Amending Soil with Black Soldier Fly Processed Food Scrap Leachate : The Life and Times of BSF (Black Soldier Flies)
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Amending Soil with Black Soldier Fly Processed Food Scrap Leachate

by Terry Green on 02/24/13

The ability of Black Soldier fly larvae (BSF, H. illucens) to mineralize and assimilate food scrap and agricultural wastes presents a tremendous economic and environmental opportunity to improve the recycling of organic wastes in an earth friendly and sustainable manner (see Black Soldier Fly Processing of Biodegradable Wastes and Black Soldier Fly Larvae | An Earth Friendly Feedstock?). Leachates from processed food scrap and agricultural waste can be applied, diluted in tap water, to plants and top soil (see Black Soldier Fly Processed Food Scrap | Foliant and Soil Applications and Are Waste Products in Your Recycled Food Scrap Inhibiting Plant Growth?). This blog discusses how to calculate the nutrient load delivered to soil on amending leachates directly into soil.

To calculate the amount of nutrient delivered, consider the chemical makeup and characteristics of the leachate. The most important chemical constituents in leachates affecting soil quality upon amendment in soil are plant available nitrogen (ammonia), inorganic phosphate, residual total polysaccharides (water soluble sugars and sugar polymers) and suspended solids (nonvolatile particulates, water insoluble minerals and salts). Of these, plant available nitrogen in the chemical forms of ammonia and nitrate or of foremost importance.

Healthy soil nitrate levels can vary depending upon soil type and location, but generally lie between  10 and 30 ppm (parts per million). Levels below about 10 ppm indicate a deficiency in plant available nitrogen. Levels in excess of about 30 ppm can be problematic because the excess nitrate easily flushes out of the soil during rainstorms, and following irrigation, finding its way into natural waterways where it poses an ecological hazard in promoting excess growth of water-born microorganisms, especially algae, depletion of dissolved oxygen, stagnation of waterways and fish die offs.

Although plants readily take up ammonia, soil microorganisms convert it to nitrate which plants also take up. So the amount of ammonia amended into soil from a practical perspective can be considered the equivalent to that of plant available nitrate. There are a number of internet sites where one can learn more about how ammonia is taken up and bound in soil. Excellent information on the internet is also available on the fate of ammonia and its use in stimulating plant growth and application rates (see, for example, NitrificationWhat Happens to Anhydrous Ammonia in Soil, and Soil Test Interpretation Guide). Having this information available, and information on the ammonia content in the leachate, figuring out the amount of leachate to be applied to soil to achieve a desired nutrient load becomes straightforward.

Start by looking closely at the chemical composition of leachates recovered from decomposing food scrap waste. Use this information to calculate how much leachate to amend into soil in raising the ppm of nitrate a known amount. For example, measurements summarized in Table 1 compare the pH, and concentration of ammonia (present as NH4+ ion in solution) in leachate recovered from food scrap with and without exposure to BSF larvae, and corresponding carbonate (CO3-2), inorganic phosphate (PO4-3), total polysaccharide (water soluble sugars and sugar polymers), and total suspended solids (nonvolatile particulates, water insoluble minerals and salts) levels. The data shows that BSF larval processing of food scrap leachate increases its ammonia content 2-fold, reduces its concentration of soluble polysaccharides by +50%, and decreases its residual nonvolatiles by roughly 4-fold.

Table 1. Changes in Chemical Composition of Food Scrap Processed by BSF Larvae

Leachate processed with BSF, combined with the rise in ammonia, accounts for the sharp rise in its pH from an average value of 3.8 (about the acidity of vinegar) to a much more alkaline pH of 8.5, and simultaneous trapping of carbon dioxide (captured in solution as carbonate). Food scrap processed directly by BSF larvae feeding on the waste however produces a leachate intermediate in pH value between these two extremes (average leachate pH,  ~6.5).

Table 2 shows (in units of gal/acre and gal/sq ft) how much BSF processed food scrap leachate should be amended into soil to a depth of one foot to increase the  plant available nitrogen in the soil 1, 5 and 10 ppm (part per million), respectively. This calculation assumes that the BSF processed leachate amended into the soil is comparable in chemical makeup to that shown in Table 1.

Table 2 image of Calculated Soil N Application Rate for Black Soldier Fly Leachate
Since other constituents in the leachate are present in a fixed ratio that can be measured relative to its ammonia content, the loading of the other constituents in soil (in this case phosphate, polysaccharides and total residual organics and minerals) can also be calculated by multiplying the volume of leachate applied for ammonia per unit area soil by the measured concentration of each of the other constituents.

The volume of leachate in gal/acre (foot depth) to be amended into soil to raise available plant nitrogen by 1 ppm in soil based on the concentration of ammonia in the leachate can be calculated by dividing 437.4 by the concentration of ammonia (use units expressed in mg/ml ammonia in solution) measured in the leachate to be applied to soil. The conversion factor assumes 3.65 lbs/acre ammonia is necessary to raise the N content by 1 ppm and incorporates corresponding mg-lbs and gal.-ml conversion units to simplify the calculation so that all one needs to do is divide the concentration of ammonia in units of mg/ml into the factor, 437.4, to arrive at the calculated volume of leachate needed to achieve a rise by 1 ppm N in soil applications. In Table 2, for example, the value of 322 gal/acre was obtained by divding 437.4 by 1.36 mg/ml ammonia (see Table 1, 80 mM NH4+ equals 1.36 mg/ml ammonia).

To calculate the lbs/acre (foot depth) of any other constituent present in the leachate carried into soil with ammonia, multiply the concentration of the constituent of interest in the leachate (expressed in mg/ml) times 0.00835, and multiply this number by the volume (in gallon units) of leachate calculated per acre obtained for amendment of ammonia into the soil. The conversion factor of 0.00835 factors in unit conversions involved in the calculation between mg/ml and ml/gal.

Table 1 also shows that BSF larvae feeding of food scrap leachate transformed approximately 92% of the mineralized inorganic phosphate into a water insoluble state of considerable interest from an environmental perspective in minimizing phosphate leaching from soils into natural waterways. Additionally, approximately 30% of the mineralized ammonia recovered in the food scrap leachate is bound up in an insoluble form. Overall these observations on the properties of food scrap leachates indicate that it is extremely unlikely that their amendment in soil poses any significant risk in overloading soil with nitrogen or phosphorous of concern regarding potential contamination of waterways or runoff problems.

While it is possible to calculate how much leachate can be applied to incrementally increase available plant nitrogen and phosphorous (or any other nutrient measured in leachates) recycled in soil, a particularly interesting aspect of  recycling leachates derived from food scrap and agricultural plant debris in soils and applied as a foliar spray to plants is that much smaller quantities of the leachate fraction, diluted upwards of 20 to 100 fold in tap water, markedly affect increased growth of plants (see, for example, Black Soldier Fly Processed Food Scrap | Foliant and Soil Applications). Given the very low application rates relative to the effect leachate treatments have on plants, it is likely that the leachate is providing other beneficial effects to the soil ecology and plant than serving solely in the capacity of providing nutrient nitrogen, or phosphorous.

It is probable that leachate applications serve to improve the efficiency of nutrient uptake by plants, possibly also synergistically improving soil microfloral activity. In this regard, water soluble humic substances present in leachates derived from decomposing food scrap and agricultural debris may account  for some of the increased efficiencies in the uptake of nutrients by plants and better soil ecology apparent on incorporating leachates back into soil. We will be following up more on these observations in a blog to follow.

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