The CSHA indicators are grouped into physical, chemical, and biological properties. The chemical indicators consist of pH and nutrients (P, K, Mg, Mn, Zn, and Fe). The cc had a variable effect on the chemical attributes in both site-years. Soil pH was greater with cc as compared to no-cc, which might be related to the role of microbial processes and cc residue decomposition on the hydrolysis of organic N (Vanzolini et al. 2017). Similar results of cc on soil pH have been reported in other studies (Duval et al. 2016; Yan et al. 2006). As per OMAFRA guidelines, range of soil pH for growing processing tomatoes in south-west Ontario is 5.0-6.5 (Garton et al. 1994), which was also observed for the cc treatments in site-year 2015. Unlike 2015, in site-year 2016, the average pH was greater (7.05) than that recommended for processing tomato production by Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA). However, similar range of soil pH (6.5-7.0) in south-western Ontario has been reported in other studies (Belfry et al. 2017; Ouellette et al. 2015; Van Eerd et al. 2015) and is not found to limit crop growth. The Zn and Mn availability is influenced by soil pH, but the response of Mn and Zn concentration was not observed to be similar as soil pH with cc in both site-years. Differences in Mn concentration due to cc were detected even when soil pH was not significant (Table 1). This result contrasted with Congreves et al. (2015) and Van Eerd et al. (2014) where similar effect of management on pH, Zn and Mn index was observed. Soil P concentration was not affected by cc. The cc can absorb and convert available P to organic forms leading to a reduction or no effect in its concentration. Villamil et al. (2006) and Hargrove (1986) found lower soil P concentration in rotations with cc than without cc, whereas Eckert et al. (1991) observed no consistent effect on soil P with cc.
Physical indicators of CSHA included aggregate stability, AWC, surface and sub-surface hardness. Chen and Weil (2010) reported that OSR can penetrate deeper soil layers due to their tap root system than grass cc, such as oat and cereal rye; resulting in alleviation of soil compaction and hardness. Our results of surface hardness contrasted with Pagliai et al. (2004), where greatest surface crusting and hardness was observed under no residue control plots. If the surface hardness measurements were taken in June instead of September, more apparent and clear differences might have been detected from cc due to greater variation in soil moisture. No differences were observed in sub-surface hardness from cc. Due to the continuous use of machinery and plowing, soil can become compacted and can lead to sub-surface hardness (Birkas et al. 2004). Roper et al. (2017) observed no effect of management (tillage) on surface and sub-surface hardness. The cc had greater aggregate stability than no-cc, which might be attributed to the (i). ability of cc in protecting soil surface from the raindrop effect leading to a reduction in soil erosion, and (ii). an increase in soil C concentrations from the cc residues (Blanco-Canqui et al. 2015) resulting in an increase in the soil fungal and bacterial communities affecting aggregate stability (Le Guillou et al. 2012). Roper et al. (2017) observed no differences due to the management on aggregate stability. The AWC was found to be greatest with cereal rye than other cc tested, which might be related to reduced soil evaporation due to residue cover (Dabney 1998), and improvement in soil physical properties, such as porosity (Villamil et al. 2006), hydraulic conductivity, water retention capacity (Blanco-Canqui et al. 2015; Kiesling et al. 1994), with the inclusion of cc.
Active C, OM, respiration, API are the biological indicators of CSHA. Active C is different from soil respiration and total soil organic C as it represents the easily and readily accessible C. The cc had greater values of the afore-mentioned soil biological attributes than no-cc, indicating the medium-term impact of cc on soil biology. Similar improvements in soil microbial activity with cc than no-cc has also been reported in other studies (Blanco-Canqui et al. 2015; Dabney et al. 2001; Jackson 2000). Role of cc in improving soil organic C concentration in the long-term has also been documented (Blanco-Canqui et al. 2013, 2015; Poeplau and Don 2015; Olson et al. 2014). Greater concentration of soil biological indicators with addition of residues has also been reported by Roper et al. (2017).