Agriculture Reference
In-Depth Information
purchasing power. Consequently, soils in these regions are highly depleted in P (Tiessen,
2005), and any solution to increasing yields in SA and SSA must address P nutrition.
Plant rhizospheres support large populations of soil microorganisms, and a number of
bacteria ( Pseudomonas spp. and Bacillus spp.) and fungi (most notably mycorrhizae but also
Aspergillus and Penicillium ) have been shown to aid P availability to plants. Microorganisms
can enhance P availability by (1) solubilization of inorganic P by acidification (e.g., produc-
tion of organic acids, carbonic acid, or H + ) or chelation; (2) mineralizing organic P by pro-
ducing extracellular phosphatases or phytases (phytate constitutes ~50% of soil organic P);
and (3) increase root growth or root hair development by production of phytohormones
(Jakobsen et al., 2005). A considerable number of organisms have been identified to have
P enhancement characteristics. However, if the work was done in the lab via screening
or even under field conditions, unless there are specific measurements to determine that
P deficiency was reduced or eliminated, a growth response may be due to other factors
(Whitelaw, 2000). Nonetheless, there are a significant number of studies that have shown
improved P nutrition and yields on a wide range of crops by inoculating with P-enhancing
microorganisms (see reviews by Whitelaw, 2000; Leggett et al., 2001; Jakobsen et al., 2005).
Besides the expected response on P-deficient soils, studies have shown P improve-
ment with P-enhancing microorganisms in the presence of rock phosphate (low-soluble
material that is used to manufacture P fertilizer) (Barea et al., 2002). This is important for
SA and SSA farmers as it may be more cost effective (indigenous sources of rock phos-
phate exist in SA and SSA that can be mined and used directly), and on high P-fixing
soils, it can have longer residual effects that last several years on acid soils (see review by
Sahrawat et al., 2001).
Symbiotic fungi that have a long research history that is worthy of special consider-
ation are mycorrhizal fungi. For crop species, endomycorrhizae form arbuscular structures
in close association with host cells; the plant provides the carbohydrates to the fungus.
The fungus in turn develops an extensive hyphal network that can transport P and other
nutrients to the plant (Smith and Read, 1997). This factor greatly increases the volume of
soil for the plant to explore; also, mycorrhizae can mobilize P by excreting phosphatases
and organic acids (Whitelaw, 2000). It should also be noted that coinoculation of plants
with mycorrhizal fungi and phosphate-solubilizing or -mineralizing microorganisms (so-
called helper microorganisms, many of the same ones mentioned) can further increase
the plant P response and therefore the effectiveness of mycorrhizal fungi (see review by
Jakobsen et al., 2005).
In conclusion, it is evident that microorganisms are an integral component of the soil P
cycle, and they can mobilize soil P and increase yields. However, exploitation of microbial
processes to increase P uptake by crops has had limited success (Jakobson et al., 2005). To
achieve optimal benefits, the following are needed:
1. Identification and genetic characterization and manipulation of microbial species
that significantly enhance P plant nutrition by either inoculation or general stimu-
lation in the rhizosphere; in particular, there is a need to identify the P-enhancing
genes and their role or interactions with plants in affecting P nutrition;
2. Basic understanding of microbial ecology and signaling between plant roots and
P-enhancing microorganisms;
3. Development of effective, low-cost, and simple inoculation technologies that can be
utilized by SA and SSA farmers;
4. Determination of microbial ecology of P-enhancing organisms and management strat-
egies that enable use of rock phosphate to eliminate P deficiency of SSA and SA crops;
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