Surface Soil Physical Properties After Twelve Years of Dryland No-Till Management

crops every 3 yr) and even continuous (annual) cropping in some instances. For example, annualized grain yields Water is the principle limiting factor in dryland cropping systems. for WCF are 75 to 100% greater than WF (Peterson et Surface soil physical properties influence infiltration and cropping al., 2000). Cropping intensification has been possible systems under no-till management may affect these properties through because no-till practices improve soil water storage effiresidue addition. The objectives of this study were: (i) to determine how cropping intensity and topographic position affect soil bulk denciencies in the early phases of fallow (Farahani et al., sity, porosity, sorptivity, and aggregate stability in the surface 2.5 cm 1998). Since nearly 75% of the annual precipitation in of soils at three eastern Colorado sites; and (ii) to relate these properthis region occurs during April to September, relatively ties to crop residue returned to the soil surface. No-till cropping small net increases in soil water storage can provide the systems had been in place on three slope positions, at three sites, for necessary water to sustain crop growth between rainfall 12 yr prior to this study. Wheat (Triticum aestivum L.)-corn (Zea events. Thus water capture via increased water infiltramays L.)-fallow (WCF) and continuous cropping (CC) systems were tion rates becomes a significant factor in maximizing wacompared with wheat-fallow (WF) on summit and toeslope positions ter storage at all points in the system. An added benefit at two sites (Sterling and Stratton), and at the third site (Walsh) of cropping intensification is that increased amounts of wheat-sorghum [Sorghum bicolor (L.) Moench]-fallow (WSF) recrop residue are returned to the soil capared with WF. placed WCF. Cropping systems (CC and WCF or WSF) that returned We believe this residue may greatly improve soil physimore crop residue decreased bulk density and increased total and cal properties resulting in increased water infiltration effective porosities compared with WF. Site and slope positions that and capture efficiency. produced more crop residue also improved these properties. However, Soil physical properties such as bulk density, porosity, sorptivity developed no significant differences as a result of cropping system. Macroaggregates made up a higher percentage of total aggresorptivity, and aggregation dictate the infiltration chargates in CC and WCF or WSF compared with WF in proportion to acteristics and potentials of the soil. Most important are residue added and were also a function of clay content of the soil at the physical properties of the surface soil (top 2.5 cm), different sites and slope positions. These factors enhance the potential as this is the initial soil-water interface. However, longfor greater infiltration and hence greater water availability for crops. term infiltration can be affected by the hydraulic conductivity characteristics of deeper soil layers. Site latitude (evaporation potential), landscape slope, and cropping W is the most important and limiting factor system intensity interact to affect surface soil physical in dryland cropping systems in eastern Colorado properties important to water capture and infiltration. By determining the extent to which these factors influ(Peterson et al., 1993). Average precipitation for this ence surface soil physical properties, we can determine region ranges from 395 mm yr 1 in southeastern Colowhich cropping systems maximize potential infiltration, rado to 440 mm yr 1 in northeastern Colorado (Peterwater availability for crops, and precipitation use effison et al., 1993; Peterson et al., 1999). Growing season ciency (PUE) of the system. (March–October) open-pan evaporation averages 1600 Numerous studies have been conducted to address mm in the northeast region of Colorado to 1975 mm in soil physical properties. Dao (1996) found that tillage the southeast region of Colorado (Peterson et al., 1993; initially decreased bulk density, but no-till had a lower Peterson et al., 1999). These conditions create a water bulk density than the more traditional methods by the deficit for crops and therefore it is important to maxend of the growing season. He also found that increased imize infiltration rates to capture as much precipitation crop residue amounts decreased bulk density in the 0as possible for plant use. This will decrease evaporation, to 0.05-m soil layer. Hammel (1989) found that no-till runoff, and potential erosion. soil had a higher bulk density relative to tillage systems Historically, the WF cropping system has dominated in the surface 0.3 m of soil. However, when 2and Central Great Plains agriculture. In the western Central 3-yr cropping systems were included in the comparison, Great Plains this system requires a 14-mo fallow period differences because of tillage were nonexistent when between the harvest and planting of wheat crops. Unforaveraged over a depth of 0 to 0.5 m. These studies also tunately, water storage efficiencies during fallow are reflect total porosity because of the direct relationship 25%. Consequently 75% of the precipitation is lost of bulk density and porosity. If no-till decreases bulk to evaporation, runoff, weed use, etc. (Peterson and density, it must also increase total porosity. Roseberg Westfall, 1996). Research over the past 12 yr has shown and McCoy (1992) found that tillage increased total that no-till management permits cropping intensification porosity, but that macropores (effective pores) decreased from one crop every 2 yr to systems like WCF (two in number, stability, and continuity compared with notill soil. T.M. Shaver, G.A. Peterson, and D.G. Westfall, Dep. of Soil and Crop Sciences, Colorado State Univ., Fort Collins, CO 80523; L.R. Macropores are responsible for the effective porosity Ahuja, L.A. Sherrod, and G. Dunn, Great Plains Systems Research Unit, USDA-ARS, P.O. Box E, Fort Collins, CO 80522. Received 20 Abbreviations: CC, continuous cropping; HMP, hexametaphosphate; July 2001. *Corresponding author ([email protected]). PUE, precipitation use efficiency; SOM, soil organic matter; WCF, wheat-corn-fallow; WF, wheat-fallow; WSF, wheat-sorghum-fallow. Published in Soil Sci. Soc. Am. J. 66:1296–1303 (2002).