Aggregate Sizes and Stability in Cultivated South Dakota Prairie Ustolls and Usterts

structure of air-dry soils, pedality refers to dry aggregates. Dry aggregation is an important phase of structure Soil structural stability often decreases as the intensity of cultivation genesis in arid and semiarid soils, where it is transient, increases. The effect of three different management systems (grass, no-till, and till) on soil aggregate stability and sizes were studied in formed only periodically at the surface. Dry aggregation six Ustolls and two Usterts on central South Dakota farms. Soil strucdetermines the resistance of the soil to wind erosion ture was morphologically described throughout the profile. Stability since aggregates and particles 1 mm are less susceptiof dry and wet aggregates in the topsoil was tested by dry and wet ble to wind transport. Dry stability measures the strength sieving. Most structural changes were observed in the top 0 to 0.20 m. of aggregates subjected to fracture and abrasion. Slight Granular structure was dominant under grass, whereas plates, blocks, differences in structural stability can be detected by dry and compacted layers were most common in conventionally tilled sieving (repeated if necessary). Dry sieving has shown and no-till soils. The largest mean weight diameters (MWD) of dry decreased macroaggregate stability after tillage (Chepil, aggregates were found in no-till soils (10 mm vs. 7 in till and 6 in 1962; Degens et al., 1996). grass). Wet aggregate stability was higher in grass (87%) than in culOn the other hand, measurements of stability to water tivated soils (70%). After about 10 yr of no-till management, no-till soil aggregates were significantly more stable (5% for wet and 32% are generally used to estimate structural changes due to for dry aggregates) than till aggregates only in the top 0 to 0.05 m. cultivation, because water is the main agent of aggregate The structural stability of cultivated soils was greater in Usterts than breakdown in agricultural soils. At the macroaggregate in Ustolls. scale, wet sieving traditionally provides a measure of size and stability of aggregates produced by the scouring action of water. Results of wet sieving tests may or may T illage operations tend to break down aggregates not include the effect of initial slaking depending on and reduce soil structural stability. Crusts and cominitial soil moisture content. Rapid immersion of airpacted layers are common in cropped soils. Blocky and dry aggregates may cause slaking, which may give a platy structures replace the granular structure of the finer separation of water stability in soils with different prairie when soil with reduced stability is subjected to cropping histories (Haynes, 1993). the external forces associated with modern farming This study evaluated the effects of management pracpractices such as high axle loads (Wiermann et al., 1999). tices on soil pedality, aggregate size, and aggregate staNo-till systems may modify soil structure toward the bility of croplands when compared with grasslands in original structure of the prairie by eliminating tillage central South Dakota. In central South Dakota, conver(Kladivko et al., 1986; Arshad et al., 1999). Evidence sion from grassland to cropland occurred during the of changes in soil structure was observed within 5 to 10 latter part of the nineteenth century and early part of yr of conversion from till to no-till (VandenBygaart et the twentieth century. Intensive tillage operations for al., 1999a). seedbed preparation have occurred annually on most Soil structure can be characterized by a variety of of this land. During the last 20 yr many producers have methods (Dexter, 1988), and different methods have started to use no-till systems (USDA-NRCS, 2003). In indicated changes in soil structure due to agricultural both till and no-till fields, dominant crops are annual management practices (Coughlan et al., 1991; Boersma species with reduced root development when compared and Kooistra, 1994). The description of some visible with perennial grasses. Annual crops may limit the pofeatures is a rapid approach that can be used for an tential for no-till to improve soil structure, especially initial global evaluation of structure. Pedality describes when noncontrolled traffic and large equipment are the structure as strength, size, shape, and arrangement used, as is common in central South Dakota. We applied of peds (Brewer, 1964). A pedality index based on scores different methods for showing changes in soil structure for size, grade, and type of structure in relation to some in fields under different management practices testing measurable property was proposed by Peerlkamp (1959). the following hypotheses: More recently, Bouma and Anderson (1973), Bouma, (1992), and Lin et al. (1999) have developed this concept 1. The pedality of grasslands is greater than the pedwith special emphasis to hydraulic properties. ality of croplands because of prevalent strong, fine, In the case of the morphological description of the granular peds; 2. The pedality of no-till soils is greater than the pedality of tilled soils due to reduced disturbance by A. Eynard, T.E. Schumacher, and D.D. Malo, 247A NPB, South Dakota State Univ., Box 2410C, Brookings, SD 57007-2141; M.J. cultivation; Lindstrom, USDA-ARS, 803 Iowa Ave., Morris, MN 56267. Received 3. Dry and wet aggregate stabilities are greater in 18 Apr. 2003. *Corresponding author (thomas_schumacher@sdstate. grasslands than in croplands and greater in no-till edu). than in tilled soils. Published in Soil Sci. Soc. Am. J. 68:1360–1365 (2004).  Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: MWD, mean weight diameter.