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Roger Weller, geology instructor

by Nurmi Ingram
Physical Geology
Fall 2010

                                                  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 west.

          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.


Figure 1

          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 materials.

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.

Particle Size. 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 .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 2.


Soil Survey.  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/figure2 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.


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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, or CSR 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.

Land Values. 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 property values.

 Figure 4. Below is another soil map. The presentation here was provided by Surety Customized Online Mapping of AgriData Inc. 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 Conservation Service.

         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 acre respectively.

          The variability of the soils is a result of the glaciation. Much of this property is 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 a road 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.


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         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.

Gillett Grove. 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.

Making “Rope”. 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.




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Figure 6                                                                Figure 7                                                                           Figure 8



List of Figures:

Figure 1.

Figure 2.

Figure 3. Soil Survey of Buena Vista County, Iowa, 1977.

Figures 4. and 5. Surety Customized Online Mapping of AgriData Inc.

Figures 6, 7, and 8. Soil samples acquired in Buena Vista and Clay County Iowa, July, 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