9.2.1.2 Phosphate (P) Nutrient
Phosphorus is an important element and limiting factor for plant growth in various
agroecosystems. Phosphorus is as main constituents in the metabolic process and
physiochemical reactions such as photosynthesis, transformation electron (energy
transfer), signal transduction, biosynthesis macromolecules, and respiration (Khan
et al. 2010). In peat soil, phosphorus is available to plants only after mineralization
because most part of them are in the form of an organic compound. The other
problem is that the mobility of phosphorous is very poor due to strong fixation in the
acidic peat environment. As Rashid et al. (2016) reported, phosphorus is abundant in
the soil in the form of organic and inorganic compounds, but mostly is in unavailable
form or immobilized. In fact, inorganic P makes a stable complex with calcium, iron,
and aluminum ions. Because of the ability of microorganisms to produce enzymes or
organic acids to solubilize the P complex, the plants can use them. According to
Sharma et al. (2013), PGPR has a function to solubilize phosphate through the
following ways: (1) releasing complex organic compounds such as organic acid
anions, protons, hydroxyl ions, and CO 2 ; (2) producing extracellular enzymes such
as phosphatase or phytase; and (3) releasing of phosphate during substrate
decomposition.
9.2.1.3 Potassium (K) Nutrient
Potassium (K) is one of the determinants of soil fertility, and its availability in
mineral soil is greatly helped by the presence of microbes that can dissolve K among
clay layers in which K binds to the phyllosilicate structure (Shelobolina et al. 2014).
PGPR is able to increase the availability of potassium by increasing the solubility of
K-containing rocks through the production and secretion of organic acids (Parmar
et al. 2016; Han and Lee 2005; Sheng and He 2006). Furthermore, Meena et al.
(2014) suggested that K among clay is solubilized by lowering the pH, enhancing
chelation of the cations bound to K, and acidolysis. Meena et al. (2015) reported that
some bacteria species of Bacillus, Burkholderia, Arthrobacter sp., Enterobacter,
Paenibacillus, P. frequentans, Cladosporium, Aminobacter, Sphingomonas, and
Paenibacillus were able to solubilize K from the silicate rocks. This process plays
a very important role in mobilizing and dissolving K from an insoluble form and
structurally in an unavailable form to become available and dissolved in the soil so
that the impact increases the availability of K in the plant root environment (Abouel-Seoud and Abdel-Megeed 2012).
However, as peat contains very low K, the bacteria function on K solubilization is
not so important in peat, or rather K deficiency affects bacteria activity and plant
growth. Thus, K-requiring plants such as sago palm can grow in a brackish water
area (seawater contains high K
+
), near a river bank (mineral soil contains high K
+ in
clay layers), and even in peatland.
9 Plant Growth-Promoting Rhizobacteria (PGPR) and Compost Materials for. . .
305
Phosphorus is an important element and limiting factor for plant growth in various
agroecosystems. Phosphorus is as main constituents in the metabolic process and
physiochemical reactions such as photosynthesis, transformation electron (energy
transfer), signal transduction, biosynthesis macromolecules, and respiration (Khan
et al. 2010). In peat soil, phosphorus is available to plants only after mineralization
because most part of them are in the form of an organic compound. The other
problem is that the mobility of phosphorous is very poor due to strong fixation in the
acidic peat environment. As Rashid et al. (2016) reported, phosphorus is abundant in
the soil in the form of organic and inorganic compounds, but mostly is in unavailable
form or immobilized. In fact, inorganic P makes a stable complex with calcium, iron,
and aluminum ions. Because of the ability of microorganisms to produce enzymes or
organic acids to solubilize the P complex, the plants can use them. According to
Sharma et al. (2013), PGPR has a function to solubilize phosphate through the
following ways: (1) releasing complex organic compounds such as organic acid
anions, protons, hydroxyl ions, and CO 2 ; (2) producing extracellular enzymes such
as phosphatase or phytase; and (3) releasing of phosphate during substrate
decomposition.
9.2.1.3 Potassium (K) Nutrient
Potassium (K) is one of the determinants of soil fertility, and its availability in
mineral soil is greatly helped by the presence of microbes that can dissolve K among
clay layers in which K binds to the phyllosilicate structure (Shelobolina et al. 2014).
PGPR is able to increase the availability of potassium by increasing the solubility of
K-containing rocks through the production and secretion of organic acids (Parmar
et al. 2016; Han and Lee 2005; Sheng and He 2006). Furthermore, Meena et al.
(2014) suggested that K among clay is solubilized by lowering the pH, enhancing
chelation of the cations bound to K, and acidolysis. Meena et al. (2015) reported that
some bacteria species of Bacillus, Burkholderia, Arthrobacter sp., Enterobacter,
Paenibacillus, P. frequentans, Cladosporium, Aminobacter, Sphingomonas, and
Paenibacillus were able to solubilize K from the silicate rocks. This process plays
a very important role in mobilizing and dissolving K from an insoluble form and
structurally in an unavailable form to become available and dissolved in the soil so
that the impact increases the availability of K in the plant root environment (Abouel-Seoud and Abdel-Megeed 2012).
However, as peat contains very low K, the bacteria function on K solubilization is
not so important in peat, or rather K deficiency affects bacteria activity and plant
growth. Thus, K-requiring plants such as sago palm can grow in a brackish water
area (seawater contains high K
+
), near a river bank (mineral soil contains high K
+ in
clay layers), and even in peatland.
9 Plant Growth-Promoting Rhizobacteria (PGPR) and Compost Materials for. . .
305