Investigating the Addition of Clay to Feedstocks for Increased Nutrient Density and Carbon Stabilization in Compost

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)

Our initial run 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 (yard debris and/or yard debris and cattle manure/bedding). 
3. Biochar addition 8% by dry weight of the feedstock. 
4. Montmorillonite addition 5% by dry weight of the feedstock.

A 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 (yard debris and/or yard debris and cattle manure/bedding). 
3. Montmorillonite addition 5% by dry weight of the feedstock.

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 about 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 is 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 which aids in even air distribution. See Figure 7. The pipes are covered by a permeable fabric so the air does not escape to the sides and instead moves up into the bins. Blowers, controlled by timers, supply the aeration pipes. Each blower’s power is 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 can 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 will wirelessly post 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 are 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.

Objective 2: To develop an efficient mechanism to disburse local clay into compost feedstocks at scale.

We are waiting for results from the experiment to proceed with this part of the project.

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.

We are waiting for the compost to cure before proceeding to this part of the project. 

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 needs to be:

1. affordable (materials under $600), 
2. easy to assemble (less than a day 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 Compost Bins: 

We began with 5x5 ft wood pallets that the bins would rest on. To create the actual cylindrical bin, we used 16x5 ft metal mesh panels lined with black fabric plastic. We used small pieces of wire to attach the plastic onto the metal panel so it would stay on. After the plastic was attached, we rolled up the panel into a cylinder. We secured the cylinder with metal rings, about an inch in diameter, and metal rods. We weaved the metal rod between the metal panel and the rings.

Cost breakdown of the 6ft compost bin

6ft wide wire mesh roll - $159.99

6 ft x 300 ft landscape plastic - $114.99

4x4 ft wood pallet - free

Temperature probe - $39

Ethernet Tag Manager - $49

B-air Koala KP 280 - $102.47 

PVC pipe attached to blower -