Soldier Fly Food Scrap Leachate | A Treasure Trove Amended in Soilby Terry Green on 03/04/13
We previously described the value of recycling fermented food scrap leachate wastes into soil, especially that processed with Black Soldier fly larvae (see Amending Soil with Leachate Recovered from Black Soldier Fly Processed Food Scrap). A very significant advantage of recycling complex organics back into soil is the improvement in soil fertility which occurs through formation of humic substances from food scrap and agricultural wastes returned to soil. This blog elaborates on how Black Soldier fly larval processing of food scrap leachate enhances the retention of plant available nitrogen and inorganic phosphate in soils.
Amending soil with N-P-K fertilizers and micronutrients does not address soil deficits associated with harvesting plants from soil. Soil also needs carbon, especially in the form of humus byproducts. These are produced from vegetal matter as it undergoes degradation. Humic substances in soil improve the efficiency of N-P-K fertilizers and enhance micronutrient uptake by plants through cation and anion exchange processes, and through chelated complexes, presenting nutrients in complexed forms that can be more readily taken up and used by plants while not washing out from soil. Humic substances vary in color from reddish brown to black. They are structurally highly complex polyphenolic and carboxylic acid molecules of varying molecular weights, making up the major fraction of organic matter in healthy soils. If you would like to read more about humic substances and their role in maintaining healthy soil, there are many excellent reviews on this subject available through the internet (see, for example, Benefits of Soil Organic Matter and Humus).
The appearance of humic substances is evident in leachates recovered from food scrap processed with and without Black Soldier fly larvae (Fig. 1). Without Black Soldier fly larval processing of the food scrap, the leachate is relatively acidic (average pH, 3.8) due to the action of acetogens on the food scrap and accumulation of organic acids in the leachate fraction. With Black Soldier fly processing of food scrap waste, the pH of the leachate becomes more alkaline as the larvae consume organic acids while also boosting its ammonia content (see Using Black Soldier Fly Larvae for Processing Organic Leachates).
Fig. 1. The appearance of humic-like substances in leachate from decomposing (fermented) food scrap. (A) without larval processing; (B) with processing using Black Soldier fly larvae. Copyright © 2013, Terry Green, All rights reserved.
Since Black Soldier fly larvae are known to accelerate the decomposition of organic wastes, it is not surprising that the average concentrations of ammonia and phosphorous recovered in decaying food scrap are higher in processing food scrap wastes using Black Soldier fly larvae (Table 1). Whether or not Black Soldier fly larvae are used in processing food scrap, approximately 30% of the total ammonia (present in leachates in its protonated form, an ammonium ion carrying a net positive charge) binds tightly with suspended solids also present in the leachate fraction. Both this bound form and that remaining in solution, upon amendment in soil, bind avidly to the soil particles, and get taken up by plants on conversion to nitrate, the principal plant available form of nitrogen used by plants.
Black Solider fly processing of food scrap also causes precipitation of upwards of 92% of the mineralized phosphate recovered in the leachate (Table 1). This mechanism of removing mineralized phosphate from the soluble fraction through processing of food scrap leachate using Black Soldier fly larvae may be attributable to the formation of insoluble hydrated salts comprised of calcium phosphate, for example brushite , hydroxyapatites and/or magnesium ammonium phosphate (struvite) deposits associated with increased accumulation phosphate in combination with the release of bound forms of calcium and magnesium present in food scrap wastes, increases in ammonia formation and progressive alkalinization of the leachate fraction.
Immobilizing phosphate in this manner before amending it into soil can markedly diminish problems associated with eutrophication (leaching from soils) of phosphates which otherwise can lead to the induction of algae and other microbial blooms, stagnation and fish die offs in situations where phosphates are allowed to flow freely from soil into natural waterways. On the other hand, the enhanced level of ammonia recovered in the leachate fraction with larval grazing and processing of the leachate is reminiscent of similar observations on the role of grazing microbes in elevating the availability of nitrogen bound humic matter which has been observed to serve as a nitrogen pool in supporting the growth of coastal phytoplankton (see Nitrogen bound to humic matter of terrestrial origin - a nitrogen pool for coastal phytoplankton?). Just as humic matter washing from organic matter falling into river ways, fjords and bays serves a natural role in providing for coastal growth of phytoplankton through its ability to bind and carry trace minerals and essential nutrients needed for the growth of phytoplankton into coastal waterways, its delivery into soil, aided by Black Soldier fly processing of decaying wastes, serves to improve the available nutrient supply and fertility of soils.
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