Each sample pedon was analyzed and described separately using standard soil survey methods US Soil Survey Staff ; Schoeneberger et al.
Horizon type, horizon depth, rock fragment content, structure, texture by feel, consistence, color, effervescence, presence of clay films, roots, pores, bulk density, pH and total soil C and N were determined for each of the horizons. Location and number of sites sampled per county across landform regions [original map from Landforms of Iowa Prior ]. The bulk of European-style, large-scale, row-crop agriculture in Iowa began between and Thompson Thus, most of the area has been farmed for more than yr.
Currently, land use in this region continues to be primarily agricultural, with corn Zea mays and soybean Glycine max as the major crops and alfalfa Medicago sativa oats Avena sativa and wheat Triticum aestivum as minor crops. The original soil survey data recorded land use at the time of sampling, and we recorded land use at the time of our sampling.
In order to determine land use change for the interim, we reviewed historical aerial photos, from s, s, s, s, s, s and s available on the Iowa Geographic Map Server IGMS Of the 82 sampling sites, 79 of the pedons were in some sort of row crop production for part or all of the time between the original sampling and current sampling date.
The remaining three pedons were forested during this time period. Using the original and current descriptions and laboratory data, each of the 82 pedons was classified using the current Canadian, US and FAO-WRB soil taxonomic systems to determine changes in soil classification after 50 yr of agricultural land use. In order to classify our sampled soils using the Canadian system, we made the following assumptions.
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An example family name is: Fine-silty, mixed, mesic, superactive Aquic Hapludoll , where Mollisol is the Order signified by the —oll suffix , Udoll is the suborder, Hapludoll is the Great Group and Aquic Hapludoll is the subgroup. We identified four major features as key in classifiying our pedons: mollic colors, redoximorphic features, depth to carbonates and the presence or absence of a clay-enriched B horizon. Mollic horizon thickness is the major diagnostic feature defining Black soils at the Order or RSG level.
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In the US system, depth to redox features is also used to make distinctions at the suborder level i. Depths to effervescence and secondary carbonates also are also used as distinguishing features for pedon classification in the Canadian and FAO-WRB system. In the FAO-WRB, the presence of secondary carbonates within 50 cm of the base of the mollic horizon is the key diagnostic feature that differentiates between Chernozems and Phaeozems.
The presence or absence of secondary carbonates within cm of the soil surface also determines whether or not the Pisocalcic prefix is used. In Canadian soil taxonomy, depth to effervescence determines whether or not the Calcareous subgroup is used. US Soil Taxonomy does not use depth to effervescence or carbonates to differentiate between the categories in which the sampled soils are classified.
The presence or absence of a clay-enriched B horizon, Bt, argillic or argic horizon was also an important factor in classifying the sampled soils. Additional Black soil classification criteria include carbon content and base saturation Table 1. The majority of the 82 sites sampled were originally classified as Black soils in each of the taxonomic systems Table 2.
The remaining pedons were classified as soils with subsurface clay accumulation Luvisols or Alfisols , minimal B horizon development Brunisols, Inceptisols or Cambisols or evidence of reducing conditions within 50 cm of the soil surface Gleysols, Canadian System. Table 2. Table 3. Table 4. Kimble et al.
Twenty-seven to seventy-one percent of the thickness of the original mollic epipedon had been lost Kimble et al. This result would be expected because they intentionally sampled moderate and severe erosion classes, where the likelihood of substantial erosional effects on mollic thickness is great.
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In contrast, by using pedons and locations that the National Cooperative Soil Survey defines as representative pedons, we sampled the sites that are assumed to be relatively stable, found on lower slopes than average and not subject to significant erosion and classification change US Soil Survey Staff Two of the three forested pedons did not change classification over the sampling period. The third forested site changed classification in the US and FAO classification systems, but did not change in the Canadian classification system it remained a Dark Grey Luvisol. In the US system, it was originally classified as a Fine-silty mixed, mesic Typic Hapludoll, and now it is classified as a Fine-silty mixed, mesic Typic Eutrudept.
These changes in classification were the result of the loss of 10 cm of mollic-colored soil. As opposed to the other two forested sites, this site was regularly grazed with cattle and located in a less stable, shoulder landscape position. Changes in the thickness of mollic horizons, depth to redoximorphic features and depth to carbonates were responsible for the majority of changes in all three classification systems. Table 5 shows the most common examples of classification changes based on those three features.
Four example pedons with their relevant data as well as the complete classification for each of the three taxonomic systems are shown in Table 6. Table 5.
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Table 6. Example pedon data and resulting classification. Using Canadian Soil Taxonomy, classification changes at the Order level were because of a deepening of redoximorphic features four pedons , b changes in thickness of mollic colored soil three pedons and c decreases in base saturation five pedons.
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These sites represent a third of the sites for which we have base saturation data. Therefore, we hypothesize that more sites likely have changed classification based on base saturation.
Ninety percent of the changes at the Great Group level were a result of subtle changes in surface soil color. Changes in Subgroup were a result of changes in depths to redox features or presence or absence of carbonates Table 5.
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Decreases in pH and decreases in total carbon also changed the taxonomy in two pedons Table 7. Table 7. Additional changes in classification specific to Canadian soil taxonomy. More than half of the changes in classification using US Soil Taxonomy were associated with changes in the thickness of mollic colors 33 pedons. Changes in soil order were exclusively related to gains or losses of mollic colored soil. Changes in the depth to redoximorphic features were responsible for the taxonomic changes at the Suborder level. While the changes at the Great Group level were related to the presence or absence of an argillic horizon, the majority of Subgroup changes were related to thickness of mollic epipedon and depth to redox features Table 5.
In the FAO-WRB taxonomic system, about half the classification changes at the RSG level were related to a change in the thickness of the mollic horizon 16 pedons , and the other half were related to changes in the depth to secondary carbonates 14 pedons. These changes are likely related to agriculturally accelerated erosion and deposition as well as enhanced microbial respiration associated with tillage and cultivation Veenstra On the surface, the three taxonomic systems appear to be very different, but with the soils we sampled, classification was predominately based on thickness of mollic colors, depth to redoximorphic features, depth to carbonates and presence or absence of a clay-enriched B horizon.
Each system focuses more heavily on one or two features, specifically, mollic epipedon thickness in the US system, depth to secondary carbonates and mollic horizon thickness in the FAO-WRB, and surface soil color and depth to redoximorphic features in the Canadian system. Despite the fact that each system emphasizes slightly different properties, all of the taxonomies seem to be equally unstable as the soil changes with agricultural land use. These changes are likely to be the result of many factors including erosion, tillage, fertilization, tile drainage, and other agricultural practices.
With the extent of these changes and the extent of agricultural land use, this study indicates that existing soil maps increasingly may not represent characteristics of the soil today, nor what conditions will be like in the future. This causes us to conclude that the veracity of soil maps diminishes over time. We advocate the development and implementation of a dynamic soil properties database that includes all properties central to soil classification and land use.
Otherwise, scientists, policy makers and the public will likely continue to assume that soil change is a minor issue that only affects one or two properties of any given soil.
And ultimately, these changes in soil classification may indicate changes in productivity of the soils as well. Advanced Search. Ministry of Forests and B.
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Ministry of Environment, Lands and Parks. Terrain and Soils Standards, Manuals and Guidelines. Field manual for describing soils in Ontario 4th ed. Denholm K. A and L. Schut Land Resource Science, University of Guelph. Field manual for describing soils. Schoeneberger, P. We wish to express our gratitude to Bill McInturff, State of Maryland, Department of Natural Resources, for his help in securing site access as well as providing site history information.
H Dost , N van Breeman pp. Advances in Geoecology No. Agronomy Monograph No. Book Series No. Part 1. A Klute pp. Section 4. WC Danke pp.
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