Are Waste Products in Your Recycled Food Scrap Inhibiting Plant Growth? : The Life and Times of BSF (Black Soldier Flies)
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Are Waste Products in Your Recycled Food Scrap Inhibiting Plant Growth?

by Terry Green on 11/17/12

Many people engaged in recycling believe rightly that recycling food scrap into soil ought to work very well as a nutrient fertilizer in stimulating plant growth in the garden. After all, the essential nutrients for all living organisms are right there in the food scrap. Carbon, the most abundant element in food scrap, and nitrogen, phosphorous, sulfur, potassium, and trace minerals, all delivered back to the soil improve its ability to support the growth of plants. Since food scrap does not breakdown readily when it is added directly to soil, it is usually composted by aerobic or anaerobic processes, or processed in other ways, for example, by using Black Soldier flies. Many people use the liquid fraction (in chemical terms designated the leachate fraction), collected as the waste decomposes, as a liquid fertilizer.  This practice can actually cause unanticipated inhibition, the stunting of plant growth, or even the loss of valuable plants in the garden unless one is aware of how to use the leachate properly in delivering nutrients into the soil. Here we describe the most common cause of this problem and how to run a very simple test on the leachate fraction that allows you to resolve the issue in applying food scrap leachate correctly to your garden for best results.

The principal problem accounting for inhibition and die-off is the presence of phytotoxic compounds (compounds toxic to the growth of plants) forming in the soil,formed or already present upon application of the leachate to plants in the garden. These compounds form in decaying food scrap and other organic wastes as intermediates as the waste undergoes decay. If you would like more specific information on the types of compounds present, please see our earlier blog Recycling Food Scrap? | What’s with the Stink

Food scrap is also rich in salt. This, alone, present in the leachate fraction, can adversely affect plant growth if added back to soil in too concentrated of form.  Anaerobic fermentation, which occurs in both aerobic and static (anaerobic) composting of food scrap, also tends to acidify the leachate fraction, which will adversely affect plant growth if the soil becomes too acidic.

A simple test to screen for phytotoxic agents in the liquid fraction is to place 10 or more tomato seeds on an absorbent paper towel, wet the towel with a couple of teaspoons of the liquid to be tested, and keep the wet seeds covered with a plate and in the dark at room temperature for about a week. Setup an identical plate of seeds with just tap water. At the end of the week, count the number of tomato seeds that have sprouted in the test sample. Prepare tests at different dilutions made up with tap water, for example, undiluted, 1 part in 5, 1 part in 10, etc. Count the seeds showing sprouts, and compare your results to those obtained with tap water (the control). The dilution where at least 8 out of 10 seedlings shows healthy sprouts should be used in your soil for best results – phytotoxins, at this dilution, have been diluted out and should no longer pose a threat to your plants allowing you to realize the benefits of still adding essential nutrients back to the soil.

image of germinating tomato seeds used in phytotoxic tests on leachate fraction
Figure 1. A simple test for phytotoxic agents present in food scrap liquid waste using tomato seeds wetted on absorbent paper used in determining how to dilute the liquid waste with tap water before application to your garden; clockwise starting in upper left corner, undiluted leachate, 10X, 100X and water control, respectively. Copyright (c) 2012, Terry Green, All rights reserved.

Figure 1 shows this test done on the liquid fraction collected following fermentation of food scrap waste just before the waste was to be buried in the ground. Starting in the upper left corner, the four inserts show 10 seedlings,  each treated (clockwise) with undiluted leachate, 10X (1 part liquid + 9 parts tap water), 100X (1 part liquid + 99 parts tap water), and a water control.

The undiluted leachate is too rich in phytotoxic agents to apply to soil. The 10X diluted sample shows sprouts in 9 of ten seedlings and is acceptable for application in soil although it is clear from the more vigorous growth of sprouts in the 100X diluted sample, and corresponding pattern of long sprouts in the control, that a dilution between 10X and 100X with tap water would work best. This test is quite reliable, inexpensive and virtually trouble free in taking the guess work out of how to apply your liquid food scrap waste safely back into your garden soil.

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