3 Microbial Endophytes: Sustainable Approach …
37
provide phosphorus to crops. The inoculation of crops with phosphate solubilizing
microbes (PSM) has the potential to reduce the application of phosphatic fertilizer
without significantly reducing crop yield (Jilani et al. 2007; Yazdani et al. 2009).
Recently, many phosphate solubilizing endophytes have been characterized from
different plant tissues to develop effective phosphatic biofertilizers. The inoculation
of these endophytes improved P solubilization capacity and resulted in increased
plant growth and crop yield.
3.2 Phosphorus Availability in Soil
Soil phosphorus exists either in inorganic or organic forms (Richardson 2001). Inorganic phosphorus accounts for 35–70% of total soil P (Sharon et al. 2016) and it
occurs mostly in insoluble mineral complexes such as apatite, hydroxyapatite, oxyapatite, mono-, di-, and tricalcium phosphates. The organic compounds making up
the humus fraction are derived from surface vegetation, microbial protoplasm, or
metabolic products of the microflora. The various inositol phosphates or related
substances in the organic matter frequently account for 20–80% of the entire organic
P fraction. Most of the organic P sources are phytin, inositol phosphates, phospholipids, nucleic acids, sugar phosphates, polyphosphates, and phosphonates. Phosphorus held within soil microorganisms constitutes a significant component of the
total soil P and is estimated to account for around 2–10% of total soil P (Achat et al.
2010). Usually, soils rich in organic matter contain abundant organic P. Moreover, a
good correlation exists between the concentrations of organic P, organic C, and total
N. Ratios of organic C to organic P of 100–300:1 are common for mineral soils. The
soil type, soil use, and management strategies differ considerably for a proportion of
different phosphorus fractions (Li et al. 2007).
Besides organic P, large quantities of the inorganic forms of P occur in minerals
where the phosphate is part of the mineral structure, as insoluble calcium iron or
aluminum phosphates (Turan et al. 2006; Vu et al. 2008). Mineral phosphate also
found related to the surface of hydrated oxides of Fe and Al, which are inadequately
dissolvable and assimilable. Inorganic phosphorus (IP) in acidic soil is associated
with Al and Fe compound, though in alkaline soil calcium phosphate is predominant
(Khan et al. 2009). According to the compilation of about 9.6 million soil tests for
available P in Indian soils, it was reported that 49.3% of areas covering different
states and union territories are in a low category, 48.8% in the medium and 1.9%
have high category phosphorus status (Hasan 1994). Therefore, the application of
phosphatic fertilizers is unavoidable in an intensive farming system. The source of
P is only from phosphatic and sulfur rocks, which are non-renewable sources and
the use of phosphatic fertilizers leads to the depletion of these resources. Thus, the
problem of P management in the soil is also very tricky and more than 70–90% of
the applied phosphatic fertilizers get fixed in the soil rendering them unavailable for
plant uptake under the ideal conditions (Holford 1997).
37
provide phosphorus to crops. The inoculation of crops with phosphate solubilizing
microbes (PSM) has the potential to reduce the application of phosphatic fertilizer
without significantly reducing crop yield (Jilani et al. 2007; Yazdani et al. 2009).
Recently, many phosphate solubilizing endophytes have been characterized from
different plant tissues to develop effective phosphatic biofertilizers. The inoculation
of these endophytes improved P solubilization capacity and resulted in increased
plant growth and crop yield.
3.2 Phosphorus Availability in Soil
Soil phosphorus exists either in inorganic or organic forms (Richardson 2001). Inorganic phosphorus accounts for 35–70% of total soil P (Sharon et al. 2016) and it
occurs mostly in insoluble mineral complexes such as apatite, hydroxyapatite, oxyapatite, mono-, di-, and tricalcium phosphates. The organic compounds making up
the humus fraction are derived from surface vegetation, microbial protoplasm, or
metabolic products of the microflora. The various inositol phosphates or related
substances in the organic matter frequently account for 20–80% of the entire organic
P fraction. Most of the organic P sources are phytin, inositol phosphates, phospholipids, nucleic acids, sugar phosphates, polyphosphates, and phosphonates. Phosphorus held within soil microorganisms constitutes a significant component of the
total soil P and is estimated to account for around 2–10% of total soil P (Achat et al.
2010). Usually, soils rich in organic matter contain abundant organic P. Moreover, a
good correlation exists between the concentrations of organic P, organic C, and total
N. Ratios of organic C to organic P of 100–300:1 are common for mineral soils. The
soil type, soil use, and management strategies differ considerably for a proportion of
different phosphorus fractions (Li et al. 2007).
Besides organic P, large quantities of the inorganic forms of P occur in minerals
where the phosphate is part of the mineral structure, as insoluble calcium iron or
aluminum phosphates (Turan et al. 2006; Vu et al. 2008). Mineral phosphate also
found related to the surface of hydrated oxides of Fe and Al, which are inadequately
dissolvable and assimilable. Inorganic phosphorus (IP) in acidic soil is associated
with Al and Fe compound, though in alkaline soil calcium phosphate is predominant
(Khan et al. 2009). According to the compilation of about 9.6 million soil tests for
available P in Indian soils, it was reported that 49.3% of areas covering different
states and union territories are in a low category, 48.8% in the medium and 1.9%
have high category phosphorus status (Hasan 1994). Therefore, the application of
phosphatic fertilizers is unavoidable in an intensive farming system. The source of
P is only from phosphatic and sulfur rocks, which are non-renewable sources and
the use of phosphatic fertilizers leads to the depletion of these resources. Thus, the
problem of P management in the soil is also very tricky and more than 70–90% of
the applied phosphatic fertilizers get fixed in the soil rendering them unavailable for
plant uptake under the ideal conditions (Holford 1997).