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).