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Human-induced Soil Change in Iowa: Two Contrasting Examples

By: C. Lee Burras, Professor of Agronomy, Iowa State University, Ames, IA, USA . Yury Chendev, Professor of Geography, Belgorod State University, Belgorod, Russia. Mostafa Ibrahim, Assistant Professor of Soil Science, Zagazig University, Zagazig, Egypt. Beth Larabee, Former Graduate Assistant in Agronomy, Iowa State University. Tom Sauer, Research Soil Scientist, USDA- ARS National Laboratory for Agriculture & the Environment 

Getting Into Soil & Water 2013   

The crops of Iowa yielded 100’s of trillion of calories in 2012. Those calories are feeding 100’s of millions of chickens, pigs, cattle and other livestock. They also help fuel millions of vehicles. So directly and indirectly they are feeding people and fueling our economy. Credit for this amazing production begins with farmers and extends through the entire agricultural sector. But part of the credit has to be given to the very land being farmed. Simply put, Iowa has amazing soils. Their natural productivity is the envy of the world. Their favorable response to farming is exceptional. Iowa is globally recognized as the place where agriculture thrives because of its soils. The recognition began in the mid-1800’s. That recognition will likely continue for another 150 years… or even longer. However, it is wrong to think Iowa’s soils are so naturally resilient that nothing changes them.  

Agriculture has, is and will continue to cause changes in our soils. That change will likely be proportional to the yields we get from our soils. To think otherwise would require ignoring the basic tenets of ecology and thermodynamics. Over 90% of the diverse prairies, forests and wetlands of Iowa have been replaced by near monocultures of row crops and forages. 

At Figure 1. A typical fencerow separating two typical fields. Note the two feet change in elevation from one field to the next even on this “flat” landscape.  
Figure 2: The new norm for the fields

At the very least this means the type, amount, and timing of humus additions and losses to soil has changed. Le Chatelier’s Principle tells us this must result in a new equilibrium for the soil. Confounding equilibrium are the other changes associated with soil use. Farming, especially tillage, has unintentionally resulted in 20-plus billion tons of soil erosion since statehood in 1846. Evidence of erosion is observable even when driving across flat landscapes (Figure 1). Another driver of soil change is land drainage. Iowa has roughly a million miles of tile lines and thousands of miles of drainage ditch- es (Figure 2). Their sole purpose is to improve the water and oxygen balance for cropping on our 10 million acres of poorly drained soils. But perhaps the least examined change is sim- ply how the soil profile has evolved due to the inputs added to insure a continual improvement in yields. Over Iowa’s history, those inputs sum to hundreds of tons per acre and billions of tons statewide.

This paper discusses two studies that examined soil change in Iowa resulting from normal agriculture uses. Both have been briefly described in earlier editions of this publication. The first study looks at the impact of up to 30 years of tile drainage and cropping on a “peat.” The sec- ond looks at the impact of 150 years of cropping on a soil that naturally formed in loess under forest. These studies were selected because their soils are very different and their response to farming has been very different. Organic soils are amazingly productive when drained, so they are almost always drained. Traditionally they are most valued for truck crops – e.g The aptly named town, Celeryville, is located in the middle of Ohio’s organic soil and truck cropping region. Organic soils develop in shallow lakes, ponds and other stillwater environments. 

These are places where cattails, sedges and other hydrophytes thrive during the growing season and then fall over and sink underwater during winter. Those submerged residues slowly and incompletely decompose. Over thousands of years the cycle of rapid growth followed by partial anaerobic decomposition produces peats that can be 10’s of feet thick. 

Figure 3. Comparison of original and final O-horizon thickness (x-axis in inches) in 14 representative Palms pedons from around north-central Iowa. Y-axis shows the county where sampling occurred followed by the number of years, in parentheses, between original and final descriptions. Data is from Larabee (2004). 

Peats and other organic soils are some of the easiest soils for humans to degrade. They have very, very low bulk densities so when drained they are incredibly sus- ceptible to wind erosion. “Black snow” in fields and ditches is a common indi- cator of a nearby wind eroded organic soil. Drainage also causes rapid subsid- ence since organic matter is buoyant in water but collapses without water. Final- ly, drainage exposes the organic matter to oxygen and aerobic decomposition, which is tens to hundreds of times faster than anaerobic decomposition. This re- leases a flush of nutrients, which the microorganisms that are doing the decomposing use to speed up decomposition.

For these reasons organic soils or, more technically, Histosols are used in soil science as an indicator for human impacts. Iowa has about 100,000 acres of organic soils around one-half of that area being the Palms series (Miller, 2010). Or at least the soil maps of Iowa show about 100,000 acres of organic soils. The amount that really exists is undoubtedly less given Beth Larabee’s (2004) findings. Larabee’s research goal was to document what if any changes had occurred in the Palms soils due to farming during the late 20th century. She did so by describing and analyzing soils collected from the exact location where NRCS had identified 15 representative pedons of Palms. Her research took her across north-central Iowa. She was most interested in the thickness and properties of the O-horizon, which is roughly synonymous with “peat” thickness. Larabee found the “average” Palms experienced about 1 inch of loss per year over the 30-year period she examined. This is substantial loss for any soil and especially for Histosols. Figure 3 shows side-by-side comparisons of the thickness of the peat (O horizons) in NRCS representative pedons and Larabee’s. Figure 4 illustrates that Palms degradation was inversely proportional to the length of time between sampling by NRCS and Larabee. All told Larabee’s research indicates the Palms is on a path of extinction with all vestiges of O-horizons being eliminated in the next 25 years. In some cases – e.g., Emmet County, the entire representative pedon is probably already gone. 

Figure 4. Rate of O-horizon loss as a percent (y-axis) of original O-horizon thickness for 14 representative Palms pedons from north-central Iowa (data from Larabee, 2004). The x-axis shows the number of years elapsed be- tween original and final soil descriptions. 

At the opposite end of organic soils are well-drained upland soils formed from loess. These soils are ideal for cropping and are highly resistant to change – provided erosion is managed. In all likelihood Iowa’s farming reputation was established based on the ease at which pioneer farmers obtained high yields on the state’s many loess-derived soils. They readily infiltrate and drain water yet retain amazing amounts of plant available moisture for the growing season. They generally have high weatherable mineral contents, which translates into high native fertility. Their textures, bulk den- sity, structure all facilitate robust root growth. And most loess soils have well developed A horizons with high organic matter contents. As a result studying change in them is probably representative of the norm for Iowa pioneer farmers obtained high yields on the state’s many loess derived soils. They readily infiltrate and drain water yet retain amazing amounts of plant available moisture for the growing season. They generally have high weatherable mineral contents, which translates into high native fertility. Their textures, bulk density, structure all facilitate robust root growth. And most loess soils have well developed A horizons with high organic matter con- tents. As a result studying change in them is probably representative of the norm for Iowa. 

This is exactly what Professor Yury Chendev did. In 2008-2009 he completed a detailed analyses of change resulting from long-term cropping on the Fayette soil at four sites in Johnson County. His research, which was part of his Fulbright Visiting Scholarship, was an exten- sion of comparable work he conducts in western Russia.

Figure 5. A Fayette profile that has never been cropped (left) and one that has been cropped for 150 years (right). 

Chendev selected the Fayette soil and Johnson County for several reasons. First, it is a common, highly pro- ductive loess-derived soil, covering over a million acres of eastern Iowa (Miller et al, 2010). Second, it is almost exclusively cropped, having a B-slope CSR off 85. Third, and why Johnson County was selected, there are fields with Fayette soil that were cleared of forest in order to crop beginning 150 years ago. Contiguous fields were converted from forest to cropping beginning 100 and 50 years ago, respectively. Equally importantly there is a contiguous forested site that has never been cropped. Finally, these sites are unusually flat for the Fayette soil. This means their erosional histories are minimal and the full impact of farming is expressed in the soil profile. 

Chendev found every property of the Fayette profile dif- fered according to duration of cropping (Chendev et al, 2012). Each difference was proportionally to time period of cropping. The A horizon thickened by about 10 inches and there was a corresponding increase in structure quality. The degree of faunal burrowing increased through 60 inches while porosity decreased in the upper 15 inches. Iron and manganese mottling became more pronounced to 60 inches. Perhaps most interesting, the thickened A horizon caused each of the other seven horizons shift deeper even as they maintained their original individual thicknesses. Taken collectively these changes – and other changes not described here- in – mean the Fayette, which is naturally an Alfisol, is becoming a Mollisol. In other words, crop- ping created a soil that is as akin to a prairie-derived soil as it is to a forest-derived one. 

Combining the research by Lara- bee and Chendev indicates that long-term cropping is definitely changing Iowa’s soils. It is too sim- ple to think all of those changes are degrading the soil. Each field has its own history both natural and agriculturally. As a result knowing a field’s history and its boundaries is becoming as important to soil science as understanding parent material, native vegetation, land- scape position, and drainage class. This finding is simply a confirmation of basic scientific tenets but it has profound ramifi- cations relative to how people see the soils of Iowa. 

References 

Chendev, Y.G., C.L. Burras and T.J. Sauer. 2012. Transformation of forest soils in Iowa (United States) under the impact of long-term agricultural development. Eurasian Soil Sci. 45:357-367. 

Larabee, B.E. 2005. Evolution of the Palms (Terric Haplosaprist) soil in Iowa, 1969-2001. Unpubl. MS thesis. 141 p. 

Miller, G.A., T.E. Fenton, B.R. Oneal, B.J. Tiffany and C.L. Burras. 2010. Iowa soil properties and in- terpretations database. ISPAID Version 7.3. Iowa State University – IA. Ag. H. Ec. Exp. Station. 30 p. 

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