areas of Kosciuszko National Park