The primary research goal is to test the hypothesis that adding clay to compost feedstock will improve the physicochemical properties of finished compost (e.g. higher nutrient concentration, increased fines, better retention of carbon) through the formation of Mineral Associated Organic Matter (MAOM) which is less susceptible to biological degradation (respiration) during the composting process. 

Objective 1:

To assess how the addition of clay to a compost feedstock alters the physicochemical properties of the finished compost. (e.g.,, nutrient concentration, particle size distribution and carbon retention)

Feedstocks

The feedstocks for the compost trials were a mixture of ground yard debris, cattle bedding and manure.  The bulk density of the feedstock was 421 lb/yard (method: TMECC 03.10 A). 

Treatments and Replicates

The first replicate was started on 10/7/22 and included four separate compost bins with the following treatments:

1. Local clay silt addition 5% clay equivalent by dry weight of feedstock. 
2. A control with only the feedstock. 
3. Biochar addition 8% by dry weight of the feedstock (this was not in our proposal and was not triple replicated)
4. Montmorillonite addition 5% by dry weight of the feedstock.

The second and third replicates were started on 11/15/22 in six bins with the following treatments:

1. Local clay silt addition 5% clay equivalent by dry weight of feedstock. 
2. A control with only the feedstock. 
3. Montmorillonite addition 5% by dry weight of the feedstock.

To blend the clay with the other compost feedstocks, we mixed the clay in 150 gallon cattle water tanks with a paint mixer connected to a drill motor.  The clay slurry was added with a bucket and then additional water was added to the feedstock with a pressure hose to reach the desired moisture content.  (roughly 60%).  The material was mixed both by hand and with a bucket loader against the mixing wall.  

Bulk Density Tests 

In our original proposal, we stated that each clay addition will be made into a slurry with 15 gallons of water and well mixed with feedstock to fill one bin. However, we instead performed bulk density tests on each of the bin mixtures in order to gauge their moisture and ensure they were all similar. We followed a Washington State University guide to perform the test (​​https://puyallup.wsu.edu/soils/bulkdensity/). 

Our goal was for each bucket to be 10 kg. After that measurement, we calculated the bulk density. 

Bulk density of Local Clay compost: 776.028 lbs/yd3

Bulk density of Control compost: 873.032 lbs/yd3

Bulk density of Biochar compost: 858.04 lbs/yd3

Bulk density of Montmorillonite Clay compost: 960.32 lbs/yd3

Aeration 

Airflow was supplied by 4” diameter PVC piping with 8 half inch (2 rows of 4 holes) aeration holes situated at the bottom of the composting bins.  The pipes were covered by a permeable fabric so the air did not escape to the sides and instead moves up into the bins. Blowers, controlled by timers, supplied the aeration pipes. Each blower’s power was 380W. 

At the beginning of the initial run, the blowers were all on the same timer. We found that there were times where one bin wouldn’t be getting up to the temperature range we wanted, so we would want to reduce air flow for that bin in particular. We then decided to put all four bins on their own timers. That way, we could adjust each bin separately if one is getting too warm or too cold. We started the air blowing at 2 min every hour and logged each change.

Temperature Monitoring 

To allow for real-time web-based temperature monitoring (and hence afford appropriate airflow control) each bin was equipped with DS18B20 outdoor water-proof probes that wirelessly posted data to a web-based data logger (Manufactured by Co Gadgets LLC). We calibrated all the probes to 31F before placing them in their corresponding bins. The probes were set to record every ten minutes. For the first four days and after each turn, we attempted to reach a range of 131º - 145º F.

Turning Compost

Each replicate was emptied, rewetted and remixed  after approximately one month and then composted an additional month. 

Sampling

After removing the cattle panel and opening the compost bin we found that the finished compost would stay in its cylindrical shape.  We used a saw to cut out two opposing eighths which we mixed together.  From there we used the standard cone and quarter process to obtain our sample. 

Testing

Samples from feedstocks and from each replicate were sent to Microbial Matrix for analysis of bacteria, fungi and protozoa and to Soil Test Labs for physicochemical analysis.

Objective 2:

To develop an efficient mechanism to disburse local clay into compost feedstocks at scale.

With the local clay and the montmorillonite we made a slurry by slaking the clay in stock tank and then mixing with a paint mixer on a drill.  In the initial run, we simply used buckets to add the slurry to the feedstock.  In a subsequent test, we used a submersible sump pump to apply the clay slurry.  This pump method worked well and could be used at scale, though the pump likely needs to be designed to withstand the abrasion.  

We spent considerable time repurposing a 500 gallon propane tank into a mixing chamber.  In the first attempts, nozzles inside the base of the standing tank were installed to create a mixing vortex. A 2 hp electric pump pressurized water into the nozzles through 1 ½ inch hoses.  The clay was added into the tank, top replaced and sealed, and the pump supplying fresh water, turned on.  It was hoped that the pressured water would create a vortex in the tank, mix with the clay, and come out of the tank as clay-infused water with sufficient pressure to allow spraying on the compost.  Unfortunately, the desired mixing didn't occur and we found it exceedingly difficult to seal the top of the tank.   

Subsequent attempts were made with this tank elevated, allowing the "clay infused" water to flow by gravity to the compost pile.  This required adding the clay to the tank with the excavator, which was difficult.  With the continued difficulty with mixing the water and clay, we added a mixer into the bottom of the tank.  It was a small propeller from a discarded outboard motor attached to a long rod which was supported in the middle of the bottom of the tank and attached to a heavy duty drill motor at the top.  This worked to mix the water and clay consistently.  The gravity system did not have enough pressure distribute the clay infused water on the feedstock in an efficient manner. 

Objective 3:

To demonstrate, compare, and document effects of growing media made with compost or co-composts on media quality and the nutrient density and yield of lettuce seedlings.

The recipe for a 6 gallon batch of our growing media was:

•  3 gallons of compost or co-compost screened to ½ inch,
• 2 gallons of aged wood fines screened to ½ inch, and
• 1 gallon of biochar
• 0.36 lbs of organic 7-2-4 fertilizer.  

For our experiment we made batches of growing media from the control, the local clay and the montmorillonite composts (we also included a biochar co-compost treatment that was not officially part of the project).

Then our process was:

•  Three 72 cell winstrip trays per treatment were filled with each growing media, which resulted in 12 trays total.  
• We seeded little gem lettuce one seed per cell into each tray, watered all trays.
• Trays were placed into the germination chamber on 8/09/2023 at 9:30am. The germination chamber settings were at 74 degrees F and 91.5% RH.
• The trays were removed from the germination chamber on 8/11/23 at 11:30 am and placed in light racks until 9/5/23.
• We sampled 25% of the 72 cell tray which was 18 plants.  We used a random number generator to choose our sample cells.  Due to observed lack of edge effect we sampled from the whole tray. There were some trays that had less germination in the interior of the tray. 
• We recorded the wet weight of the lettuce samples and sent them to Soil Test Labs for tissue analysis.  

Objective 4:

To prove a design of an easily replicated aerated compost system suitable for small batch (approximately 3 yds) farm and experimental use. 

The design perameters:

1. affordable (materials under $600), 
2. easy to assemble (less than 4 hours for two people), 
3. easy to fill and empty, 
4. mobile (with forks on a tractor), and 
5. have a mass to surface area ratio large enough to assure the compost will get up to 131℉ during the winter. 

Building the Experimental Compost Bin

We began with 5'x5' ft wood pallets that the bins would rest on.  (This required adding onto a more readily available 4'x4' pallet.)  For the cylindrical bin, we used 16'x5' ft galvanized metal mesh cattle panels lined with woven black plastic landscape fabric. We used a piece of high tensil wire to attach the plastic onto the metal panel by loosely threading the fabric onto the panel. After the plastic was attached, we rolled the panel into a cylinder. To create the cylindrical shape we inserted a metal rod through metal rings attached to the panel.  (Full details are described in the video.)

Cost breakdown of compost bin, aeration and wireless temp monitoring equipment