Agricultural research communication centre

Soil acidity: 
Plant growth and most soil processes, including
nutrient availability and microbial activity, are favoured by
a soil pH range of 5.5 – 8. When pH of the soil declines,
aluminium solubility in soil solution increases which is toxic
to plants as a result of which root growth is retarded. In
India, nearly 25 million hectares of cultivated lands with pH
less than 5.5 are critically degraded. The productivity of these
soils is very low (one tonne/ha) due to deficiencies of P, Ca,
Mg, Mo and B and toxicities of Al and Fe (IISS Vision 2030).
Low pH in top layer of the soil may affect microbial
activity, significantly decreasing legume nodulation. Most
leguminous plants require a neutral or slightly acidic soil
for growth. Soil acidity limits symbiotic nitrogen fixation
by reducing the chances of 
Rhizobium
survival and its
persistence in soils, and ultimately reducing nodulation in
legumes (Taylor 
et al,
1991). In most of the cases, a pH
sensitive stage in nodulation occurs early in the infection
process and that 
Rhizobium
attachment to root hairs is one
of the stages affected by acidic conditions in soils. At or
below pH 4.8, aluminium will reduce root growth while
manganese disrupts photosynthesis and other functions of
growth, resulting in the reduction of nitrogen fixation by
rhizobia (Zahran, 1999)
Temperature: 
High soil temperature is one of the most
critical factors which can prevent the development of a
symbiotic association between the host plant and micro-
symbiont especially in arid and semi-arid regions. For most
of rhizobia, optimum temperature range for growth is 28 to
31
o
C and majority of them are unable to grow at 38
o
C
(Graham, 1992). Temperature influences the survival of free
rhizobia as well as the molecular dialogue between host and
rhizobia. Elevated temperature can have inhibiting effect on
microsymbiont adherence to root nodules, on nodule
structure and on legume root nodule functioning (Zahran,
1999).
Rupela and Rao (1984) reported that with increase
in the temperature to 35
o
C there was significant decrease in
nodule number as well as nodule dry mass which may be
attributed to drastic decrease in photosynthesis at high
temperature and hence reduced supply of photosynthates to
Rhizobium
. These findings are in line with that of Jain 
et al
(2014) who also revealed the negative effects of high
temperature on nitrogen fixation in chickpea. However, it
has been reported that when plant is subjected to high
temperature conditions specific set of proteins are over
produced, termed as heat shock protein, which are important
for survival during temperature stress conditions (Rodrigues,
2006).