Homeowner Guidance for using Food Grounds

This article, written by Romy Carpenter and Andrew Carpenter at Northern Tilth, LLC., outlines the results of a comprehensive study on how you can use Food Grounds at home.

Mill partnered with Northern Tilth, LLC. to run a series of trials to evaluate the effective use of Food Grounds (the dry grounds generated in Mill) in your backyard. This article outlines the results of the study and how you can use Food Grounds at home.

Northern Tilth, LLC. specializes in working with complex regulatory issues related to soil-based recycling, such as determining nutrient-balanced soil blending recipes, assessing potential contaminants in soils, and improving overall soil health through recycling programs.

Introduction  

The Food Grounds (FG) generated from the Mill have many of the attributes of organic matter-based soil amendments. The dry grounds have high concentrations of nutrients that plants require to grow. They are also an excellent source of soil organic matter, which is residue that comes from once-living plants or animals and helps improve long-term soil health. 

Food Grounds are not the same as finished compost, but if managed appropriately, Food Grounds can provide the same benefits to soil as compost. The biggest difference between Food Grounds and compost, is that the grounds contain fresh organic matter, whereas compost contains mature, stable organic matter. Fresh organic matter in the soil or in a compost pile is food for the microbes (including fungi, bacteria and mold) that drive the organic matter transformations. Because most food waste is material that was once alive, Food Grounds typically contain 85-90% organic matter whereas most agricultural soil contains 3-8% organic matter and compost typically contains anywhere from 30-70% organic matter. After microbes break down the fresh organic matter, the decomposed grounds will provide a long-term pool of plant nutrients and will build soil health. You can think of Food Grounds as rocket fuel to drive microbial processes in the soil. They should be managed accordingly.

The characteristics of Food Grounds are partly dependent on what foods are put into the Mill. However, as compared to compost, Food Grounds will almost always have a high organic matter and high plant nutrient content, low pH, and a slightly elevated salt content. The fertilizer value of Food Grounds has a similar nitrogen content and slightly lower phosphorus and potassium content than dried and pelletized chicken manure, which is often used by organic farmers to build soil fertility. Options for utilizing Food Grounds as a soil amendment at home include composting Food Grounds or directly applying Food Grounds to lawns and gardens. Guidance for these options is provided in this report. 

Using Un-Composted Food Grounds to Build Soil Health  

Spreading FG on lawns, or mixing FG directly into soil is less labor-intensive than composting FG and is a  great way to increase soil fertility. In these methods, FG is broken down by microbes in a lawn or  garden, rather than in the compost pile.  

Applying Food Grounds on a Lawn

On an existing lawn, FG may be spread on the lawn as a fertilizer. We recommend an application rate of 15 lbs (approximately 2 Mill buckets) of FG spread over 100 ft² of lawn. This translates to approximately 3 gallons of FG per 100 ft², or 5 cups of FG per 10 ft² – a light sprinkling of the material. If the FG is visible, gently rake it to let it settle under the grass. In a few months, the material should be mostly broken down. Due to the salt content of FG and potential disturbance to the sod that may be caused by microbial activity, FG should not be applied to the same area of lawn more than three times per year. 

FG are slightly acidic, and for most homeowners, this is not a concern. However, if the pH of the soil is important, one pound of lime (which is pulverized limestone) or two pounds of wood ash should be applied to the lawn for every 3-4 pounds of FG. 

Mixing Food Grounds with Garden Soil

For garden application, we recommend applying 5 gallons (approximately 3 Mill buckets) of FG per 100 ft² or 8 cups of material per 10 ft².  Thoroughly incorporate FG with the top six inches of garden soil using a shovel and/or metal rake. Ideally, FG should be applied at least two weeks prior to planting to allow for microbial transformations of the material. Depending on garden conditions (i.e. moisture and temperature), the blended FG and soil may form a slight crust on the soil surface. If this occurs, the crust should be broken up with a metal rake prior to planting.

Burying Food Grounds

An alternative to spreading FG on top of lawns or gardens, is burying FG under lawns or gardens. To bury FG, dig a small pit, approximately 8 inches deep in garden soil or lawn; if digging in lawn, use a tile spade or knife to cut a rectangle into the sod, then peel back the sod so that it may be replaced after burying FG. Place FG into the pit at an application rate of 3 cups per ft², or 1 gallon per 5 ft². Place an equal amount of loose soil (the material that was just dug up) into the pit and mix with FG. Replace the remaining soil and, if in a lawn, replace the sod. Field trials indicate that in less than a year, grass roots can access the nutrients in buried FG as evidenced by increased growth in the following growing season.

Composting Food Grounds

Just like food scraps, FG may be composted in a backyard compost pile. The benefits of composting FG instead of composting unprocessed food scraps include a faster composting time, due to the small particle size of FG, and more control over the compost recipe and timing. FG are “shelf-stable” until moistened – this allows homeowners to choose when they’d like to build a compost pile, and to build a compost pile at once, rather than iteratively. 

Successful composting of organic wastes requires a proper mix of the following basic components.  

1. Fresh organic matter (food for the microbes), in the form of food scraps (including FG), leaves and yard wastes, manures, wood wastes, etc. The organic matter materials should provide a good balance of carbon and nitrogen to drive the microbial process. 

2. Moisture and air (oxygen) in the right balance to provide a hospitable environment to the microbes. 

3. A bulk density of the mix of compost feedstocks that is “fluffy” enough to provide good air and water exchange through the compost pile.

Relative to the proper mix of carbon and nitrogen in compost, food waste tends to be rich in nitrogen; it has a low carbon-to-nitrogen ratio, and consequently, composts most effectively when blended with carbon-rich (often brown) materials such as dead leaves, mulch or wood shavings. Grass clippings are a good ingredient in compost piles but do not provide as much carbon as dead leaves and wood waste. FG contains the same nutrients as unprocessed food waste, but due to the loss of moisture and volume reduction in the mill, a little bit of FG goes a long way in the compost pile. One pound of FG contains the same amount of nutrients and organic matter as approximately four pounds of unprocessed food waste.  Additionally, FG has a smaller particle size than unprocessed food waste, meaning the material is more readily available to soil microbes and, consequently, quicker to compost.

When building new compost piles with FG, use the following as a rough guide for making a good mix:

• When building a pile with FG and primarily “brown”, or, high carbon materials, such as leaves, twigs, wood chips or sawdust, use (by volume):

  • 1 part FG

  • 10 parts brown materials

• When building a pile with FG with a mix of brown materials and grass clippings or other green vegetation, use (by volume):

  • 1 part FG

  • 14 parts brown/green mix

In all cases, while making your compost blend, mix the ingredients together then grab a handful of the material. Check the moisture of the material using the squeeze test: squeeze the handful of material in your hand. If the material feels dry and doesn’t form a ball in your hand, it’s too dry – spray the pile with a hose for a few minutes, mix it up, and try again. If the material forms a ball in your hand and very little water drips between your fingers when you squeeze the ball, it’s perfect. If free water is running out of the material when you squeeze it, it's too wet – try mixing in dry ingredients like dead leaves or wood shavings. 

Check the temperatures in the weeks after building the pile (either with a compost thermometer, or more qualitatively, by hand).  In a well-built pile, the temperature should go up noticeably within 24 hours after the initial blending, and the temperatures should stay high for at least a week, if not more.  When the pile temperatures start to drop again, it is time to turn/fluff up the pile to introduce more air to the microbes.  Repeat this process until the pile is no longer heating up after turning.

When the compost is no longer heating up and it looks like a nice, uniform brown or black color, it’s ready to use. The finished compost may be mixed into garden soil, spread on top of a garden, mixed into potting soil, or lightly spread across a lawn (consider raking over the compost after spreading it, to allow the compost to settle under the grass rather than on top of it). Most plants grow best in a mix of compost and soil, rather than compost alone, but it’s hard to apply too much compost.

Food Grounds and Mother Nature  

Microbes

Microbial activity is an essential and unavoidable part of breaking down FG into plant-available nutrients and soil-building organic matter. There are tens of thousands of species of fungi, molds, algae, and bacteria present in soils, with billions of individual organisms present in each handful. Without these microbes, FG wouldn’t provide much benefit to the soil and the plants growing in it. It’s possible that homeowners will see mold and/or fungi after incorporating FG in soil, and may even catch a whiff of the FG breaking down. For this reason, it is not recommended to use FG indoors (i.e. in potted plants). However, in lawns, gardens and outdoor compost piles, the microbes are harmless and they’re a sign that FG are on their way to improving soil health and fertility. In field trials performed with the FG, where high application rates of FG were used, even the plots that molded significantly showed increased grass growth relative to areas that did not receive any FG. If composting FG, the finished compost should be stable and should not mold when applied to lawns and gardens.

Vectors

Before FG are fully broken down by microbes, they may attract backyard animals such as raccoons (a.k.a. vectors). Composting FG (following the guidance in this report) is a method of recycling FG at home that’s less likely to attract vectors than spreading FG over lawn or mixing FG into garden soil. When FG are fully broken down, they will no longer attract animals.