Relationship among soil properties
The positive correlations among ASA, WASI, IAWHC, and SHC at Froid and Sidney suggest that well aggregated soils can also enhance soil water retention and infiltration capacities. As soil aggregation is related to SOC (Blanco-Canqui and Benjamin 2013), it is possible that increased SOC in enhanced aggregated soil helped to retain more water. Also increased soil aggregation may have increased macroporosity which increased SHC. In contrast, negative correlations among BD, ASA, and IAWHC at Froid suggest that increased soil compaction can reduce slaking resistance of soil aggregates and water holding capacity. Blanco-Canqui and Benjamin (2013) reported that well aggregated soils have lower BD, better consistency, and greater macropores and water retention capacity than poorly aggregated soils. Benjamin et al. (2007) found that soil aggregation was positively correlated to SHC, but negatively correlated to BD. Manrique and Jones (1991) reported that BD was negatively correlated to soil water content. Our result of positive correlation between clay concentration and WASI was similar to that observed by Shaver et al. (2002).
The positive associations among ASA, WASI, WSA, AS, VWC33, IAWHC, and VWCFM at Froid and Sidney suggest that increased soil aggregation enhanced aggregate stability, slaking resistance, and water holding capacity. Water-stable aggregates with enhance stability have higher slaking resistance and water holding capacity and are less erodible than aggregates with reduced stability (Blanco-Canqui and Benjamin 2013). The close association between IAWHC and VWC33 at Froid indicates that water holding capacity increased as soil water content increased. Similarly, the close association between SHC and VWCFM suggests that water infiltration rate increased as soil water content at saturation increased. The negative association between clay concentration and BD suggests that soils with high clay concentration are less dense than soils with less concentration. Similarly, the negative association between clay and sand concentrations at both sites suggests that clay concentration increased as sand concentration decreased.
The positive associations among ASA, WASI, and some nutrients at Froid and Sidney suggest that improved soil aggregation can favor retention and availability of nutrients probably by enhancing nutrient cycling. As soil aggregation is closely related to SOC (Blanco-Canqui and Benjamin 2013; Blanco-Canqui et al. 2011), it is likely that increased SOC that enhanced soil aggregation also favored nutrient cycling. Similarly, the positive associations among IAWHC and some nutrients indicate that increased water holding capacity of soil also may help to retain nutrients that may be leached out of soil profile. Increased clay concentration also may have favored retention of nutrients, as clay concentration was positively associated with nutrients at both sites. Negatively charged clay particles may have attracted cations, helping to retain positively charged nutrients. The BD and SHC had minor associations with soil chemical properties, as soil compaction and water infiltration capacity may be less important for affecting chemical properties and nutrient levels.
Greater number of soil chemical properties were associated with ASA, BD, Clay, IAWHC, SHC, and WASI at Froid than at Sidney (Fig. 2). This suggests that the longer duration of the experiment conducted in coarse-textured soil may have altered both soil physical and chemical properties that were more related to each other than the shorter duration of the experiment conducted in fine-textured soil. This indicates that long duration experiments may be more ideal to evaluate soil physical properties as potential soil health indicators relating to chemical properties than short duration experiments in dryland cropping systems.
The positive associations among ASA, WASI, and most biological and biochemical properties reveal that soil aggregation is related to SOM, microbial abundance and activity, nitrogen mineralization, and enzyme activities. The binding agents from soil organic matter and fungal growth enhance soil aggregation (Blanco-Canqui and Benjamin 2013), as fungi contribute more to soil aggregation than bacteria because of their greater abundance (Hattori and Hattori, 1976; Paul & van Veen, 1978). Some researchers (Blanco-Canqui and Benjamin 2013; Blanco-Canqui et al. 2011; McVay et al. 2006) reported that AS was positively correlated, but BD was negatively correlated to SOM. The increased SOM associated with improved soil aggregation enhances soil microbial biomass and activity and N mineralization (Blanco-Canqui and Benjamin 2013; Rivero et al. 2020). Soil aggregation has been shown to be positively correlated to C mineralization (Liebig et al. 1995; van Es and Karlen 2019), WEC (Veum et al. 2012), and BG activity (Stott et al. 2010). Norkaew et al. (2019) reported that WSA was positively related to SOM, STN, PNM, MAC, but BD was negatively related to these parameters. It is likely that soils get less dense, as SOM is increased, favoring soil biological and biochemical properties. The positive associations among IAWHC, WETN, PNM, WEON, CO2-1d, NO3-N, and PLFA suggest that increased soil water holding capacity can enhance N mineralization and availability by stimulating microbial biomass and activity. Rice and Havlin (1994) reported that soil water content was linearly related to PNM. Similarly, the positive associations among SHC, NO3-N, and WEON indicate that increased water infiltration capacity may enhance N availability by increasing N mineralization through increased aeration. In contrast, the negative associations among clay concentration and ACEP, WEON, and BG indicates that increased clay concentration might inhibit N availability by reducing enzyme activity.
Relationship between soil physical properties and crop yields
The positive relationships among ASA, WASI, IAWHC, clay concentration, and mean crop yields across years suggest that improved soil aggregation, water holding capacity, and fine-textured soils increased crop yields probably by enhancing soil water retention and availability, aeration, nutrient cycling, and root growth, as soil aggregation promotes these variables (Blanco-Canqui and Benjamin 2013; Rivero et al. 2020). Several researchers (Maddoni et al. 1999; Shang et al. 2018) found that soil aggregation and stability as well as soil water content were positively related to crop yields. Our results are in contrast to those reported by Sene et al. (1985) who observed that soil aggregation was negatively related, but SHC was positively related to crop yield. In contrast, the negative relationship between BD and crop yield indicates that increased soil compaction reduced crop yield, likely due to reduced porosity, aeration, water and nutrient movements, and root growth. Many researchers (Abu-Hamdeh 2003; Rubio et al. 2021; Shang et al. 2018) reported that BD was negatively related to crop yield. However, Logsdon and Karlen (2004) demonstrated that BD had no effect on crop yield.
Implications of soil physical properties
The ASA, WASI, and IAWHC were associated with most soil physical, chemical, biological, and biological properties, followed by clay concentration (Figs. 1, 2, and 3). The BD and SHC were minimally associated with soil properties. The ASA, WASI, and IAWHC were also weakly to very strongly related to mean crop yields across years at Froid and Sidney (Figs. 4 and 5). Clay concentration, BD, and SHC were either not related or weakly to strongly related to crop yield.
These results suggest that ASA, WASI, and IAWHC can be regarded as potential soil health indicators that were associated with most soil physical, chemical, biological, and biochemical properties and related to mean crop yields across years. However, it takes long time to measure WASI and IAWHC. Soils need to be oven dried for 2–3 d and aggregates needs to be separated in a stack of sieves in water to measure WASI. Similarly, soil water contents need to be determined at various hydraulic pressures and soils oven dried for several days to determine IAWHC. In contrast, the determination of ASA is simple, rapid, and inexpensive where the slaking resistance of aggregates is measured by a smart phone loaded with SLAKES software that calculates areas covered by aggregates before after slaking in water for 10 min. Therefore, ASA can be considered as a potential soil health indicator under dryland cropping systems in arid and semiarid regions.