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This webpage is an abstract from the Aquaponics list archive

Soil bacteria

Ted,
any ideas why I could have a buildup of phosphorus in my wastewater beds with no apparent phosphorus inputs?? Is it the fact that I am not really doing any bed maintenance etc? (as in leaves falling and rotting in the water?) Any ideas?
My nitrates are the easy ones to get rid of..

Mike


Mike, my assertion is that although insoluble inorganic compounds of phosphorus are largely unavailable to plants, many microorganisms can bring the phosphorus into soluble form. Again, citing Alexander's Intro to Soil Microbiology, this attribute is not rare since up to one half of soil bacterial isolates tested usually are capable of solubilizing calcium phosphates, and the counts of bacteria solubilizing insoluble inorganic forms of phosphorus may be as high as 10^7 (10 million) per gram of soil. Such bacteria are often especially abundant on root surfaces. (Raghu, K. and I.C. MacRae, 1966 Journal of Applied Bacteriology, 29:582-586). Species of Pseudomonas, Mycobacterium, Micrococcus, Bacillus, Flavobacterium, Penicillium, Sclerotium, Fusarium, Aspergillus, and others are active in the conversion of insoluble inorganic phosphorus into soluble phosphorus. Many soil microorganisms, both bacteria and fungi, can grow in culture media with only apatite (ahem),
that is, Ca3(PO4)2, or similar insoluble inorganic phosphorus materals as the sole phosphours sources.

I assert that not only do the microorganisms assimilate the phosphorus but that they also make a large portion soluble, releasing quantities in excess of their own nutritional demands. Which is remarkable and causes some of us prairie pioneer dreamers and Bio-Bards to ponder, on the prairie, such things as grand intelligent designs, and, ergo, Grand Designer(s) with Grand Intelligence. These ponderings can lead the pilgrim to discoveries of rivers of grandeur, or many Rio Grandes, one might say. Yee Haw.

If the inorganic insoluble phosphorus mineral source is suspended in an agar microbiological growth medium, the strains of microbes in the soil responsible for converting the phosphorus to soluble forms can be readily detected by the zone of clearing produced around the colony formed on the agar. This solubilizaton of phosphorus minerals is not restricted to calcium salts of phosphorus. Iron, aluminum, magnesium, manganese, and other phosphorus salts are biologically acted on also.

Alexander writes that the major microbiological means by which insoluble phosphorus compounds are mobilized is by the production of organic acids. Acetic acid or vinegar is an organic acid, for example. In the speical case of the ammonium- and sulfur-oxidizing chemoautotrophs, nitric acid and sulfuric acids are responsible. The organic or inorganic acids convert the Ca3(PO)4 to di-and monobasic phosphate speices with the net result of an enhanced availability of the element to plants. (But only if the bugs have the apatite for it!) The amount of P 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.

The oxidation of elemental sulfur (a substance which is recognized and approved as A-OK in at least some organic certification programs, including the US national program, as I understand it) is a simple and effective means of providing utilizable phosphorus from inorganic insoluble forms of P present in the soil. For example, a mixture may be prepared with soil or manure, elemental sulfur, and rock phosphate, which contains radioactive Polonium, naturally. Anyways, as the sulfur is oxidized to sulfuric acid by Thiobacillus sp., there is an increase in acidity and a the apatite begins to dissolve. So, if you have acid, you begin to lose your apatite, see. The nitrification process by our good buddies Nitosomonas et.al, also leads to a slight but significant liberation of phosphorus from composts to which rock phosphate has been added. Biological sulfur or ammonium oxidation has not been adopted by conventional commercial agriculture because of the availablity of cheap and efficient menas of preparing and applying fertilizer salts to the soil. What do you say about what we already have going for us in an aquaponics gravel bed when you think about that comparitively and by contrast, eh?

Alexander writes that although phosphate solubilization commonly requires acid production, other mechanisms may account for ferric phosphate mobilization. Uh oh... now it is all starting to form a great web of aquaponic science connectivity. Bill Nye's got nothing on us here, so.....let's read on in rapt fascination and curiosity.....In flooded soil, the iron in insoluble ferric phosphates may be reduced, a process leading to the formation of soluble iron with a concomitant release of phosphate into solution (Patrick, , Gotoh, and Williams, 1973. Science, 179:564-565). Such increases in the availability of phosphorus subsequent to flooding a soil, sediment, wetland, (or gravel growing bed?)may explain why rice cultivated under water often has a lower requirement for fertilizer phosphorus than crops grown in dry-land agriculture on the same soil.

Inorganic phosphorus may also be made more available for plant uptake by certain bactera that liberate H2S (hydrogen sulfide), a product that reacts with ferric phosphate to yield ferrous sulfide, liberating the phosphate (Sperber, J.I. 1957, Nature 180:994-995).

Alexander shows data supporting the assertion that the many phosphorus dissolving microbes in the vicinity of plant roots appreciably enhances P assimilation by the plants. He shows that the yield of oats grown in sterile and non-sterile conditions with the additon of phosphorus as ferrophosphate, CaHPO4, Ca3(PO4)2, and bonemeal is consistently greater in the nonsterile plots due to the microbes role in solubilizing the P for the plants.

It is interesting that the rate of P mineralization from the organic forms of P in the soil is "ENHANCED BY ADJUSTING THE pH TO VALUES CONDUCIVE TO GENERAL MICROBIAL METABOLISM, AND A SHIFT FROM ACIDITY TO NEUTRALITY INCREASES PHOSPHATE RELEASE"...from the organic forms of P. Furthermore, the rate of mineralization is directly related to the quantity of substrate; hence soils rich in organic phosphorus will be the most active. The microbial degradation of organic P is not inhibited by inorganic P so that mineralization proceeds rapidly even when the soil has "adequate" phosphate (Daughtrey, Gilliam, and Kamprath. 1973, Soil Science, 115:18-24). The P uptake by plants is correlated with the mineralization rate of P from organic forms of P in the soil (Sekhon and Black , 1968, Plant Soil, 29:299-304).

As you might suspect, there is a correlation between the biologically mediated mineralization rates of Carbon, Nitrogen, and Phosphorus from their organic forms in the soil. The ratio of C:N:P microbial mineralization RATES at an equilibrium condition is similar to the ratio of those 3 elements in the soil humus (Thompson, Black & Zoellner, 1954, Soil Science, 77:185-196). So, if soil ratios of C:P are around 100 or 200 to 1, then rates organic carbon decomposition as measured by CO2 evolved by microbial decomposition of the organic matter, would be about 100 or 200 times the mineralization rate, roughly, of organic P mineralization.

It is interesting how this might play out in aquaponic gravel beds. One point of contrast is that fish feed and aquaculture waste may have different C:N:P ratios than some soils that have been studied. Another interesting point of potential research is that in greenwater systems, or in other systems in which we deliberately play with the protein content of the feed, and thus, subsequently, change the C:N:P ratios in the waste going into an aquaponics biofilter-growbed. Many aquaculture feeds have inorganic phosporus added as dicalcium phosphate, and some feeds have been experimented with that have phytase added to enhance phosphorus availability and thus digestibility for the animals of the plant-based phosphorus (phytic acid or its calcium-magnesium salt, phytin) in the soybean or other plant ingredients in the feed. So, all these things could have interesting consequences or considerations for P mineralization and availability to plants in aquaponic systems, and they could potenially serve as doctoral dissertation topics....for some enterprising young scholars, or even older scholars...ahem...

Later.

Tedzo


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