Soil acidity has become a principal constraint in dry land
crop production systems of acidic Ultisols in tropical and subtropical
regions of southern China, where winter wheat and canola are cultivated as
important rotational crops. There is little information on the determination
of critical soil pH as well as aluminium (Al) concentration for wheat and
canola crops. The objective of this study is to determine the critical soil
pH and exchangeable aluminium concentration (Al
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
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 km
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 Al
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.
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.
The two Ultisols used in this study were collected from cropland areas in
Qiyang, Hunan province (26
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
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)
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)
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 %
H
Relationship between soil pH and KCl extractable Al
(cmol kg
All the crop growth components were measured after 28 days. Plant height was
measured using a ruler with an error of
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
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).
The range of KCl extractable Al was from 8.49 to 0.09 cmol kg
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
Commonly, Al
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
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
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
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
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
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
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
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
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
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
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
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
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
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.
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
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.
The results of this study demonstrate that wheat and canola growth were
significantly reduced at low soil pH values and high Al concentrations.
Plant height, shoot dry weight, root dry weight, and chlorophyll content in
leaves were significantly decreased below the critical soil pH. A negative
correlation was found between plant growth parameters and soil exchangeable
Al. Plant height, shoot dry weight, root dry weight, and the chlorophyll
content in leaves were decreased below the threshold soil Al concentration.
The critical soil pH and Al concentration differed between locations as well
as crop species. At the Hunan site, the critical soil pH and Al
concentration for wheat were 5.29 and 0.56 cmol kg
The data are not publicly available due to copyright issues. However, the data set can be obtained from the corresponding author through e-mail (rkxu@issas.ac.cn).
M. Abdulaha-Al Baquy and Ren-Kou Xu designed the experiments and M. Abdulaha-Al Baquy carried them out. M. Abdulaha-Al Baquy and Ren-Kou Xu prepared the manuscript with all co-authors.
The authors declare that they have no conflict of interest.
This study was supported by the National Key Basic Research Program of China (grant number: 2014CB441003) and the National Natural Science Foundation of China (grant number: 41230855). The first author gratefully acknowledges the Chinese Academy of Sciences – The World Academy of Sciences President's Fellowship for his PhD studies in China. Edited by: A. Jordán Reviewed by: A. Cerdà and two anonymous referees