Introduction
Soil is a key component of the Earth system as it controls the geochemical,
biological, erosional and hydrological cycles and offers services, goods and
resources for human kind (Keesstra et al., 2012; Brevik et al., 2015; Decock
et al., 2015; Smith et al., 2015). Soils also play an important role in
global food security, water security, biofuel security and human
health (Brevik et al., 2015; Keesstra et al., 2016). However, many soils are
under threat and unable to fulfil the food demand due to loss of soil
fertility, erosion, drought and climate change (Muluneh et al., 2015;
Tsozué et al., 2015; Mwango et al., 2016; Potopová et al., 2016;
Singh et al., 2016). This situation might worsen due to increased population
pressure on soil worldwide and thus enhance the degradation of soil.
Moreover, soil degradation is due to intensive cropping, overgrazing, and
unsustainable land use, and desertification further aggravates the soil,
making it unfavourable for cropping (de Moraes Sá et al., 2015; Symeonakis et al.,
2016; Yan and Cai, 2015). There is a need to find solutions to improve the
crop yield. It is important to know the detrimental effect of intensive
agricultural practices as well as their interaction with different kind of
soils to ensure the security of food (Beyene, 2015).
Soils in tropical and subtropical regions undergo a natural acidification
process due to intensive weathering and leaching under hot and humid climate
conditions (Krug and Frink, 1983; Adams, 1984; Ulrich and Sumner, 1991). In
the initial stage, prolonged intensive leaching and abundant precipitation
deplete cations (especially base cations such as Na+, K+,
Ca2+, and Mg2+) adsorbed on negatively charged sites of soil
particles and then the leached ions are replaced by protons (H+)
originating from H2O, H2CO3, or organic acids (van Breemen et
al., 1984). The exchangeable H+ ions on soil minerals are reactive and can
dismantle the mineral lattices by reacting with structural Al3+. The
releases of Al3+ ions from mineral structure occupy some soil cation
exchangeable sites to form exchangeable Al3+ (Reuss and Johnson, 1986;
Huang, 1997). Therefore, exchangeable Al3+ is the main form of
exchangeable acidity in acidic soils (Yu, 1997). The rate of soil
acidification process is generally very slow under natural conditions.
However, in recent decades, various anthropogenic activities have
accelerated soil acidification to a great extent. Acid deposition resulting
from air pollution is a major cause for increased soil acidity (Reuss and
Johnson, 1986; Blake et al., 1999). At present, acid deposition is still a
serious factor that accelerates soil acidification in China (Vogt et al.,
2006; Zhao et al., 2009). Soil acidification can also be accelerated by
applying excessive NH4+- or R-NH2-based fertilizers (Bolan et
al., 1991; Malhi et al., 1998; Xu et al., 2002; Schroder et al., 2011).
Under the intensive land use in China, the sharp increase in application of
N fertilizer in crop systems has greatly accelerated soil acidification in
the last three decades (Guo et al., 2010).
Soil acidification is a serious process of agricultural land degradation,
which leads to the decrease in soil pH and the increase in soil acidity
(Behera and Shukla, 2015). Soil acidity is a principal obstacle for crop
production in many regions of the world (Sumner and Noble, 2003).
Approximately 30 % of the world's total land area consists of acid soils
and it has been estimated that over 50 % of the world's potential arable
lands are acidic (von Uexküll and Mutert, 1995). There are 203 million km2 of acid soils distributed in tropical and subtropical regions of
southern China and account for about 21 % of arable land in the country
(Hseung and Li, 1990). This huge area is needed for crop production to meet
the demand of food. Intensive use of land for agriculture and clearing of
vegetation for fuel further aggravate the degradation process by declining
fertility of soil and changing dynamics of phosphorus (Wu and Tiessen,
2002). Typically, acidic Ultisols are low in organic matter content, cation
exchange capacity and high in Al concentration, which makes the soils more
susceptible to acidification.
In acidic soils, Al toxicity to plants and soil infertility are the main
limiting factors for crop growth (Adams, 1984; Kochian, 1995; Ulrich and
Sumner, 1991; Kidd and Proctor, 2000; Eimil-Fraga et al., 2016; Elisa et
al., 2016). Soil acidity directly affects crop growth through acidic
reactions and shows indirect effects on crop growth by affecting nutrient
availability. The concentrations of cations such as Al and Mn are high
enough to be toxic to plants in acid soils, and the solubility of Al and Mn
increases with increasing soil acidity (Pavan et al., 1982; Robson, 1989).
On the other hand, N, K, S, Ca, Mg, Mo, and P are deficient in acid soils
when the soil pH falls below 5.5. For these reasons, the majority of crop
plants produce yields less than their potential. It is well documented that
acid soils possess toxic concentrations of Al3+ and Mn2+,
deficient concentrations of P, and a low availability of bases, which
together cause a reduction in crop yield (Adams, 1984; Robson, 1989;
Schroder et al., 2011).
The issue of soil acidification is of principal concern when considering the
sustainable agricultural crop production system. Liming of acid soils can
increase soil pH and alleviate Al toxicity to plants and thus maintain a
suitable pH for the growth of a variety of crops (Slattery and Coventry,
1993; Mullen et al., 2006; Lollato et al., 2013; Mamedov et al., 2016). To
establish which acid soils need to be ameliorated for plant growth and the
target status of soil acidity after amelioration, the parameters of critical
soil pH and soil Al concentration must be determined, and methods to achieve
this need to be developed.
Some initial properties of the two Ultisols from Hunan and Anhui.
Location
Soil pH
Organic matter
CEC
Exchangeable
Exchangeable
(Soil : water = 1 : 2.5)
(g kg-1)
H+
Al3+
(cmol(+) kg-1)
Hunan
4.06
15.5
13.5
0.40
6.41
Anhui
3.97
18.1
15.5
0.40
4.88
CEC: cation exchange capacity.
The threshold or critical soil pH value, defined as the highest soil pH
level at which the addition of liming materials increases plant growth, as
well as yield, varies among soil types, plant species, and cultivars of the
same plant species (Adams, 1984; Rhoads and Manning, 1989). To advise
growers on the need for liming, the identification of the critical soil pH
for a particular crop species is essential (Adams, 1984). The development of
crop varieties with an Al tolerance for a particular locality a critical
soil pH is also crucial for plant breeders. The critical soil pH and KCl
extractable Al for the same crop (wheat, sunflower, sorghum, and canola)
varies with soil types and even between different cultivars within the same
crop species (Kariuki et al., 2007; Lofton et al., 2010). The tolerable soil
pH of winter wheat is 5.5 or lower, although this depends on the soil and
weather characteristics, and crop growth failure usually occurs at a soil pH
of 4 (Lollato et al., 2013). It is very important to know the effects of a
wide range of soil pH values on crop growth. Ultisols are acidic and humid
in nature and contain a high level of Al. It is believed that Al toxicity is
a serious agricultural problem in Ultisols in southern China. However, there
have been few investigations on the critical pH and Al concentration of
these Ultisols reported for various crops. There has been a growing interest
in wheat and canola crops in China, and due to the combination of these above
factors it is essential to investigate the critical soil pH and Al
concentration for southern China. Therefore, the objective of this study was
to investigate the critical soil pH and Al tolerance for wheat and canola
crops using two Ultisols collected from Hunan and Anhui provinces, China.
Materials and methods
Site and soil characteristics
The two Ultisols used in this study were collected from cropland areas in
Qiyang, Hunan province (26∘45′12′′ N, 111∘52′32′′ E), and
Langxi, Anhui province (31∘6′ N, 119∘8′ E), China in
2015. Some of the initial soil properties are given in Table 1. The soil
samples were collected from the top soil layer (0–15 cm), air-dried, and
finally ground to pass through a 2 mm sieve.
Both Ultisols were derived from Quaternary red earth. Ultisols derived from
Quaternary red earth are widely distributed in subtropical regions of
southern China. The profile depth of this type of soils is normally more
than 2 m or, sometimes, deeper than 10 m (Hseung and Li, 1990).
The clay content in the soils was more than 40 %. Langxi, Anhui province,
is located in the northern part of subtropical region in China. The average
annual rainfall and temperature are 1300 mm and 15.5 ∘C at this
sampling site. Qiyang, Hunan province, is located in the middle part of
subtropical region in China. The average annual rainfall and temperature are
1431 mm and 18 ∘C at this sampling site. The greater precipitation
and higher temperature at Qiyang led to higher weathering extent of the
Ultisol from this site than that from Langxi. Therefore, the cation exchange
capacity (CEC) of the Ultisol from Langxi is greater than that of the
Ultisol from Qiyang (Table 1).
Incubation experiment to obtain the target soil pH
A soil incubation experiment was executed for each location before
conducting the pot culture to achieve the target soil pH level. To determine
the actual amount of quick lime (Ca(OH)2) and aluminum sulfate
(Al2(SO4)3) needed to reach a given target soil pH level, a
soil incubation experiment in the laboratory was conducted to establish a
standard curve. Briefly, 100 g air-dried and 2 mm ground soil was placed in
a plastic cup and mixed with five incremental rates (0.1, 0.2, 0.3, 0.4, and
0.5 g) of Ca(OH)2 and Al2(SO4)3. The soils were then
moistened with distilled water, with a field capacity of 60 %, and placed
under a polyethylene cover containing a hole. After 2 weeks, soil pH was
measured. The relationships between soil pH and the amounts of Ca(OH)2
and Al2(SO4)3 were established.
Treatments, experimental design and pot culture
In this study, two pot experiments were conducted in a controlled
environment and different soil pH gradients were considered as a treatment.
There were seven target soil pH levels ranging from 3.7 to 6.5 (i.e. 3.7,
4.0, 4.5, 5.0, 5.5, 6.0, and 6.5) for the Ultisol from Hunan, and six target
soil pH levels ranging from 3.97 to 6.5 (i.e. 3.97, 4.5, 5.0, 5.5, 6.0, and
6.5) for the Ultisol from Anhui. Each treatment was replicated three times
and for the experimental design we used a complete randomized design. In
each pot, 550 g soil from either Hunan or Anhui was amended with
Ca(OH)2 and Al2(SO4)3 to obtain the target soil pH
levels. After mixing the soil with Ca(OH)2 or Al2(SO4)3,
the samples were incubated at 25 ∘C. The mixtures were pulverized
every 5 days to mix the Ca(OH)2 and Al2(SO4)3 with
the soil. The field capacity of the incubated soil was maintained at about
60 % throughout the 15-day incubation period.
Wheat (Scout 66) and canola (Qinyou 11) were used as test crops in this
study. The seeds of both crops were surface sterilized with 10 %
H2O2 for 10 min, washed with running tap water, distilled
water, and then allowed to germinate without light at 25 ∘C in
distilled water. After 15 days of soil incubation, eight 1-day germinated
seeds of wheat in the Ultisol from Hunan and nine seeds in the Ultisol from
Anhui were sown at the same depth in each pot. In the case of canola crops,
eight 1-day germinated seeds were sown in each pot and after coming out the
seedlings from soil were thinned to five plants. Both crops were grown in a
controlled environment growth chamber (Percival, Perry, IA, USA) with 60%
field capacity, day/night temperatures of 20/15 ∘C, a day length
of 14 h, light intensity of 400 µmol photon m-2 s-1, and
day/night relative humidity of 70/60 %.
Relationship between soil pH and KCl extractable Al
(cmol kg-1). The fitted equations were significant at
P < 0.01.
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Plant growth parameters
All the crop growth components were measured after 28 days. Plant height was
measured using a ruler with an error of ±0.1 cm. The chlorophyll
content (SPAD value) was measured using a SPAD-502 plus chlorophyll meter
(Konica Minolta Sensing, Tokyo, Japan). Shoots and roots were harvested
separately, washed with running tap water and then distilled water, and
finally dried in a forced-air oven at 80 ∘C to constant weight and
weighed.
Soil analysis
After the crop harvest, soil samples were collected from each pot, air-dried,
and finally ground to pass through a 0.3 mm sieve. Soil pH was determined
with a pH combination electrode in a 1 : 2.5 soil : water suspension. The
total soil exchangeable acidity (H+ and Al3+) was extracted with
1.0 M KCl and then titrated by 0.01 M NaOH to pH 7.0 (Pansu and
Gautheyrou, 2006). The exchangeable Al3+
was the difference between exchangeable acidity and exchangeable H+
(Bertsch and Bloom, 1996).
Data analysis
Data were analysed using OriginPro 2015 software. To attain the critical
points, piecewise models were evolved using a nonlinear curve fitting
procedure. The Levenberg–Marquardt method was used for the segmented linear
function (PWL2).
Results and discussion
Relationship between soil pH and exchangeable Al
The range of KCl extractable Al was from 8.49 to 0.09 cmol kg-1 for the
Ultisol from Hunan and from 4.98 to 0.06 cmol kg-1 for the Ultisol from
Anhui, respectively (Fig. 1). There were differences in the Al content
between the two Ultisols at a given pH. For example, at pH 4.5 the
concentration of exchangeable Al was 3.0 and 2.30 cmol kg-1 for the
Ultisols from Hunan and Anhui, respectively. This was probably due to the
different soil types and other soil chemical properties, such as the organic
matter content and CEC of the soils.
There was an inverse exponential relationship between soil pH and KCl
extracted exchangeable Al for both soils. The concentration of exchangeable
Al decreased with increased soil pH, which was consistent with both
theoretical prediction and previous reports (Evans and Kamprath, 1970;
Chartres et al., 1990; Kariuki et al., 2007). With a decrease in soil pH,
more Al ions were released from the soil mineral structure and occupied the
exchangeable sites on soil surfaces, thus increasing soil exchangeable Al
(Yu, 1997). Therefore, the relationship between soil pH and exchangeable Al
was quiet strong for both Ultisols, and the coefficient of the correlation
was 0.95 for both soils.
Plant heights of wheat and canola as a function of soil pH of the
Ultisols from Hunan and Anhui. The fitted equations were significant at
P < 0.01.
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Commonly, Al3+ is missing in soils with pH 5.3 or upper. However, the
exchangeable Al3+ was still detected above pH 5.3 in present study. This
may be due to the indirect method used, in which the exchangeable Al3+
was the difference between exchangeable acidity and exchangeable H+
(Bertsch and Bloom, 1996).
Plant heights of wheat and canola as a function of KCl extracted
exchangeable Al of the Ultisols from Hunan and Anhui. The fitted equations
were significant at P < 0.01.
![]()
Effect of soil acidity on plant height
Wheat plant height was adversely affected by soil acidity. The range of
plant height was 4.55 to 30.67 and 9.37 to 30.52 cm for the Ultisols from
Hunan and Anhui, respectively (Fig. 2). There was a negative response of
plant height to the decreased soil pH. The plant height was also affected by
the soil Al concentration. With the increased soil exchangeable Al
concentration, the plant height was decreased. The breaking point was the
threshold soil pH and exchangeable Al concentration, which was obtained by
two intersected linear lines. For the Ultisol from Hunan, the breaking point
occurred at pH 5.23. On the other hand, the threshold soil pH was at 4.66
for the Ultisol from Anhui. The breakpoints for the exchangeable Al
concentration were detected at 0.56 and 2.56 cmol kg-1 for the Ultisol
from Hunan and Anhui, respectively (Fig. 3). We can also calculate the
critical Al concentration from Fig. 1 based on the critical soil pH. It was
0.90 and 1.72 cmol kg-1 for the Ultisol from Hunan and Anhui,
respectively. Therefore, 0.56 and 1.72 cmol kg-1 were determined as the
critical Al concentration for wheat in the two Ultisols.
Canola plant height ranged from 3.2 to 6.21 and 2.48 to 6.22 cm for
the Ultisol from Hunan and Anhui, respectively (Fig. 2). The critical soil
pH obtained from Fig. 2 was 5.65 for the Ultisol from Hunan and 4.87 for the
Ultisol from Anhui. The breaking point of exchangeable Al was 2.72 cmol kg-1 for the Ultisol from Anhui, and no critical point was found from
Fig. 3 for the Ultisol from Hunan.
Dry weights of plant shoots and roots of wheat and canola as a
function of soil pH of the Ultisols from Hunan and Anhui. The fitted
equations were significant at P < 0.01.
![]()
The results of a comparison between the two soils indicated that there was a
different threshold soil pH and exchangeable Al concentration in wheat and
canola production. This was probably due to the different Al content in the
soil as well as the cation exchange capacity. The plant root system is
affected by high Al concentrations because Al interferes with the uptake,
transport, and utilization of essential plant nutrients such as P, K, Ca,
Mg, and water, as well as enzyme activity in the roots (Lofton et al., 2010).
Wallace and Anderson (1984) reported that DNA synthesis in plant roots was
inhibited by Al and was followed by root elongation. Due to the lower cation
exchange capacity and higher Al content of the Ultisol from Hunan, compared
with the Ultisol from Anhui at the same soil pH, the threshold soil pH
differed and was higher for the Ultisol from Hunan. Moreover, the results
also indicated that the critical soil pH values for canola in two Ultisols
were higher than these for wheat in the same soils, which suggested that
canola was more sensitive to soil acidity than wheat.
Dry weights of plant shoots and roots of wheat and canola as a
function of KCl extracted exchangeable Al of the Ultisols from Hunan and
Anhui. The fitted equations were significant at P < 0.01.
![]()
Effect of soil acidity on the dry weight of shoots and roots
Soil acidity had a negative impact on the biomass dry weight of the wheat
and canola crops. The range of wheat shoot dry weights for the Ultisols from
Hunan and Anhui was 0.03 to 0.78 and 0.12 to 1.10 g, respectively (Fig. 4). Similar to plant height, shoot dry weight increased with the increased
soil pH. The reverse trend was observed in the case of soil exchangeable Al.
Shoot dry weight was enhanced with the reduced soil Al concentration. At a
soil pH of 5.27, the breaking point was obtained for the Ultisol from Hunan.
In contrast, the breaking point for the exchangeable Al concentration was
0.65 cmol kg-1 in the same location. On the other hand, the threshold
soil pH was at 4.66 for the Ultisol from Anhui, but there was no breaking
point for exchangeable Al. A negative linear response was identified with
increased soil exchangeable Al (Fig. 5).
Similar to plant height and shoot dry weight, there was a negative impact of
soil acidity on wheat root dry weight. The root dry weight for the Ultisols
from Hunan and Anhui at the different soil pH gradients was 0.04 to 0.89 g
and 0.07 to 0.97 g, respectively (Fig. 4). Root dry weight increased with an
increase in soil pH in both locations. At a soil pH of 4.99, the breaking
point was reached for soil from Hunan. In contrast, for soil from Anhui, the
breaking point was observed at a soil pH of 4.66. Root dry weight decreased
with an increase in exchangeable Al for both locations (Fig. 5). At Hunan,
the breaking point was found at 2.27 cmol kg-1 of exchangeable Al,
while the breaking point was 2.39 cmol kg-1 for soil from Anhui.
Canola shoot growth had also a negative response to soil acidity. Shoot dry
matter yield ranged from 0.09 to 0.34 g for the Ultisol from Hunan and 0.04
to 0.39 g for the soil from Anhui (Fig. 4). The critical soil pH of Hunan
and Anhui was 5.14 and 4.57, respectively. This indicates that there was a
strong relationship between soil pH and shoot dry weight. The shoot dry
weight was reduced at lower soil pH due to soil acidity for both the
Ultisols. A negative linear response was observed with the increased soil
exchangeable Al for Hunan. The threshold point of soil exchangeable Al at
2.71 cmol kg-1 was identified for Anhui (Fig. 5).
Canola root dry matter yield ranged from 0.02 to 0.16 g for Hunan and 0.01
to 0.13 g for Anhui, respectively (Fig. 4). For Hunan, the critical soil pH
was obtained at 5.30 in the case of root dry weight. On the other hand, at soil
pH 4.86, the breaking point for Anhui was found. Root dry matter yield was
greatly affected by soil exchangeable Al for both the Ultisols. At Hunan,
the response of root biomass yield to Al concentration followed a negative
linear trend, with higher Al concentration resulting in higher reduction in
root dry matter yield. A threshold point for soil exchangeable Al was
acquired at 2.72 cmol kg-1 for Anhui (Fig. 5).
Similar to plant height, the threshold pH of the Ultisol from Hunan was
higher than for the Ultisol from Anhui. This was probably due to the high Al
concentration as well as the low cation exchange capacity in Hunan soil.
Because Al interferes with root growth and then nutrient and water uptake,
plant growth was reduced at a lower soil pH due to the high solubility of
Al, and ultimately plant shoot dry weight was also reduced at a lower soil
pH. A previous study conducted by Joris et al. (2013) reported that the
density of root length, shoot biomass, grain yield, and the nutrition of
corn were increased due to the reduction of soil acidity through liming.
Poolpipatana and Hue (1994) reported that the dry matter yield of legume
crops was decreased at lower soil pH values due to the presence of a high Al
concentration. These findings are in agreement with those of our study.
Leaf chlorophyll contents (SPAD value) of wheat and canola as a
function of soil pH of the Ultisols from Hunan and Anhui. The fitted
equations were significant at P < 0.01.
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The primary and most evident symptom of Al toxicity is that the root growth
of plants decreases (Rengel and Zhang, 2003), which reduces the plant uptake
of nutrients from soils. Watanabe et al. (2006) reported that the absence of
phosphate due to the presence of Al decreased the weight of roots. These
findings are consistent with the results of this study, in which the dry
matter yield of roots was reduced at high Al concentrations. Low soil pH
with high concentration of Al showed adverse effects on roots of both crops.
Stunted, thick, bent, brownish roots, deformed root tips, and no or very few
lateral roots were observed in our pot experiments.
Leaf chlorophyll contents (SPAD value) of wheat and canola as a
function of KCl extracted exchangeable Al of the Ultisols from Hunan and
Anhui. The fitted equations were significant at P < 0.01.
![]()
Effect of soil acidity on chlorophyll content
As well as the growth components, the chlorophyll contents in wheat and
canola leaves were also affected by soil acidity. Wheat leaf chlorophyll
content (SPAD value) range for the Ultisols from Hunan and Anhui was from
8.4 to 37.8 and 10.1 to 46.2, respectively, for the different soil pH
treatments (Fig. 6). At a soil pH of 5.29, the breaking point was achieved
for the Ultisol from Hunan location. For Anhui, at a soil pH of 4.66 a
linear plateau was found, which indicated that there was little response in
the chlorophyll content at higher soil pH values. At Hunan, the threshold
soil exchangeable Al was 1.85 cmol kg-1, while for Anhui it was found
at 2.36 cmol kg-1 (Fig. 7).
The range of chlorophyll content (SPAD) in the leaf of canola varied from
20.4 to 35.6 for the Ultisol from Hunan, whereas it was 24.1 to 36.0 for the
Ultisol from Anhui (Fig. 6). The threshold soil pH was detected at 4.60 for
the Ultisol from Hunan. In contrast, the critical soil pH was observed at
4.86 for the Ultisol from Anhui. The breaking point for soil exchangeable Al
was 3.82 and 4.56 cmol kg-1 for the two Ultisol from
Hunan and Anhui, respectively (Fig. 7). However, these values of soil
exchangeable Al were too high for canola growth and cannot be set as the
critical soil exchangeable Al for canola.
The presence of Al in plant tissues interferes with Ca and Mg uptake from
soil, as well as damaging the chloroplast and mitochondrial membrane
(Meriño-Gergichevich et al., 2010). The results of this study suggest
that the chlorophyll content in leaves was lower at a lower soil pH and
higher at a higher soil pH. Zhang et al. (2007) also found that chlorophyll
content in leaves was reduced due to the presence of a high Al concentration
in soils, which confirms the findings of this study.
General discussion
The critical soil pH and Al concentrations were different for the same crop
at different Ultisols. The growth of canola will not be affected at or above
soil pH of 5.65 and 4.87 for Hunan and Anhui, respectively. On the other
hand, wheat crop will not be damaged by acidity at or above soil pH of 5.29
and 4.66 for Hunan and Anhui, respectively. The difference was also found for
critical exchangeable Al for wheat varied from 0.56 cmol kg-1 in Hunan
to 1.72 cmol kg-1 in Anhui. The differences of critical values between
two locations were mainly due to the difference in soil CEC. The CEC of the
Ultisol from Anhui was greater than that from Hunan (Table 1). Thus, at the
same exchangeable Al level, the Al saturation (percentage of exchangeable Al
in CEC) was lower at the Ultisol from Anhui than that at the Ultisol from
Hunan, while the base cation saturation (percentage of exchangeable base
cation in CEC) was higher at the Ultisol from Anhui than that at the Ultisol
from Hunan. Base cations can alleviate Al toxicity to plants
(Meriño-Gergichevich et al., 2010; Liu and Xu, 2015). Therefore, the
higher CEC and greater base cation saturation of the Ultisol from Anhui led
to the lower critical values of soil pH and the higher exchangeable Al in the Ultisol
from Anhui compared with that from Hunan. A similar relationship between plant
growth and soil Al saturation was observed by other investigators (Lollato et
al., 2013).
The critical values of soil pH and Al content varied with crop species.
Canola was more sensitive to soil acidity than wheat and thus has higher
critical soil pH in both locations than wheat. Canola was also more
sensitive to Al toxicity and less tolerant to toxic Al. This may be the
main reason why the critical soil Al contents were not obtained for canola in
present study. The critical soil pH and Al values varied with soil types and
crop species and thus the two parameters obtained in this study cannot be
extended for other crops or the same crops for other soil types.
In the present study, the critical soil pH and Al levels for wheat and canola
were obtained with pot experiments in only one crop season. Better Al and
pH levels in a soil should be reasoned considering a crop rotation and not
only one crop. Thus, better Al and pH levels will be defined for the more
sensitive crop in the crop rotation adopted in future.
Soil pH and Al are important indicators of soil quality assessment in acidic
Ultisols. Soil quality assessment is a large and challenging issue due to its high
variability in properties and functions. According to Brevik and Sauer (2015), soil has a distinct impact on human health. The
availability of food and contamination with various chemicals and pathogens
from human input are influenced by soil. However, priority should be given to
developing new technologies for maintaining soil quality not only for
productivity but also human health (Zornoza et al., 2015). According to our
results and findings, the critical values of soils vary among both
locations for a particular crop. Different crop species have different
sensitivity to soil acidity. These obtained critical values are only for
specific soil types and crops. It is suggested that liming should be done
according to the critical values for the growth of same species in different
soil types. Hence, site-specific agricultural management practices including
liming can be applied judiciously with proper crop selection, provided these are
economically as well as environmentally sound. Judicious application of lime
is necessary in order to protect not only the soil from degradation but also human
health.