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Investigation of Soil Conditioning Index values for Southern High Plains agroecosystems.

As new questions and concerns arise about our ability to sustain our limited land and water resources, the importance of adequate assessment tools for evaluating the effects of land management practices on soil, air, and water resources grows. The USDA Natural Resources Conservation Service (NRCS) has adopted the Soil Conditioning Index (SCI) to evaluate cropland management systems in the United States. The SCI is a tool used to predict the consequences of management actions on the state of soil organic carbon (SOC). Soil organic carbon (also called soil organic matter [OM]) is a primary indicator of soil quality and an important factor in carbon (C) sequestration and global climate change.

The SCI predicts qualitative changes in SOC in the top 10 cm (4 in) of soils based on the combined effects of three determinants of OM using the following equation:

SCI = [OM x (0.4)] + [FO x (0.4)] + [ER x (0.2)], (1)

where OM represents the organic material from animal or plant sources produced and returned to the soil, FO signifies field operations including tillage and other field procedures that stimulate OM breakdown and decomposition, and ER corresponds to the influences of wind and water erosion (USDA NRCS 2003). Note that OM and FO each account for 40% of the final SCI value (total of 80% combined) and wind and water erosion represent 20%.

The SCI is an important soil management index and is required by several USDA NRCS criteria of practice standards, including the Conservation Crop Rotation (328) practice standard and as an additional criteria in the Residue and Tillage Management-No Till/Strip Till/Direct Seed (329) practice standard; it is also specified for use in the Conservation Security Program of the Farm Security and Rural Investment Act of 2002. However, only one published study testing the SCI for various conservation systems has been reported (Hubbs et al. 2002).

The SCI was developed based on research conducted from 1948 to 1959 in a humid region with high clay soils at Renner, Texas, USA. The SCI was originally developed as a soil rating system for soils of the western US (USDA SCS 1974). A shorter version of this rating system was prepared for use by the USDA Soil Conservation Service (SCS), Southern National Technical Center region (USDA SCS 1987). Further testing of the concept was provided using data from Iowa and Montana to develop the index currently in use (USDA NRCS 2002, 2003). An evaluation of SCI using nine long-term C studies found that positive trends in C followed positive trends in SCI and negative SCI trends were associated with negative C trends (Hubbs et al. 2002). Correlations of C and SCI were improved when data were separated by states.

The SCI assumes field operations reduce SOC by stimulating decomposition and that maintaining organic residues will maintain and increase soil organic levels. The amount of reduction of SOC due to field operations and erosion depends on the native level of carbon that may be sustained for a given site and region. Research studies have evaluated the amount of SOC and other soil quality indicators for loamy or clayey Southern High Plains (SHP) soils (Potter et al. 1997; Unger 2001; Bronson et al. 2004), but no studies investigating the SCI and its relation to SOC and other soil quality indicators are available for sandy soils. Studies have related microbial enzymes and microbial community structure to a variety of semiarid agroecosystems in sandy soils of SHP of Texas (Acosta-Martinez et al. 2003a and 2003b). In addition, previous research from a sandy soil in the SHP has shown that tillage of long-term grassland will reduce SOC levels by 50% (Zobeck et al. 1995). In a companion study of the effects of SHP cropping systems on carbon pools (Bronson et al. 2004), the total soil carbon in the upper 30 cm (12 in) was 34 Mg [ha.sup.-1] (15 t [ac.sup.-1]) for native rangeland and 23 Mg [ha.sup.-1] (10 T [ac.sup.-1]) for cropland soils. Total soil C in conservation grassland was greater than cropland soils only in the 0 to 5 cm layer, and was 24 Mg [ha.sup.-1] (11 t [ac.sup.-1]) in the upper 30 cm (12 in).

Particulate organic matter (POM) is plant material in various stages of decomposition that represents active fractions of C and N in soil (Cambardella and Elliott 1992; Cambardella 1998; Wander and Bidhart 2000) and may also be related to SCI. Native prairie and conservation grasslands have significant fractions of total C as POM carbon (Cambardella and Elliott 1992; Huggins et al. 1997). The POM fractions of soil C have been reported to be active (labile) fractions of C (Cambardella and Elliott 1992). Particulate organic matter carbon was greater in native grassland than in cropped soils in the upper 30 cm (12 in) profile in SHP soils (Bronson et al. 2004). Conservation grassland had similar levels of POM-C as cropped soils in the surface 0 to 10 cm (0 to 4 in) depths (Bronson et al. 2004). The relation of total C or POM-C with SCI was not investigated.

Considerable uncertainty still exists in the application of the SCI concept and its relation to SOC and other soil quality parameters in warm, semiarid regions, particularly in sandy soils such as those that occupy millions of acres in the SHP. In this study, we relate SCI values with other soil quality parameters for a wide variety of SHP land management systems in sandy soils of this semi-arid, hot region.

Materials and Methods

Soils and Site Management. We identified 16 field locations in six counties (Crosby Cochran, Hockley, Howard, Lubbock, and Terry) across the SHP that represented major cropping systems and conservation-planted and native grasslands (figure 1). Twelve agroecosystem combinations were sampled (table 1) including native rangeland, conservation grassland, different cotton (Gossypium hirsutum L.) and wheat (Triticum aestivum L.) rotations, and a high residue sorghum (Sorghum bicolor L.), with various combinations of irrigation and tillage intensity. The sites (grower fields and pastures) were selected with the assistance of USDA Natural Resources Conservation Service personnel and were classified as Aridic Paleustalfs or Aridic Paleustolls (Soil Survey Staff 1996).

Native rangelands consisted of native warm-season grasses, shrubs, and forbs that had never been tilled for crop production (main species: blue grama, Bouteloua gracilis [Kunth] Lag. Ex Griffiths; sand drop-seed, Sporobolus cryptandrus [Torr.] A. Gray; Kleingrass, Panicum coloratum L.; honey mesquite, Prosopis glandulosa Torr.; yucca, Yucca spp.; silverleaf nightshade, Solanum elaeagnifo-lium [Cav.]; and goldaster, Heterotheca canescens [D.C.] Shinners). Conservation grassland fields were formerly in crop production but had been planted with warm-season grasses (main species: switchgrass, Panicum virgatum L.; sand drop-seed; Canada wild rye, Elymus Canadensis L.; little bluestem, Schizachyrium scoparium [Michx.] Nash; sideoats grama, Bouteloua curtipendula [Michx.] Torr.; blue grama; Old World bluestem, Bothriochloa isch-aemum L.; and weeping lovegrass, Eragrostis curvula [Schrad.] Nees) that had been in place for at least 10 to 15 years.

In conventionally tilled (CT) cotton systems, cotton stalks were usually shredded and incorporated into the soil using a disc plow in December, the fields were chisel plowed in February, herbicide was incorporated with a spring-tooth chisel followed by raising beds in March, and rod-weeding was performed before planting in early May. After planting in May, a rotary hoe was used for wind erosion control and for breaking crusts after rain events in May and June. Field cultivation using sweep cultivators was done in June and July. Irrigation generally consisted of providing a preplant irrigation in March and April and 10 to 20 cm (4 to 8 in) per month from May through August. There was some variation in these irrigation amounts and distributions depending on crop and site conditions.

Sites under cotton and wheat rotations, including dryland and irrigated fields, had cotton planted into the previous wheat crop residue. Terminated wheat-cotton systems often had wheat planted in the fall. Cover crops such as rye (Secale cereale L.) were sometimes used in place of wheat. Typically, these systems were chiseled to a depth of about 10 cm (4 in) with sweeps in November and bedded with a lister or bedder. Wheat was then drilled in November or December. The cover crop was usually killed (terminated) by herbicide in April, and cotton was planted in early May.

The limited-tillage (LT) systems were generally ridge tilled with ridges made in the winter or spring as the only significant tillage. The dryland, high residue agroecosystem consisted of a continuous forage sorghum crop that was sprayed with herbicide in April and cultivated with a cultivator/rod weeder combination and planted in May. The beds were cultivated with undercutting blades in July, and the crop was harvested in October. The field was tilled into ridges using a lister in late November or December.

Soil Sampling and Analyses. Each field site was sampled at the following depths: 0 to 5 cm, 5 to 10 cm, 10 to 15 cm, 15 to 30 cm, and 30 to 60 cm (0 to 2 in, 2 to 4 in, 4 to 6 in, 6 to 12 in, and 12 to 24 in). Many of the same sites were used in a previous study to report carbon and nitrogen (N) pools of SHP cropland and grassland soils (Bronson et al. 2004). The study reported here considers the average or cumulative sums of soil properties from 0 to 10 cm (0 to 4 in) (tables 1 and 2), corresponding to the depths modeled by the SCI. Three replications were sampled in each site. Samples were collected with a Giddings probe, sampling five cores per replication. Samples were combined by depth.

Bulk density was determined using the soil cores (Blake and Hartge 1986). Bulk samples were collected using a shovel for determination of the wet aggregate stability. Wet aggregate stability was measured on 4 g (0.07 oz) of 1 to 2 mm (0.04 to 0.08 in) diameter aggregates by the method described by Kemper and Rosenau (1986). Soil subsamples were air-dried and ground overnight in a roller mill. Total C and N were determined using the Vario Max Elementar CN analyzer (D-63452 Hanau, Germany). Soil texture was determined using a Beckman-Coulter LS230 (Zobeck 2004). The pH values were determined on air-dried soil (<2 mm [<0.08 in]) using a 1:1 soil: water ratio (Watson and Brown 1998). Soil nitrate nitrogen (N[O.sub.3]-N) was determined by flow injection analysis (Lachat Instruments 2000). Soil phosphorus was measured using the Olsen (NaHC[O.sub.3]) procedure (Frank et al. 1998). Particulate organic matter carbon was determined according to the method of Gregorich and Ellert 1993.

Data Analyses. Details of the calculation of SCI and the SCI sub-factors are described in the USDA NRCS National Agronomy Manual, Part 508 (USDA NRCS 2002). In this study, the SCI values and sub-factors (equation 1) for OM, field operations, and water erosion were determined using the Revised Universal Soil Loss Equation (RUSLE2) version 1.25.8 (Dec. 2005) for each agroecosystem and site (tables 3 and 4). Individual field management practices were established using information on producer surveys. Wind erosion estimates are also needed to determine SCI for fields where wind erosion is active. However, wind erosion is not determined by RUSLE2 and must be provided by another method. Wind erosion was estimated using an MS Excel spreadsheet program (Sporcic et al. 1998), written by USDA NRCS agricultural engineers and agronomists, based on the Wind Erosion Equation (WEQ) (Woodruff and Siddoway 1965). The program calculates erosion using the management period method. The observed values for the crop/plant residues for each management system were determined by clipping rangeland and grassland plots and using producer survey crop yield results for cropped fields (table 3). Plot clipping followed the procedures outlined by the USDA NRCS National Range and Pasture Handbook (USDA NRCS 2006).

RUSLE2 calculates the OM sub-factor of SCI based on the RUSLE2 predicted biomass production of the test site, scaled to the biomass produced at the Renner site (D. Yoder, personal communication 2006). This sub-factor is based on the amount of OM returned to the soil as residue, roots, cover crops, green manure, etc. for OM restoration, called the residue equivalent value (REV) (USDA NRCS 2002). In RUSLE2, the OM factor includes a parameter to account for the effect of texture on decomposition and also estimates root biomass to a depth of 10 cm (4 in) based on the ratio of maximum root mass to aboveground residue produced at harvest, as found in the RUSLE2 database (USDA NRCS 2002). The observed values of crop/plant residues were used in WEQ and RUSLE2 to adjust the yields for the determination of REV.

Statistical analyses were performed using procedures of SAS version 9.1 (SAS 2002). Analyses of variance of the soil physical and chemical properties were performed using Proc Mixed with (sites*reps within sites) as a random effect. Analysis of variance of SCI factor results were not performed due to lack of sufficient sample size. However, meaningful evaluations of the results were possible by comparing means and correlations. Statistical significance tests were performed at the p = 0.05 level of significance unless indicated otherwise.

Results and Discussion

Physical and Chemical Soil Properties. The 16 field locations consisted of 51 fields sites that represented a total of twelve different combinations of crops/plant communities, irrigation levels, and tillage intensities (called agroecosystems in this paper) (tables 1, 2 and 3). Some of the agroecosystems in this study had very few observations but were included in this study because limited information is currently available in the literature (table 3). Details of the data collected by agroecosystem are presented in table 4.

The physical soil properties that were observed can be found in tables 1 and 4. Although the soil surface textures of individual sites varied from fine sand to clay loam (figure 2), the majority of the agroecosystems in this study were fine sandy loams and loamy fine sands. The only exception was the dryland wheat-cotton CT rotation which had a sandy clay loam soil surface (table 1). Mean soil surface bulk density had little deviation, varying from 1.26 Mg [m.sup.-3] (78.7 lb [ft.sup.-3]) for the CT terminated wheat-cotton field to 1.41 Mg [m.sup.-3] (88.1 lb [ft.sup.-3]) for the native rangeland. Although soil bulk density can be modified by management, it is not currently considered in SCI determinations. Bulk density was used in this study to determine soil C and N mass.

Soil wet aggregate stability may also be changed by management and has been correlated with OM (Kemper and Koch 1966; Chaney and Swift 1984). Mean wet aggregate stability was much more variable and depended upon soil management, varying from 4.7% for the irrigated wheat-cotton rotation CT field to 40.0% for the native rangeland (figure 3). The native rangeland wet aggregate stability was significantly greater than all other agroecosystems tested. The conservation grassland, with at least ten years in grassland after cultivation, had the next highest wet aggregate stability (24.9%), about 62% of the native rangeland stability. The conservation grassland had a greater aggregate stability than all other systems with the exception of the dryland high residue, dryland wheat-cotton CT, and terminated wheat-cotton CT agroecosystems which had similar aggregate stability values. Conservation grassland also had a significantly greater aggregate stability than the dryland cotton no-tillage (NT) and wheat-cotton NT rotation but at the p = 0.10 level of significance. These results suggest that even long-term NT practices may not restore wet aggregate stability in SHP sandy soils to the levels found in native rangelands. The stability was partially restored in the conservation grassland but was still only 62% of the native rangeland. Part of the lack of significant differences in some of these sites is attributed to sampling variation and low observation numbers. For example, the mean terminated wheat-cotton CT aggregate stability appears much lower than that of the conservation grassland, (table 1) but large standard errors and low observation numbers did not allow statistical separation of means. Additional sites are needed to verify this result.

The chemical properties of the soils are shown in tables 2 and 4. Some of the chemical properties of the surface soils may have been related to tillage method or intensity and fertility management. The NT, dryland cotton field had the lowest pH, and the NT and LT sites had the highest soil surface phosphorus content (table 2). These results were probably related to surface application of fertilizers with no or very limited incorporation. Detailed investigation of fertility management effects on SCI was beyond the scope of this study.

Total surface soil N and C showed few clear trends for the soils in this study (figure 3). Total soil N of native rangeland was significantly greater than dryland cotton CT and NT (p < 0.10) and terminated wheat-cotton LT (p < 0.10). Total soil N in conservation grassland was significantly greater than only dryland cotton CT. All other sites had statistically similar (p > 0.10) total soil N values. Although total SOC seemed to vary among agroecosystems in a manner similar to the total soil nitrogen, the data had considerable variation, and no statistical differences among sites were detected (figure 3).

Particulate organic matter carbon by agroecosystem was less than about one-third the amount of total SOC (table 1 and figure 3 and 4). Due to experimental constraints, POM-C was not measured on all sites. Particulate organic matter C of the surface soil of native rangeland was significantly greater than all other agroecosystems with the exception of the conservation grassland and the dryland wheat agroecosystems. Conservation grassland had statistically greater surface soil total POM-C contents than dryland cotton CT, irrigated cotton CT (p < 0.10), and the terminated wheat-cotton LT agroecosystems. These results are similar to the results found for wet aggregate stability. These results suggest that even long-term NT practices may not restore POM-C values in SHP sandy soils to the levels found in native rangelands. The POM-C was partially restored in the conservation grassland but was only 57% of the native rangeland. Again, part of the lack of significant differences in some of these sites is attributed to sampling variation and low observation numbers. Additional sites are needed to verify this result.

Soil Conditioning Index Subfactors. The variable response of different soil properties to management emphasizes the importance of adopting an index (such as SCI) to indicate soil quality changes by considering several site and management characteristics simultaneously. The soil conditioning index sub-factor values, by agroecosystem, are reported in tables 3 and 4. In general, the more positive the sub-factor value, the greater assumed potential to build OM.

The SCI-OM factor generally had a negative value with the exception of the conservation grassland and the wheat-cotton NT rotation. Unexpectedly, the native rangelands had a negative mean SCI-OM value. The native rangelands were representative of common native sites in the region. Most were grazed in the past but were not grazed during the year of sampling. The amount of residue observed for these rangelands was less than the maintenance level required to maintain OM in three out of the five sites tested (table 4). Although the REV may have exceeded the maintenance amount of OM, residue was removed during grazing, resulting in a negative OM factor. All of the conservation grassland grew biomass with no removal, resulting in positive OM factors.

Field operations for most agroecosystems in this study were generally less aggressive than the conventional systems represented by the irrigated and dryland cotton CT systems. Both irrigated cotton CT systems had negative SCI-FO values, and 7 out of 17 of the dryland cotton systems had negative SCI-FO values (table 4). However, other agroecosystems in this study had less aggressive field operations as reflected in the positive SCI-FO values.

The SCI-ER values include consideration of the wind and water erosion sediment losses predicted by the WEQ spreadsheet and RUSLE2 (tables 3 and 4). Both erosion models estimate erosion based on soil properties, field operations, crop growth and residue cover, and climatic conditions. Water erosion was estimated to be very low for all agroecosystems tested (soil loss tolerance T value is 11 Mg [ha.sup.-1], 5 t [ac.sup.-1]). Mean wind erosion estimates varied from a high of 40.5 Mg [ha.sup.-1] (18.1 t [ac.sup.-1]) for dryland cotton CT to zero for the native rangeland and conservation grassland.

Residue returned to the soil and residue needed to maintain OM levels in the soil are important concepts embodied in the determination of the SCI. The REVs were developed to convert all crop residues to a common standard for each crop group (USDA NRCS 2002). The REV of any plant material is its mass expressed as the equivalent mass of the standard crop group to which it belongs, based on a relative decomposition rate found in the database for that crop group. In this study, the REVs were determined based on the clipped residue values (REVs in table 3 and REVs in table 4). The clipped masses (or crop yield multiplied by a harvest index) were multiplied by a root adjustment and texture factor found in the RUSLE2 database. This result was then multiplied by a crop group conversion factor to obtain the REV.

The amount of residue assumed in SCI to maintain constant levels of OM in a given climate and soil texture is called the maintenance residue level (MRL) (MRLs in table 3 and MRL in table 4). Research at the Renner, Texas (Blacklands Farming Systems studies), 1948 to 1959 was used to determine the amount of organic residues needed to maintain constant soil OM levels (USDA NRCS 2002). Maintenance levels for other locations are determined in RUSLE2 by adjusting to account for differences in climate and annual decay rate of the crop group. The REV and MRL amounts listed in tables 3 and 4 were determined using previous versions of an MS Excel-based SCI calculation program called the Soil Conditioning Index Worksheet, version 24 (March 2003) or version 25 (April 2003). The MRL amounts calculated in these worksheets included a texture scaling factor to correct for site texture differences.

The results for REV and MRL are difficult to reconcile in this study of the SHP of Texas. The REV for the conservation grassland was generally 25% greater than the native rangeland values. The conservation grassland included an improved variety of grasses that usually exceed the growth of native species. None of the SHP cropping systems had REV values that exceeded the estimated MRL, including all NT systems. In addition, only 40% of the grazed native rangelands and 50% of the conservation grasslands had REV values that exceeded the MRL values. Yet, the native rangeland had the highest SOC, exceeding the conservation grassland by 18%. This result suggests the maintenance level of OM may be higher than needed to maintain SOC levels at levels found in native grasslands commonly found in the SHP.

Soil Conditioning Index, Soil Management, and Soil Properties. The computed SCI values seem to be closely associated with field operations and subsequent erosion estimates. The SCI values were negative for all conventionally tilled sites and positive for the native rangeland, conservation grassland and all NT agroecosystems (table 3 and 4, figure 5). In addition, for each agroecosystem, the signs of the SCI values for all sites tested were either all positive or all negative (table 4), indicating similar trends in OM accumulation or depletion, respectively.

The results presented in tables 3 and 4 and figure 5 demonstrate the effect of soil management on SCI values among cropping agroecosystems. For example, this study included both dryland and irrigated wheat-cotton CT rotations. The dryland wheat-cotton CT rotations had negative SCI values while the irrigated wheat-cotton CT rotations had positive SCI values. The irrigated wheat-cotton CT rotation returned more OM to the soil and had less erosion because the systems had two years of wheat and one year of cotton in the rotation. Conversely, the dryland wheat-cotton CT rotation returned less OM and had one year of wheat and two years of cotton in the rotation, resulting in more wind and water erosion (table 3).

A more subtle effect of soil management on SCI is seen in the terminated wheat-cotton agroecosystems. These agroecosystems were all irrigated and had cotton planted into wheat or other small grain crops that had been killed with herbicides prior to planting cotton. However, these agroecosystems had positive SCI values for the LT systems and negative SCI values for the CT systems. The terminated wheat-cotton CT in this study area uses a form of minimum tillage as the conventional tillage system. The only tillage consists of the use of undercutting sweeps prior to bed formation, and a disk-bedder is often used to bed the fields prior to planting. In the terminated wheat-cotton CT in this study, bedding was done in the spring, exposing the soil surface during the wind erosion season. As a result, wind erosion sediment loss was estimated to range from 8.5 to 15.7 Mg [ha.sup.-1], (3.8 to 7.0 t [ac.sup.-1]) with the SCI varying from -0.28 to -0.47, respectively. In the terminated wheat-cotton LT systems, sweeps were not used, no bedding occurred in the wind erosion season, and less than 3.6 Mg [ha.sup.-1] (1.6 t [ac.sup.-1]) wind erosion sediment loss was estimated for these systems. These details in management can be considered in RUSLE2 and WEQ and used in erosion estimates.

Since the SCI is a tool used to predict the consequences of management actions on the state of SOC, it is expected that the SCI values would be correlated with changes in soil organic carbon/matter-related properties. To explore this effect, we correlated the SCI value with other variables (table 4). The SCI was strongly correlated with aggregate stability, POM-C, total nitrogen, carbon mass, and the REV (table 5).

All parameters tested in table 5 were significantly correlated with SCI. However, some parameters were more strongly related to the SCI than others. Of the five parameters listed, soil carbon mass in the soil surface was the least related to SCI. This seems reasonable since total carbon mass includes humified, recalcitrant organic carbon with a long residence time in soils as well as more labile forms that may be more representative of current management. Particulate organic matter C and REV represent OM of recent origin. Thus, POM-C and the REV were much more highly correlated with SCI (0.57 and 0.68, respectively). In addition, wet aggregate stability was also correlated with SCI (r = 0.47). This is expected since the correlation of aggregate stability and POM-C was 0.79.

The USDA NRCS currently uses SCI as part of the criteria for consideration of participation in certain farm programs. The results of this study suggest that SCI is not a precise tool and may include significant variation in index estimates. Although we had 52 sites for comparison in this study, SCI error values listed in table 3 and other statistics describing the SCI variation do not generally support establishment of precise SCI cut-off limits. The standard error of our most numerous system, the dryland cotton CT, was 0.35 (table 3). Although the standard error was rather low, varying from 0 to 0.10 for all other cropped agroecosystems, the average coefficient of variation was 32%. In addition, the average standard deviation of all cropped agroecosystems was 0.29.

Summary and Conclusions

The SCI program implemented in RUSLE2 successfully associated the conservation grasslands, native rangelands, and NT, limited (minimum) tillage and high residue croplands with positive SCI values and the conventionally-tilled fields with negative SCI values. In addition, the general trends seemed reasonable. The conservation grasslands had the highest SCI value, and the conventionally-tilled dryland cotton had the most negative SCI value. Rather subtle changes in soil management were detected by using RUSLE2 and a spreadsheet version of the WEQ to determine SCI.

Although the stated purpose of the SCI is to predict the consequences of management actions on the state of soil organic carbon, the SCI values were not strongly correlated with total SOC. The SCI values were more strongly associated with a specific and more labile form of soil organic carbon, POM-C. The SCI was even more strongly correlated with a measure of residue production, the REV, which serves to add OM to the soil and to protect the soil from the forces of erosion.

Caution should be used when using SCI in a very precise manner. Due to variability in SCI estimates, it may be advisable to have a buffer of plus or minus 0.2 or 0.3 considered when assigning SCI values. The buffer is suggested to account for the variation in SCI suggested by the standard error values listed in table 3. This buffer may be particularly necessary in western states where the OM sub-factor in SCI may often be less than 0, even in situations with adequate cover. For example, in this study only the conservation grassland and NT wheat-cotton rotations had positive SCI OM sub-factors. Further field testing of SCI over a wide range of climatic and agroecosystems is recommended.

Acknowledgements

The authors are indebted to Dean Holder (USDA ARS) and Deanna Halfmann Faubian (formerly USDA ARS, currently USDA NRCS) for technical assistance with field work and laboratory analysis; Lucia Barbato and Ada Rosenbaum Warren, Center for Geospatial Technology, Texas Tech University, for providing the site map; and Daniel Yoder, University of Kentucky, for helpful information on RUSLE2.

Disclaimer

Mention of trade names or commercial products is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA ARS, USDA NRCS, or Texas A & M University.

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USDA NRCS. 2003. Interpreting the soil conditioning index: A tool for measuring soil organic matter trends. US Department of Agriculture, Natural Resources Conservation Service, Soil Quality-Agronomy Tech. Note No. 16. http://soils.usda.gov/sqi.

USDA NRCS. 2006. US Department of Agriculture--NRCS National Range and Pasture Handbook, Chapter 4 http://www.glti.nrcs.usda.gov/technical/publications/nrph.html.

USDA SCS (Soil Conservation Service). 1974. Soil conditioning rating indices for major irrigated and non-irrigated crops grown in the western Unites States. Technical Note No. 27., West Technical Service Center, Portland, Oregon.

USDA SCS. 1987. Soil Conditioning Rating Indices. Technical Note Agronomy TX-8. Temple, TX: USDA SCS.

Wander, M.M., and M.G. Bidhart. 2000. Tillage practice influences on the physical protection, bioavailability and composition of particulate organic matter. Biology and Fertility of Soils 32:360-367.

Watson, M.E., and J.R. Brown. 1998. pH and Lime Requirement. In Recommended Chemical Soil Test Procedures for the North Central Region, ed. J.R. Brown, 13-16. North Central Regional Publication No. 221 (revised). Columbia, MO: University of Missouri Agricultural Experiment Station.

Woodruff, N.P., and F.H. Siddoway. 1965. A wind erosion equation. Soil Science Society of America Journal 29:602-608.

Zobeck, T.M. Rapid particle size analyses using laser diffraction. 2004. Transactions of the ASAE 20:633-639.

Zobeck, T.M., N.A. Rolong, D.W. Fryrear, J.D. Bilbro, and B.L. Allen. 1995. Properties and productivity of recently tilled grass sod and 70-year cultivated soil. Journal of Soil and Water Conservation 50(2):210-215.

Ted M. Zobeck is a research soil scientist for the USDA Agricultural Research Service (ARS) in Lubbock, Texas. James Crownover is a retired agronomist and Monty Dollar is a retired agronomist for the USDA Natural Resources Conservation Service in Lubbock, Texas. R. Scott Van Pelt is a soil physicist for the USDA ARS in Big Spring, Texas. Veronica Acosta-Martinez is a soil microbiologist and biochemist for the USDA ARS in Lubbock, Texas. Kevin F. Bronson is a soil fertility specialist at Texas A & M University in Lubbock, Texas. Dan R. Upchurch is a southern plains area director for the USDA ARS in College Station, Texas.
Table 1 Selected average soil physical properties by agroecosystem.

Agroecosystem             Observations  Sand         Clay

Native rangeland          13            67.6% (2.8)  16.1% (1.4)
Conservation grassland    30            72.4% (3.1)  15.2% (1.5)
Dryland cotton CT         41            73.0% (2.6)  15.5% (1.3)
Dryland cotton NT          3            80.0% (1.5)  12.7% (0.7)
Irrigated cotton CT        6            84.2% (2.6)   9.8% (1.3)
Dryland high residue       3            83.5% (1.4)   9.4% (0.8)
Terminated wheat-cotton    2            76.0% (8.4)  14.1% (3.7)
  CT
Terminated wheat-cotton    8            83.5% (2.5)  10.3% (1.3)
  LT
Dryland wheat              3            73.9% (1.9)  15.6% (1.1)
Irrigated wheat-cotton     7            84.0% (2.1)  10.2% (1.1)
  rotation CT
Dryland wheat-cotton       3            48.9% (2.4)  25.9% (1.4)
  rotation CT
Wheat-cotton rotation NT   9            70.8% (5.3)  15.6% (2.1)

                                           Bulk density     Aggregate
Agroecosystem             USDA texture     (Mg [m.sup.-3])  stability

Native rangeland          Fine sandy loam  1.41 (0.02)      40.0% (4.1)
Conservation grassland    Fine sandy loam  1.35 (0.02)      24.9% (2.8)
Dryland cotton CT         Fine sandy loam  1.32 (0.01)       9.6% (0.9)
Dryland cotton NT         Fine sandy loam  1.32 (0.03)       6.9% (1.3)
Irrigated cotton CT       Loamy fine sand  1.35 (0.02)       7.0% (1.3)
Dryland high residue      Loamy fine sand  1.34 (0.01)      14.8% (2.9)
Terminated wheat-cotton   Fine sandy loam  1.26 (0.05)      10.8% (--)
  CT
Terminated wheat-cotton   Loamy fine sand  1.35 (0.01)       9.7% (1.9)
  LT
Dryland wheat             Fine sandy loam  1.29 (0.04)       8.7% (0.7)
Irrigated wheat-cotton    Loamy fine sand  1.37 (0.04)       4.7% (0.9))
  rotation CT
Dryland wheat-cotton      Sandy clay loam  1.35 (0.01)      15.9% (5.1)
  rotation CT
Wheat-cotton rotation NT  Fine sandy loam  1.36 (0.02)      12.3% (2.1)

Notes: CT = conventional tillage; NT = no tillage; LT = limited tillage.
Standard errors are given in parentheses; "--" indicates no data
available.

Table 2 Selected average soil chemical properties by agroecosystem.

                                   N[O.sub.3]-N  Phosphorus
Agroecosystem           pH         (ppm)         (ppm)

Native rangeland        7.3 (0.2)   5.0 (2.1)     5.0 (0.4)
Conservation            7.4 (0.1)   1.6 (0.3)     7.5 (0.9)
  grassland
Dryland cotton CT       7.4 (0.1)   9.3 (1.4)    12.3 (0.8)
Dryland cotton NT       6.3 (0.1)  10.2 (1.0)    52.0 (2.0)
Irrigated cotton CT     7.7 (0.1)   5.9 (1.2)     6.8 (0.7)
Dryland high residue    7.9 (0.1)   4.5 (0.4)    13.7 (1.6)
Terminated              7.8 (0.2)  18.5 (10.4)    8.5 (1.0)
  wheat-cotton CT
Terminated              7.8 (0.1)   4.4 (1.2)    25.4 (3.5)
  wheat-cotton LT
Dryland wheat           7.4 (0.3)   9.3 (1.8)     9.3 (1.1)
Irrigated wheat-cotton  7.6 (0.1)   5.9 (1.4)    20.6 (2.3)
  rotation CT
Dryland wheat-cotton    8.2 (0.1)   7.9 (2.3)    11.5 (1.3)
  rotation CT
Wheat-cotton rotation   7.2 (0.1)  15.3 (3.4)    38.5 (4.6)
  NT

                            Total nitrogen
                                       Sum
                        Mean           0 to 10 cm
Agroecosystem           0 to 10 cm     (Mg [ha.sup.-1])

Native rangeland        0.07% (0.01)   0.87 (0.13)
Conservation            0.06% (0.01)   0.84 (0.08)

  grassland
Dryland cotton CT       0.04% (0.004)  0.54 (0.05)
Dryland cotton NT       0.02% (0.001)  0.23 (0.01)
Irrigated cotton CT     0.04% (0.01)   0.59 (0.17)
Dryland high residue    0.04% (0.01)   0.57 (0.10)
Terminated              0.05% (0.01)   0.64 (0.15)
  wheat-cotton CT
Terminated              0.03% (0.01)   0.45 (0.08)
  wheat-cotton LT
Dryland wheat           0.06% (0.01)   0.73 (0.07)
Irrigated wheat-cotton  0.04% (0.01)   0.53 (0.09)
  rotation CT
Dryland wheat-cotton    0.07% (0.004)  0.95 (0.05)
  rotation CT
Wheat-cotton rotation   0.05% (0.01)   0.68 (0.17)
  NT

                                                        Particulate
                        Total organic carbon            organic matter
                                      Sum               Sum
                        Mean          0 to 10 cm        0 to 10 cm
Agroecosystem           0 to 10 cm    (Mg [ha.sup.-1])  (Mg [ha.sup.-1])

Native rangeland        0.86% (0.14)  11.31 (1.81)      5.4 (0.6)
Conservation            0.68% (0.07)   9.57 (0.96)      3.1 (0.6)
  grassland
Dryland cotton CT       0.48% (0.06)   6.41 (0.84)      1.0 (0.2)
Dryland cotton NT       0.23% (0.01)   3.05 (0.10)      1.2 (--)
Irrigated cotton CT     0.46% (0.10)   6.36 (1.47)      1.1 (0.4)
Dryland high residue    0.35% (0.04)   4.86 (0.51)      --
Terminated              0.54% (0.18)   6.80 (2.05)      0.9 (--)
  wheat-cotton CT
Terminated              0.50% (0.18)   6.76 (2.41)      1.1 (0.0)
  wheat-cotton LT
Dryland wheat           0.64% (0.09)   8.37 (1.08)      2.1 (--)
Irrigated wheat-cotton  0.43% (0.05)   6.04 (0.83)      0.97 (--)
  rotation CT
Dryland wheat-cotton    0.63% (0.04)   8.60 (0.51)      0.97 (--)
  rotation CT
Wheat-cotton rotation   0.50% (0.12)   6.74 (1.54)      2.4 (0.6)
  NT

Notes: CT = conventional tillage; NT = no tillage; LT = limited tillage.
Standard errors are given in parentheses; "--" indicates no data
available.

Table 3 Soil conditioning index (SCI) subfactors by agroecosystem.

                   Fields
Agroecosystem      evaluated  OM            FO            ER

Native rangeland    5         -0.43 (0.15)   1.00 (0.00)   0.94 (0.02)
Conservation       12          3.88 (0.63)   1.00 (0.00)   1.00 (0.00)
  grassland
Dryland cotton CT  17         -0.41 (0.13)   0.08 (0.06)  -6.79 (1.67)
Dryland cotton NT   1         -0.17          0.97          0.58
Irrigated cotton    2         -0.52 (0.03)  -0.28 (0.01)  -3.55 (0.05)
  CT
Dryland high        1         -0.48          0.73         -1.6
  residue
Terminated          2         -0.62 (0.07)   0.63 (0.03)  -1.85 (0.25)
  wheat-cotton CT
Terminated          4         -0.29 (0.12)   0.48 (0.05)   0.39 (0.21)
  wheat-cotton LT
Dryland wheat       1         -0.88          0.93         -0.33
Irrigated           2         -0.12 (0.07)   0.52 (0.00)  -0.56 (0.08)
  wheat-cotton
  rotation CT
Dryland             1         -0.72          0.72         -1.5
  wheat-cotton
  rotation CT
Wheat-cotton        3          0.05 (0.24)   0.93 (0.04)   0.89 (0.08)
  rotation NT

                   Wind erosion      Water erosion     Residue
                   estimate          estimate          equivalent values
Agroecosystem      (Mg [ha.sup.-1])  (Mg [ha.sup.-1])  (Mg [ha.sup.-1])

Native rangeland    0.0 (0.0)        0.3 (0.1)         5.44 (1.41)
Conservation        0.0 (0.0)        0.0 (0.0)         8.24 (1.20)
  grassland
Dryland cotton CT  40.5 (9.6)        3.8 (0.4)         1.67 (0.18)
Dryland cotton NT   2.24             0.2               3.20
Irrigated cotton   23.1 (0.0)        2.9 (0.4)         1.94 (1.67)
  CT
Dryland high       13.7              1.2               1.19
  residue
Terminated         12.1 (3.6)        4.2 (2.0)         5.33 (1.16)
  wheat-cotton CT
Terminated          1.7 (1.0)        1.7 (0.2)         4.44 (0.84)
  wheat-cotton LT
Dryland wheat       4.7              2.9               0.46
Irrigated           7.8 (0.2)        1.0 (0.2)         1.72 (0.53)
  wheat-cotton
  rotation CT
Dryland            10.5              3.8               1.59
  wheat-cotton
  rotation CT
Wheat-cotton        0.1 (0.07)       0.6 (0.4)         4.61 (0.72)
  rotation NT

                   Maintenance
                   residue level
Agroecosystem      (Mg [ha.sup.-1])  SCI

Native rangeland   7.64 (0.42)        0.41 (0.06)
Conservation       7.78 (0.15)        2.15 (0.25)
  grassland
Dryland cotton CT  7.85 (0.21)       -1.49 (0.35)
Dryland cotton NT  8.77               0.43
Irrigated cotton   8.57 (0.0)        -1.00 (0.00)
  CT
Dryland high       7.90              -0.22
  residue
Terminated         8.04 (0.53)       -0.38 (0.10)
  wheat-cotton CT
Terminated         9.29 (0.37)        0.16 (0.04)
  wheat-cotton LT
Dryland wheat      7.51              -0.05
Irrigated          8.57 (0.0)         0.05 (0.01)
  wheat-cotton
  rotation CT
Dryland            7.51              -0.31
  wheat-cotton
  rotation CT
Wheat-cotton       9.28 (0.88)        0.57 (0.10)
  rotation NT

Notes: OM = SCI organic matter factor; FO = SCI field operations factor,
ER = SCI erosion factor. CT = conventional tillage, NT = no tillage, LT
= limited tillage. Standard errors are given in parentheses.

Table 4 Mean physical and chemical properties of study sites by
agroecosystem.

                                                  N[O.sub.3]-N
Agroecosystem                       Sand   Clay   (ppm)         P (ppm)

Native rangeland                    79.9%  10.3%   1.1           3.8
Native rangeland                    72.5%  13.3%   7.6           6.3
Native rangeland                    65.6%  17.3%  10.4           5.0
Native rangeland                    54.6%  19.7%   3.1           6.5
Native rangeland                    56.8%  22.2%   1.6           4.2
Conservation grassland              87.0%   7.9%   3.8           4.0
Conservation grassland              86.3%   8.4%   0.4           3.8
Conservation grassland              86.2%   8.7%   0.8          16.0
Conservation grassland              83.1%   9.9%   1.2          13.0
Conservation grassland              81.6%  10.8%   1.0           5.8
Conservation grassland              84.0%  10.8%   0.8           8.8
Conservation grassland              65.4%  16.7%   4.3          12.0
Conservation grassland              69.9%  17.0%   0.7           4.5
Conservation grassland              62.0%  19.3%   0.5           5.0
Conservation grassland              62.0%  20.5%   4.3           7.8
Conservation grassland              46.2%  26.3%   0.6           1.8
Conservation grassland              42.2%  30.8%   1.0           7.0
Dryland cotton CT                   90.7%   6.4%   6.1          18.5
Dryland cotton CT                   89.5%   7.7%   7.3           9.7
Dryland cotton CT                   84.0%   8.3%  15.8           8.5
Dryland cotton CT                   88.3%   8.4%   5.0           6.5
Dryland cotton CT                   86.7%   8.4%   6.5          15.8
Dryland cotton CT                   88.5%   8.9%   2.3          13.3
Dryland cotton CT                   85.9%   9.4%  24.9          11.8
Dryland cotton CT                   82.7%  10.7%  10.3          19.3
Dryland cotton CT                   79.5%  12.1%   9.8           5.5
Dryland cotton CT                   74.3%  14.4%  12.7           6.5
Dryland cotton CT                   71.8%  16.5%  13.9           7.0
Dryland cotton CT                   72.3%  17.0%   2.6           6.8
Dryland cotton CT                   61.3%  19.8%   4.1          10.8
Dryland cotton CT                   59.0%  20.0%  14.7          12.8
Dryland cotton CT                   51.4%  25.2%  19.7          10.0
Dryland cotton CT                   51.2%  25.8%   7.4          17.5
Dryland cotton CT                   46.3%  32.9%   2.6           9.8
Dryland cotton NT                   80.0%  12.7%  10.2          52.0
Irrigated cotton CT                 89.5%   7.3%   7.9           5.8
Irrigated cotton CT                 78.9%  12.3%   3.9           7.8
Dryland high residue                83.5%   9.4%   4.5          13.7
Terminated wheat-cotton CT          84.4%  10.4%   8.1           9.5
Terminated wheat-cotton CT          67.6%  17.8%  28.9           7.5
Terminated wheat-cotton LT          88.4%   7.7%   3.9          21.3
Terminated wheat-cotton LT          84.3%   9.8%   2.9          31.0
Terminated wheat-cotton LT          82.0%  11.4%   3.6           8.0
Terminated wheat-cotton LT          68.1%  18.5%  11.7          38.0
Dryland wheat                       73.9%  15.6%   9.3           9.3
Irrigated wheat-cotton rotation CT  87.4%   8.3%   4.6          20.4
Irrigated wheat-cotton rotation CT  79.5%  12.6%   7.7          21.0
Dryland wheat-cotton rotation CT    48.9%  25.9%   7.9          11.5
Wheat-cotton NT                     80.9%  11.6%  25.1          33.7
Wheat-cotton NT                     81.3%  11.6%   4.2          54.0
Wheat-cotton NT                     50.3%  23.7%  16.4          27.8

                                         Aggregate  Carbon mass
Agroecosystem                       pH   stability  (Mg [ha.sup.-1])

Native rangeland                    7.8  43.2%       6.99
Native rangeland                    6.7  46.8%      19.56
Native rangeland                    6.8  20.9%       5.17
Native rangeland                    7.6  55.2%      17.86
Native rangeland                    8.0  44.0%      11.36
Conservation grassland              7.5  15.6%      13.87
Conservation grassland              7.1  32.4%       5.30
Conservation grassland              7.3   7.2%       5.13
Conservation grassland              7.0   7.1%       3.90
Conservation grassland              7.8  41.3%       5.17
Conservation grassland              7.0   5.6%       3.60
Conservation grassland              7.5  21.1%      10.90
Conservation grassland              7.4  26.8%      19.71
Conservation grassland              7.8  30.0%      13.65
Conservation grassland              8.0  31.9%      14.68
Conservation grassland              7.1  42.2%      15.09
Conservation grassland              7.5  39.4%      14.18
Dryland cotton CT                   7.1   4.8%       2.91
Dryland cotton CT                   7.8   7.2%       5.08
Dryland cotton CT                   6.1   6.9%       4.19
Dryland cotton CT                   7.6   3.1%      21.70
Dryland cotton CT                   6.9   9.8%       3.36
Dryland cotton CT                   6.0   3.9%       2.19
Dryland cotton CT                   6.4   3.5%       1.40
Dryland cotton CT                   6.6   8.1%       5.12
Dryland cotton CT                   7.7  16.3%      30.20
Dryland cotton CT                   8.0   2.2%       4.48
Dryland cotton CT                   8.0   5.9%       7.17
Dryland cotton CT                   8.2  15.0%       3.70
Dryland cotton CT                   8.3  14.7%       5.40
Dryland cotton CT                   7.2  18.1%       9.87
Dryland cotton CT                   7.7  16.2%       9.71
Dryland cotton CT                   8.2  13.1%       9.27
Dryland cotton CT                   8.2   5.4%       6.96
Dryland cotton NT                   6.3   6.8%       3.05
Irrigated cotton CT                 7.5   4.5%       4.00
Irrigated cotton CT                 7.9   9.5%       8.72
Dryland high residue                7.9  14.8%       4.86
Terminated wheat-cotton CT          7.6  10.8%       4.75
Terminated wheat-cotton CT          7.9   -          8.85
Terminated wheat-cotton LT          7.6  11.6%       3.24
Terminated wheat-cotton LT          8.0   5.2%       3.70
Terminated wheat-cotton LT          7.7  17.7%      22.24
Terminated wheat-cotton LT          7.7   -         11.04
Dryland wheat                       7.4   8.6%       8.37
Irrigated wheat-cotton rotation CT  7.7   5.6%       4.63
Irrigated wheat-cotton rotation CT  7.5   3.6%       7.91
Dryland wheat-cotton rotation CT    8.2  15.9%       8.60
Wheat-cotton NT                     7.1   9.0%       4.39
Wheat-cotton NT                     7.5  15.3%       3.02
Wheat-cotton NT                     7.1  12.6%      12.82

                                    Nitrogen mass     POM-C
Agroecosystem                       (Mg [ha.sup.-1])  (Mg [ha.sup.-1])

Native rangeland                    0.58              -
Native rangeland                    1.32              6.49
Native rangeland                    0.31              4.50
Native rangeland                    1.61              5.19
Native rangeland                    1.04              -
Conservation grassland              1.38              2.96
Conservation grassland              0.64              -
Conservation grassland              0.48              1.31
Conservation grassland              0.20              2.30
Conservation grassland              0.54              -
Conservation grassland              0.25              0.85
Conservation grassland              1.42              2.94
Conservation grassland              0.80              2.28
Conservation grassland              1.10              3.27
Conservation grassland              1.15              7.54
Conservation grassland              1.33              4.91
Conservation grassland              1.28              2.51
Dryland cotton CT                   0.23              -
Dryland cotton CT                   0.40              0.78
Dryland cotton CT                   0.46              1.23
Dryland cotton CT                   0.61              0.67
Dryland cotton CT                   0.28              -
Dryland cotton CT                   0.17              0.36
Dryland cotton CT                   0.12              0.30
Dryland cotton CT                   0.56              0.75
Dryland cotton CT                   0.77              0.57
Dryland cotton CT                   0.49              0.73
Dryland cotton CT                   0.69              -
Dryland cotton CT                   0.50              -
Dryland cotton CT                   0.59              -
Dryland cotton CT                   0.96              2.55
Dryland cotton CT                   0.95              1.67
Dryland cotton CT                   0.84              1.00
Dryland cotton CT                   0.82              -
Dryland cotton NT                   0.23              1.17
Irrigated cotton CT                 0.35              0.76
Irrigated cotton CT                 0.84              1.47
Dryland high residue                0.57              -
Terminated wheat-cotton CT          0.49              0.90
Terminated wheat-cotton CT          0.79              -
Terminated wheat-cotton LT          0.35              1.01
Terminated wheat-cotton LT          0.35              1.11
Terminated wheat-cotton LT          0.49              1.04
Terminated wheat-cotton LT          0.95              1.17
Dryland wheat                       0.73              2.09
Irrigated wheat-cotton rotation CT  0.36              0.97
Irrigated wheat-cotton rotation CT  0.76              -
Dryland wheat-cotton rotation CT    0.95              0.97
Wheat-cotton NT                     0.51              3.41
Wheat-cotton NT                     0.21              1.20
Wheat-cotton NT                     1.32              2.51

                                         SCI subfactors
Agroecosystem                       OM     FO     ER      SCI

Native rangeland                    -0.06   1.00    0.99   0.57
Native rangeland                    -0.91   1.00    0.85   0.20
Native rangeland                    -0.54   1.00    0.96   0.37
Native rangeland                    -0.46   1.00    0.97   0.41
Native rangeland                    -0.17   1.00    0.95   0.52
Conservation grassland               4.80   1.00    1.00   2.50
Conservation grassland               3.30   1.00    1.00   1.90
Conservation grassland               4.70   1.00    1.00   2.50
Conservation grassland               2.40   1.00    1.00   1.60
Conservation grassland               3.70   1.00    1.00   2.10
Conservation grassland               0.60   1.00    0.99   0.84
Conservation grassland               5.80   1.00    1.00   2.90
Conservation grassland               3.70   1.00    1.00   2.10
Conservation grassland               3.50   1.00    1.00   2.00
Conservation grassland               7.60   1.00    1.00   3.60
Conservation grassland               6.30   1.00    1.00   3.10
Conservation grassland               0.12   1.00    0.97   0.64
Dryland cotton CT                   -0.72   0.22   -7.10  -1.60
Dryland cotton CT                   -0.79  -0.29   -7.10  -1.80
Dryland cotton CT                   -0.62  -0.15   -5.00  -1.30
Dryland cotton CT                    0.22  -0.03   -3.80  -0.68
Dryland cotton CT                   -0.72   0.22  -26.00  -5.40
Dryland cotton CT                   -0.29   0.41   -5.90  -1.10
Dryland cotton CT                   -0.54  -0.10   -4.90  -1.20
Dryland cotton CT                   -0.75  -0.29   -7.00  -1.80
Dryland cotton CT                   -0.69   0.43   -1.80  -0.47
Dryland cotton CT                   -0.70   0.04  -23.00  -4.90
Dryland cotton CT                   -0.80  -0.17   -2.80  -0.95
Dryland cotton CT                    0.78   0.50   -5.10  -0.51
Dryland cotton CT                   -0.64  -0.19   -3.70  -1.10
Dryland cotton CT                   -0.36   0.13   -2.80  -0.66
Dryland cotton CT                   -0.36   0.13   -2.80  -0.66
Dryland cotton CT                   -0.77   0.03   -2.60  -0.81
Dryland cotton CT                    0.78   0.50   -4.10  -0.31
Dryland cotton NT                   -0.17   0.97    0.58   0.43
Irrigated cotton CT                 -0.55  -0.29   -3.50  -1.00
Irrigated cotton CT                 -0.49  -0.27   -3.60  -1.00
Dryland high residue                -0.48   0.73   -1.60  -0.22
Terminated wheat-cotton CT          -0.69   0.60   -2.10  -0.47
Terminated wheat-cotton CT          -0.55   0.65   -1.60  -0.28
Terminated wheat-cotton LT          -0.34   0.51    0.79   0.23
Terminated wheat-cotton LT          -0.58   0.34    0.72   0.05
Terminated wheat-cotton LT          -0.02   0.53    0.09   0.22
Terminated wheat-cotton LT          -0.22   0.53   -0.03   0.12
Dryland wheat                       -0.88   0.93   -0.33  -0.05
Irrigated wheat-cotton rotation CT  -0.19   0.52   -0.48   0.04
Irrigated wheat-cotton rotation CT  -0.05   0.52   -0.64   0.06
Dryland wheat-cotton rotation CT    -0.72   0.72   -1.50  -0.31
Wheat-cotton NT                     -0.37   0.98    0.95   0.43
Wheat-cotton NT                      0.45   0.96    0.99   0.76
Wheat-cotton NT                      0.07   0.86    0.74   0.52

                                    Wind erosion      Water erosion
Agroecosystem                       (Mg [ha.sup.-1])  (Mg [ha.sup.-1])

Native rangeland                      0.0             0.1
Native rangeland                      0.0             0.9
Native rangeland                      0.0             0.2
Native rangeland                      0.0             0.2
Native rangeland                      0.0             0.3
Conservation grassland                0.0             0.0
Conservation grassland                0.0             0.0
Conservation grassland                0.0             0.0
Conservation grassland                0.0             0.0
Conservation grassland                0.0             0.0
Conservation grassland                0.0             0.1
Conservation grassland                0.0             0.0
Conservation grassland                0.0             0.0
Conservation grassland                0.0             0.0
Conservation grassland                0.0             0.0
Conservation grassland                0.0             0.0
Conservation grassland                0.0             0.2
Dryland cotton CT                    43.7             2.1
Dryland cotton CT                    43.7             2.2
Dryland cotton CT                    30.0             4.0
Dryland cotton CT                    23.1             1.8
Dryland cotton CT                   151.9             2.1
Dryland cotton CT                    37.9             1.2
Dryland cotton CT                    30.7             2.9
Dryland cotton CT                    42.3             3.4
Dryland cotton CT                    13.0             3.1
Dryland cotton CT                   132.2             5.2
Dryland cotton CT                    15.7             6.0
Dryland cotton CT                    31.6             3.4
Dryland cotton CT                    21.7             5.2
Dryland cotton CT                    15.2             6.5
Dryland cotton CT                    15.2             6.5
Dryland cotton CT                    14.3             5.8
Dryland cotton CT                    26.2             2.7
Dryland cotton NT                     2.2             0.2
Irrigated cotton CT                  23.1             2.5
Irrigated cotton CT                  23.1             3.4
Dryland high residue                 13.7             1.2
Terminated wheat-cotton CT           15.7             2.2
Terminated wheat-cotton CT            8.5             6.3
Terminated wheat-cotton LT            0.0             1.2
Terminated wheat-cotton LT            0.0             1.6
Terminated wheat-cotton LT            3.4             1.8
Terminated wheat-cotton LT            3.6             2.2
Dryland wheat                         4.7             2.9
Irrigated wheat-cotton rotation CT    7.6             0.8
Irrigated wheat-cotton rotation CT    8.1             1.3
Dryland wheat-cotton rotation CT     10.5             3.8
Wheat-cotton NT                       0.0             0.3
Wheat-cotton NT                       0.0             0.1
Wheat-cotton NT                       0.2             1.3

                                    REV               MRL
Agroecosystem                       (Mg [ha.sup.-1])  (Mg [ha.sup.-1])

Native rangeland                     8.30              7.90
Native rangeland                     1.25              7.51
Native rangeland                     3.47              8.95
Native rangeland                     8.65              7.51
Native rangeland                     5.54              6.34
Conservation grassland               8.63              7.51
Conservation grassland               6.36              8.77
Conservation grassland               9.24              7.51
Conservation grassland               4.42              7.66
Conservation grassland               6.09              7.90
Conservation grassland               1.58              8.95
Conservation grassland              18.00              7.51
Conservation grassland               5.99              7.51
Conservation grassland              11.98              7.51
Conservation grassland              11.00              7.51
Conservation grassland               7.50              7.51
Conservation grassland               8.09              7.51
Dryland cotton CT                    2.01              8.77
Dryland cotton CT                    1.21              7.51
Dryland cotton CT                    1.01              7.51
Dryland cotton CT                    1.51              7.51
Dryland cotton CT                    2.01              8.77
Dryland cotton CT                    3.72              8.95
Dryland cotton CT                    2.65             10.16
Dryland cotton CT                    1.21              7.51
Dryland cotton CT                    1.61              7.51
Dryland cotton CT                    0.60              7.51
Dryland cotton CT                    2.39              7.51
Dryland cotton CT                    1.49              7.90
Dryland cotton CT                    1.51              7.51
Dryland cotton CT                    1.48              7.51
Dryland cotton CT                    1.48              7.51
Dryland cotton CT                    1.01              7.51
Dryland cotton CT                    1.49              6.34
Dryland cotton NT                    3.20              8.77
Irrigated cotton CT                  0.27              8.57
Irrigated cotton CT                  3.61              8.57
Dryland high residue                 1.19              7.90
Terminated wheat-cotton CT           6.49              7.51
Terminated wheat-cotton CT           4.17              8.57
Terminated wheat-cotton LT           5.89             10.02
Terminated wheat-cotton LT           5.89             10.02
Terminated wheat-cotton LT           3.27              8.57
Terminated wheat-cotton LT           2.72              8.57
Dryland wheat                        0.46              7.51
Irrigated wheat-cotton rotation CT   2.25              8.57
Irrigated wheat-cotton rotation CT   1.19              8.57
Dryland wheat-cotton rotation CT     1.59              7.51
Wheat-cotton NT                      5.49             10.16
Wheat-cotton NT                      5.16             10.16
Wheat-cotton NT                      3.19              7.51

Notes: P = phosphorus; POM-C = particulate organic matter carbon; OM =
organic matter; FO = field operations; ER = erosion; SCI = soil
conditioning index; REV = residue equivalent value; MR = maintenance
residue; CT = conventional tillage; NT = no tillage; LT = limited
tillage.

Table 5 Pearson correlations of the soil conditioning index with
selected study variables.

                              Particulate                    Residue
                   Aggregate  organic      Nitrogen  Carbon  equivalent
Source             stability  matter       mass      mass    value

SCI Pearson         0.47       0.57         0.41      0.29    0.68
  correlation, r
SCI                 0.0006     0.0002       0.002     0.037  <0.0001
  probability > r
Number of          49         38           51        51      51
  observations
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Article Details
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Author:Zobeck, T.M.; Crownover, J.; Dollar, M.; Van Pelt, R.S.; Acosta-Martinez, V.; Bronson, K.F.; Upchurc
Publication:Journal of Soil and Water Conservation
Article Type:Report
Geographic Code:1USA
Date:Nov 1, 2007
Words:10120
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