Handling Soybean ( Glycine max L.) Under Stress - Crop Improvement: New Approaches and Modern Techniques - page 425

ing different soil processes including microbial activities and nutrient availability
(Joris et al. 2012 ).
In addition to the high H + concentration, the high level of Al can also adversely
affect plant growth under acidic conditions. Under such conditions high Al decreas-
es plant growth by reducing root growth, the photosynthetic ability and competing
with nutrients such as N, Mg, P and Fe. In acid soils the reaction of P with Al and
Fe, results in the precipitation of P compounds and hence significantly decreases P
availability (Akaya and Takenaka 2001 ; Shamsi et al. 2008 ).
Al presence increased the activities of antioxidant enzymes including malondi-
aldehyde (MDA), super oxidase dismutase (SOD) and peroxidase (POD). Shamsi
et al. ( 2008 ) indicated that the differences in soybean genotypes under the high lev-
els of elements such as Al are determined by the root ability to absorb such metals.
Fluctuations in soil acidity can affect the physiological properties such as enzymatic
structure and activities in plants and microbes. Accordingly, neutral pH is the most
optimum acidity for the growth of crop plants and activity of soil microbes (Marti-
nez et al. 2012 ; Bissoli et al. 2012 ).
The most suitable method to adjust high soil hydroxyl concentration is the use
of elemental sulfur inoculated with the chemo-autotrophic bacteria, Thiobacillus
spp. These bacteria are able to acquire the energy necessary for their activities by
oxidizing sulfur, resulting in the production of hydrogen and sulfate ions, and hence
decreasing soil pH. Use of tolerant plant species may also be another alternative to
alleviate acidity stress on plant growth and yield production (Miransari and Smith
2007 ).
Soil acidity can adversely affect the process of N-fixation in leguminous plants
by affecting both the host plant and rhizobium. In a 17-year experiment under acidic
conditions Popescu ( 1998 ) found that pH's less than six decreased soybean yield.
Usually the leguminous family is sensitive to high levels of soil acidity. High soil
acidity decreased root nodulation in white clover (Wood et al. 1984 ) subclover
(Whelan and Alexander 1986 ), pea (Lie 1969 ; Evans et al. 1980 ), cowpea (Keyser
1979 ), alfalfa (Munns 1968 , 1970 ) and bean (Wolff et al. 1993 ; Vassileva et al.
1997 ) even in the presence of high rhizobial population. Soil acidity, lower than
five, inhibits nodule formation (Appunu and Dhar 2006 ).
There are plants, which are able to accumulate high rate of alkaline cations in
their tissues, while their growth and performance remains unaffected. Soybean
plants grow the best at pH's around seven and under high or low acidity their growth
may be adversely affected. Under acidic conditions plants produce lower amounts
of hydrogen ions affecting the uptake of nutrients by plant roots (Marschner 1995 ).
It can also affect the cytoplasmic pH and hence result in the reduction of shoot and
root growth (Schubert et al. 1990 ; Yan et al. 1992 ).
Appunu and Dhar ( 2006 ) evaluated the survival of different strains of acid toler-
ant Bradyrhizobium japonicum under high acidity (pH = 4) and found that they can
tolerate high acidity level. Interestingly, the strains showed higher tolerance to acid-
ity in soil than in YEM broth medium. The slow growing Bradyrhizobium are more
tolerant to acidity than the fast growing rhizobium (Cooper et al. 1985 ; Graham
et al. 1994 ; Miransari and Smith 2007 ; Ferreira et al. 2012 ). All the tolerant spe-