BIOFERTILIZERS AS COMPLEMENTS TO SYNTHETIC AND ORGANIC FERTILIZATION - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 158

biocontrol activities against insects (Azevedo et al. , 2000) and nematodes (Hallmann et al. ,
1997; Ryan et al. , 2009) that are harmful to plants.

b) Phytostimulators or Biostimulators

This category includes microorganisms that promote plant growth usually by hormonal
action. It has been elucidated that the main mechanism of the growth promoting bacterium
Azospirillum brasilense is related to its ability to stimulate root growth through the production
of indole acetic acid (IAA; Mascarua-Esparza et al. , 1988 ). In addition to its effective
capacity for fixing nitrogen, Gluconacetobacter diazotrophicus can produce IAA (Fuentes-
Ramírez et al., 1993; Bastian et al. , 1998) and cytokinins (Jiménez-Salgado et al., 1994),
which could magnify the growth promoting activity of this bacterium. Likewise, some species
of the genera Pseudomonas, Azotobacte r and Bacillus release indole acetic acid, gibberellins
and cytokinins into the rhizosphere of plants, exerting a growth stimulation which is specially
marked at the seedling stage of plants (Lugtenberg & Kamilova, 2009).
Vessey (2003) considers that although biopesticides can stimulate plant growth by
controlling pathogenic organisms, they do not have a direct effect on the nutritional status of
plants and, therefore, should not be considered as biofertilizers. Based on this premise, some
authors propose to differentiate the plant growth promoting bacteria (PGPB) into biocontrol-
PGPB's and PGPB's depending on whether or not exhibit antagonism against pathogenic
organisms (Bashan & Holguin, 1997).
Vessey (2003) also mentions that biofertilizers should include any microorganism that
promotes plant growth by increasing the supply of primary nutrient availability to the host
plant, either by promoting the resupplying of soil nutrients ( e.g. through biological nitrogen
fixation), increasing the availability of nutrients ( e.g. solubilization of phosphates) or
extending the physical access of the plants to these nutrients ( e.g. increasing the volume or
changing root architecture).
Under this definition, biofertilizers should include microorganisms incorporating new
nutrients into the plant-soil system as well as microorganisms increasing the uptake and
availability of nutrients. However, this raises a problem because many organisms have more
than one mechanism to promote growth and can exhibit complementary biocontrol activities
(Raupauch & Kloepper, 2000; Manjula & Podile, 2001; Guo et al. , 2004). Based on these
premises, some authors propose to use the term in a broader sense to include any biological
product (or microorganism) capable of promoting plant growth regardless the mechanism
used for this purpose; this is the meaning used for ―biofertilizer‖ in this chapter of the topic.
The benefits of using biofertilizers in agriculture include:

 Increased capacity of plants to absorb water and nutrients from the soil.

 Reduced demand of irrigation and fertilization doses in crops.

 Increased growth and seedling establishment.



Increased rooting of cuttings.



Increased vigor of seedlings and adult plants.

 Biocontrol of pathogens.

 Reduced time of harvest (in some cases between 7 and 9 days) and extension of the

productive time of crops (Dibut & Martínez, 2004).