4.3. Soil quality
The causes for low or higher cane yield across the cane producing zones of Uttar Pradesh varied, but changes in nutrient index, microbial activity and soil quality might be the main reasons (Kang et al., 2005). About 25 physical, chemical and biological soil attributes were used to set up a minimum data set (MDS) based on higher factor loading value sort out by using PCA. These attributes have been used by several researchers (Bhaduri & Purakayastha, 2014; Masto et al., 2007; Qi et al., 2009) for selecting potential indicators to assess soil quality as each could affect crop productivity. Among them, Na, SOC, AZO and Sa were emerges as common quality indicators across the cane producing zones, but their contribution towards SQI development were fluctuated which depend upon the management factors, site specific characters and ecological factors. Na is one of the most crucial factors for achieving sustainable cane productivity, but it is most limiting nutrient across the cane producing zones as 98.2% soil samples showed low categories (<280 kg N ha-1) (Brady & Well, 2000). Na contributes 13.5 and 4.67% more in CZ soils than WZ and EZ towards SQI development linked to greater Na, Pa and Ka, AZO counts, urease activity and normal pH range. Our results are in line with the results of Landell et al. (2003) and Otto et al. (2017) as significant strong positive correlation of Na with Pa, Ka, AZO, β-glucosidase and urease activity (Table 3S). SOC was retained as quality indicators across the cane producing zones due to its greater factor loading values than other soil variables. In addition, SOC is the most extensively used as a soil quality indicator owing to its multifunction role including filtering and buffering, physical stability and support, nutrient recycling and as primary food source for microorganisms (Carter, 2002; Gregorich et al., 1994). The lowest SOC contribution in WZ soils due to 57.4% soil samples falls in low category than EZ (32.8%) and CZ (56.6%). The contribution ofAzotobacter counts were 5.01, 14.1 and 9.11% towards the SQI development in EZ, CZ and WZ, respectively. Fluctuations in their contribution related to changes in edaphic variables that alter by following management practices, and ecological condition. The lowestAzotobacter contribution in EZ soil linked to low O2 availability in low-lying areas as highly aerobic in nature and very sensitive at low O2 levels, hence affected their growth and activity. Esmailpour et al. (2013) suggest that Azotobacter is free living-N2 fixer that have the ability to fix ⁓20 kg N ha-1 per y-1 or per season thereby synthesis their cell protein notably in N-deficient soil, which mineralized after the death of their cells (Wani et al., 2016). Fe was pondered as soil quality indicators in EZ and CZ cane producing zones which contributed 8.70 and 6.50% towards SQI development, respectively. Because, EZ soils prone to water-logging resulting decreasing redox potential resulting excess electron changes from Fe3+ and Mn4+ to Fe2+ and Mn2+ (Singh & Setter, 2017). Ka was also retained as highly weighted factor in EZ and WZ, but contributed more in EZ soils (11.6%) than WZ (6.98%) in SQI development owing to 87.8 and 28.7% soil samples falls under low K category (<120 kg ha-1), respectively. Imbalance fertilization and excess K removal from soil through surface runoff, leaching and plants acquisition are the causes of more K-deficiency in EZ than WZ soil. However, it could not surface as quality indicators in CZ soils due to more K availability (Alemayehu, 2010; Shukla et al., 2009; Kingston, 2014). Available P was retained in EZ, whereas ALP were chosen in CZ and WZ as quality marker in MDS, and their share were 8.28, 10.7 and 6.66% towards SQI development, respectively. Since, Pa has been accepted as the limiting factor in EZ soil due to 74.8% soil samples showed its deficiency (<10 kg P ha-1), but current Pa levels in CZ (27.8 kg ha-1) and WZ (18.7 kg P ha-1) are not in the deficiency range. Hence, greater P is required for cane growth notably in wherein excess aerial root occurs for their survival under water logging conditions (Tarafdar & Claassen, 1988). Landon (1984) suggested that sugarcane is a moderate P demanding crops, but its deficiency occurs only if Olsen P levels <7 ppm, whereas for K, the critical value ranges from 78-125 ppm (Filho, 1985). The values obtained under this study were above the critical levels both in CZ and WZ due to trash mulching practices are common in cane production system that return P and K back to soil, and thus mitigated exhaustion of these nutrients below critical levels. However, P and K levels below the critical levels in EZ soils might be excess P and K losses through surface-runoff in EZ soils (Khan and Qasim, 2008). Similarly, ALP retained as weighted variables in CZ and WZ despite having medium to high P levels as it played a vital role in enhancing Pa by secreting phosphomonoesterases enzymes by enhancing microbial activity. Nakas et al. (1987) reported that soil bacteria, fungi and fauna responsible for secreting extracellular phosphatases due to their large combined biomass, high metabolic activity and short lifecycles. Variations of ALP towards SQI developments are related to management practices, changes in hydrothermal conditions, microbial activity and nutrient availability in soil. Olander &Vitousek (2000) and Deng et al. (2017) attributed that high N fertilization accelerated phosphatase activity and P mobilization, since N is used as an essential nutrient for the synthesis of soil enzymes. Sa was chosen as the representative of stability function for the MDS in each EZ, CZ and WZ as 43.3, 58.8 and 27.9% soil samples falls in low categories (<10 mg kg-1 soil), respectively. The Sa contribution were 5.01, 5.67 and 7.06% towards the SQI in EZ, CZ and WZ soil, respectively indicated their importance in each cane producing zones. Contribution of Sa was greater in WZ over CZ and EZ soil directly linked to more S availability in WZ soil. Soil available Zn was retained as MDS in WZ due to 47.5% soil samples falls under low Zn categories (<0.6 mg kg-1) along with higher factor loading value. Casarin et al. (2001) report that in low fertile soil, which are exploited for many years without Zn fertilization, the occurrence of its deficiency becomes more aggravated as sugarcane frequently exhibits “hidden hunger” a situation in which the symptoms do not appear visually (Orlando Filho et al., 2001). Thus, zinc fertilization mainly in Zn-deficient sugarcane grown soils might be important to maintain adequate Zn level for affording higher internodes, top growth, stalk length and cane yields (Alloway, 2008). BSR is the most used attributes as biological indicators of soil, since, it produces through decomposition of microbial biomass which is the most active living part of the SOM formed mainly by bacteria, fungi and actinomycetes (Kaschuk et al., 2009). Hence, BSR retained as soil quality indicators in CZ and WZ soils due to higher factor loading value in respected PCs. BSR contributed 5.11 and 6.42% towards the SQI development in CZ and WZ soils, respectively. The WZ soils had more BSR thereby producing more biomass, MBC and microbial activity (Galdos et al., 2009).