Black Soldier Flies & Food Scrap |Putting the Leachate to Good Use : The Life and Times of BSF (Black Soldier Flies)
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Black Soldier Flies & Food Scrap |Putting the Leachate to Good Use

by Terry Green on 09/03/13

Black Soldier fly (BSF) larvae are exceptionally efficient in recycling food scrap and agricultural waste (see Black Soldier Fly Processing of Biodegradable Wastes).  They grow off of nutrients in the waste and are well adapted in chewing and macerating waste materials. They play a critical role in working in synergy with microbes in recycling wastes.  This blog describes observations on applying BSF leachate (the liquid fraction) recovered from processed food scrap to soil and leafy plants.

Fig. 1 shows the effect of amending leachate (0.09 gal/cu ft of soil, Helvetia silt loam) into soil on the soil’s pH, plant available nitrogen, and phosphate levels, respectively, over a one month interval beginning in early Spring.  The pH of the soil remained constant (about 6.5 to 7), and phosphate levels remained mid-range and constant. Control (untreated) soil drawn from the garden was found to be deficient in plant available nitrogen. After adding leachate, plant available nitrogen sufficient to meet the needs of growing plants was restored even as heavy rains washed through the soil  during the Spring when these measurements were made (see Figs. 1A, 1B and 1C).



Fig. 1. Soil pH, and available plant nitrate and phosphate levels, respectively, measured after treating soil in mid-Spring with leachate recovered from BSF processed food scrap. Left set of three of tubes in each series - treated soil; right set of three  tubes - untreated (control) soil; analysis based on Mosser Lee Company’s SoilMaster Soil Testing Kit. (A), left three tubes, test results obtained 1 week after addition of leachate ( 0.09 gal./sq ft mixed into garden soil to a depth of 12 inches); right three tubes, control results; (B) soil tests drawn two weeks later – note markedly improved restoration of nitrate (bright pink-red test result - tube 2 in left  set), and persistent deficiency of nitrate in the corresponding control sample (tube 2 in right set); (C) results one month later confirming persistent availability of nitrate in treated soil (tube 2 in left set), and much weaker albeit detectable nitrate in control soil (tube 2 in right set). Some washout of phosphate from the soil is evident in the control set by the end of the study (tube 3 in right set) For further details see text. Copyright (c) 2013 Terry Green, All rights reserved.   

By the end of April, some plant available nitrogen in the control soil, albeit in trace amounts, was detected (see Fig. 1B and 1C). It is not clear whether this small rise in nitrogen availability was the result of increased mineralization activity occurring as the soil warmed up from the long and cooler winter months, the result of some washout of plant available nitrogen from sections of the garden treated with leachate, or from some other source washed into the test site with the heavy rains.

Transient phytotoxic agents can accumulate in undiluted food scrap leachates (see Are Waste Products in Your Recycled Food Scrap Inhibiting Plant Growth?).  For this reason, we generally recommend diluting the leachate at least 10 fold (1 part concentrate mixed with 9 parts tap water) before applying it directly to plants.

However, in some instances we have observed that well-established plants, especially perennials do well even when exposed to undiluted leachate. Grass, for example, tolerates leachate applications without dilution of the leachate in tap water. Fig. 2, for example, shows a square 10 ft x10 ft test patch of ordinary grass treated with undiluted BSF food scrap leachate (application rate, 0.09 gal per sq ft). The grass greened up and became vibrant and healthy with a single application. These results indicate that certain perennial plants with established root systems are able to tolerate leachate sprayed or poured over the plant without adverse effect. The restoration of a lush green color to the test patch of grass is an indication that the soil in this test plot was deficient in nitrogen, restored with application of leachate as it percolated into the soil, and consistent with the results of the soil tests shown in Fig. 1.

Similar results with direct application of the leachate as a foliar spray were also observed on other well-established perennials, for example,  on box wood (Buxus), and ground cover plants such as leadwort (Plumbago capensis), and different varieties of Ajuga (Ajuga reptans) on which tests were run.

Applying leachate diluted in tap water as a foliar spray on the leaves of some vegetable plants, for example on the leaves of radishes once a week, increased their root mass and yield over that of controls grown in parallel.  In this study, 33 radishes were planted from seeds in parallel rows, one receiving a weekly foliar application (30-fold diluted leachate), and the other water in place of spray. At harvest (Fig. 2), the leaves of the radishes were cut from the roots, and root masses recorded. The cumulative harvested mass of the treated set was 128% greater than that of the control group. The experiment was repeated measuring the full mass without separation of roots and leaves. The yield of treated radishes in this instance (n=37) was 113% of the control group.

The lesser overall stimulatory effect on a whole plant basis for the treated radishes relative to the control set may be an indication that in the control set (untreated radishes) more energy goes into leaf production. These results suggest that leachate applied as a foliar spray may be improving the efficiency of leaf photosynthesis, possibly through direct uptake of plant available nitrogen in the leaves of the radishes, and its incorporation into chlorophyll needed for photosynthesis and growth of healthy plants.

Fig. 2. Effect of applying BSF food scrap leachate directly to grass undiluted,  and as a foliar spray diluted 30-fold in tap water applied to the leaves of radishes grown and harvested from seeds. Upper Panel,  A grassy area of 10 ft x 10 ft marked by blue flags was treated with undiluted leachate at an application rate of 0.09 gal per square foot. The photograph contrasting the treated test area against grass on the same plot of land left unexposed to leachate was taken approximately 2 weeks after application showing the grass treated with leachate had substantially greened up relative to adjacent grass left untreated outside the test area; Lower Panel, Radishes grown from seed comparing control set grown without foliar spraying with BSF food scrap leachate (left) to radishes grown in parallel row (right) having received foliar treatment once a week. For additional information see text. Copyright © 2013 Terry Green, All rights reserved.

Fig. 3 shows that on some plants, in this instance Black Fountain grass (Pennisetum alopecuriodes), that leachate markedly and positively affected plant growth and vigor in a manner commensurate with the dose of leachate delivered to the plants. In this study potted plants were cut back at the root ball to 5 cm, and the foliage cut to 3.5 cm length at the beginning of the experiment. The plants (triplicate sets) were repotted in 2 gal. planters with fresh potting soil, and watered and cared for over the course of approximately 2 months.

During this interval the triplicate sets of plants received once a week a watering of leachate diluted in tap water 10-, 100- and 1000-fold. Controls used in the study received only water. Outcomes are as in Fig. 3. Leachate positively affected the growth of both the root structure on these nursery plants and foliage, and in a dose dependent manner, indicating that it could be used very effectively as a non-synthetic organic-based fertilizer in managing nursery plants.

Fig. 3 Outcome of varying applications of BSF food scrap leachate on the growth rate of Black Fountain grass root and foliage. Fountain grass plants of the same age grown in 2 gal. planters were trimmed of foliage to within 3.5 cm of the ground, their roots trimmed to 5 cm lengths, then repotted in potting soil and watered over the course of approximately 2 months with varying doses of leachate included in the watering regimen once a week. At the end of the study, the plants were washed of soil and laid out side by side for comparison of growth impact on root and foliage. Photo is typical of results obtained in running each set of tests in triplicate.  Left to right, control (no leachate treatment), 10-fold, 100-fold and 1000-fold dilution of BSF leachate in tap water, respectively, applied once per week. Copyright © Terry Green, All rights reserved.

We earlier noted several beneficial aspects of the leachate fraction recovered from BSF processed food scrap added back to soil and leafy plants (see Black Soldier Fly Processed Food Scap | Foilant and Soil Applications, Amending Soil With Black Soldier Fly Processed Food Scrap Leachate, Soldier Fly Food Scrap Leachate | A Treasure Trove Amended in Soil and Is Composting an Earth Friendly Method of Recycling Plant and Food Scrap Waste?).  The present observations lend further support and insight on the beneficial aspects of BSF-mediated recycling of food scrap and agricultural wastes.

In the not too distant future it may become practical to produce sufficient quantities of BSF leachate so that it can be introduced into large scale farming and nursery operations in managing crops and nursery plants. This byproduct is easily obtained as BSF larvae process food scrap waste, and its use in place of NPK fertilizers could markedly reduce the runoff of nitrogen and phosphates from soils into waterways since the amount needed in eliciting growth of healthy plants appears to be but a fraction of the quantity of nitrogen and phosphate applied using current NPK synthetic fertilizers.

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Comments (2)

1. Chris said on 7/21/14 - 04:22PM
Thank you for the great information here. Is there a clear winner in benefits between applying leachate as a soil drench versus as a foliar spray? Thanks.
2. Terry Green said on 7/23/14 - 01:23PM
No clear winner! It depends - Foliar applications are generally more labor intensive, but cover more plants because foliar applications use less liquid per plant relative to the volume of leachate required in soil applications. Not all plants are good candidates for foliar applications however. Plants with waxy cuticles (kale for example) do not readily absorb the aqueous leachate. Percolating undiluted leachate into soil where young seedlings have just begun to grow can overpower seedlings causing stunted growth, or even die off. Well-established plants and perennials however generally tolerate and respond well to direct application of the leachate, even undiluted, into soil. Additionally, there is some circumstantial evidence that soil-born nitrogen fixing Clostridia, present in the leachate added back to soils, stimulate biological fixation of nitrogen at the application site which is of great benefit in sustaining and enhancing plant growth in nitrogen depleted soils.


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