Phosphorus - Insoluble inorganic compound of phosphorus are largely unavailable to plants, but many microorganisms can bring the phosphate into solution. This attribute is apparently not rare since one-tenth to one-half of the bacterial isolates tested usually are capable of solubilizing calcium phosphates and count of bacteria solubilizing insoluble phosphates may range from 10^5 to 10^7 per gram. Such bacteria are often especially abundant on root surfaces. Species of Pseudomonas, Mycobacterium, Micrococcus, Bacillus, Flavobacterium, Penicillium, Sclerotium, Fusarium, Aspergillus, and other are active in the conversion.
These bacteria and fungi grow in media with Ca3(PO4)2, apatite or similar insoluble materials as sole phosphates sources. Not only do the microorganisms assimilate the element but they also make a large portion soluble, releasing quantities in excess of their own nutritional demands. If the insoluble phosphate is suspended in an agar medium, the responsible strains are readily detected by the zone of clearing produced around the colony. The solubilization is not restricted to calcium salts for iron, aluminum, magnesium, manganese and other phosphates are acted on also.
The major microbiological means by which insoluble phosphorus compound are mobilized is by the production of organic acids. In the special case of the ammonium and sulfur oxidizing chemoautotrophs, nitric and sulfuric acids are responsible. The organic or inorganic acids convert Ca3(PO4)2 to di-and monobasic phosphates with the net result of an enhanced availability of the element to plants. The amount brought into solution by heterotrophs varies with the carbohydrate oxidized, and the transformation generally proceeds only if the carbonaceous substrate is one converted to organic acids.
Nitric or Sulfuric acids produced during the oxidation of nitrogenous materials or inorganic compounds of sulfur react with rock phosphate, thereby effecting an increase in soluble phosphate. The oxidation of elemental sulfur is a simple and effective means of providing utilizable phosphates. For example, a mixture may be prepared with soil or manure, elemental sulfur and rock phosphate as the sulfur is oxidized to sulfuric acid by Thiobacillus, there is a parallel increase in acidity and a net released of soluble phosphate. Nitrification of ammonium salts also leads to a slight but significant liberation of soluble phosphorus from rock phosphate compost. Biological sulfur or ammonium oxidation has never been adopted on a commercial scale because of the availability of cheaper and more efficient means of preparing fertilizers.
Although phosphate solubilization commonly requires acid production, other mechanism may account for ferric phosphate mobilization. In flooded soil, the iron in insoluble ferric phosphate may be reduced, a process leading to the information of soluble iron with a concomitant release of phosphate into solution. Such increase in the availability of phosphorus on flooding may explain why rice cultivated under water often has a lower requirement for fertilizer phosphorus than the same crop grown in dry land agriculture. Phosphorus may also be made more available for plant uptake by certain bacteria that liberate hydrogen sulfide, a product that reacts with ferric phosphate to yield ferrous sulfide, liberating the phosphate. The many phosphate dissolving microorganism in the vicinity of roots may appreciably enhance phosphate assimilation by higher plants.
These bacteria and fungi grow in media with Ca3(PO4)2, apatite or similar insoluble materials as sole phosphates sources. Not only do the microorganisms assimilate the element but they also make a large portion soluble, releasing quantities in excess of their own nutritional demands. If the insoluble phosphate is suspended in an agar medium, the responsible strains are readily detected by the zone of clearing produced around the colony. The solubilization is not restricted to calcium salts for iron, aluminum, magnesium, manganese and other phosphates are acted on also.
The major microbiological means by which insoluble phosphorus compound are mobilized is by the production of organic acids. In the special case of the ammonium and sulfur oxidizing chemoautotrophs, nitric and sulfuric acids are responsible. The organic or inorganic acids convert Ca3(PO4)2 to di-and monobasic phosphates with the net result of an enhanced availability of the element to plants. The amount brought into solution by heterotrophs varies with the carbohydrate oxidized, and the transformation generally proceeds only if the carbonaceous substrate is one converted to organic acids.
Nitric or Sulfuric acids produced during the oxidation of nitrogenous materials or inorganic compounds of sulfur react with rock phosphate, thereby effecting an increase in soluble phosphate. The oxidation of elemental sulfur is a simple and effective means of providing utilizable phosphates. For example, a mixture may be prepared with soil or manure, elemental sulfur and rock phosphate as the sulfur is oxidized to sulfuric acid by Thiobacillus, there is a parallel increase in acidity and a net released of soluble phosphate. Nitrification of ammonium salts also leads to a slight but significant liberation of soluble phosphorus from rock phosphate compost. Biological sulfur or ammonium oxidation has never been adopted on a commercial scale because of the availability of cheaper and more efficient means of preparing fertilizers.
Although phosphate solubilization commonly requires acid production, other mechanism may account for ferric phosphate mobilization. In flooded soil, the iron in insoluble ferric phosphate may be reduced, a process leading to the information of soluble iron with a concomitant release of phosphate into solution. Such increase in the availability of phosphorus on flooding may explain why rice cultivated under water often has a lower requirement for fertilizer phosphorus than the same crop grown in dry land agriculture. Phosphorus may also be made more available for plant uptake by certain bacteria that liberate hydrogen sulfide, a product that reacts with ferric phosphate to yield ferrous sulfide, liberating the phosphate. The many phosphate dissolving microorganism in the vicinity of roots may appreciably enhance phosphate assimilation by higher plants.
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