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

Progress report for FW22-389

Project Type: Farmer/Rancher
Funds awarded in 2022: $24,745.00
Projected End Date: 03/31/2024
Host Institution Award ID: G383-22-W9210
Grant Recipient: Midnight's Farm
Region: Western
State: Washington
Principal Investigator:
David Bill
Midnight's Farm
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Project Information


Can the addition of clay to compost feedstocks increase fines, and carbon and nutrient retention in finished compost?  We asked this question after a talk by Dr.Griffin LaHue on soil carbon and clay’s role in carbon sequestration coupled with our observation of a loss of fines around aeration pipes in finished compost in our positively aerated compost system.  Challenges meeting customer expectations regarding the ratio of fines to woody material in our finished compost prompted further investigation. We discovered research claiming that clay integrated with compost feedstocks increases compost quality and its ability to sequester carbon in soils (Barthod et al., 2016; Bolan et al., 2012; Chen et al., 2018; Coleman 1995).

Montmorillonite clay and farm-sourced clay will be added to compost feedstocks in a controlled, replicated experiment in specially made compost bins. The finished composts will be tested for agronomic mineral concentrations, carbon content, particle size distribution and cation exchange capacity (CEC). These composts will be utilized in growing media for lettuce seedlings in a triple replicated trial, with lettuces tested for nutrients and yield assessed by wet and dry weight. 

Through professional videos, social media, conference presentations and on-farm demonstrations, Midnight’s Farm will share research results, and enable other farmers to build and use aerated static pile compost systems.

Building on successful past collaborations, our team is uniquely qualified to carry out both the research and associated outreach to bring new understanding of carbon stability to the age-old practice of making and using compost.


Project Objectives:

In consultation with Dr. Collins and other soil scientists, our primary objective is to test the hypothesis that adding clay to compost feedstock will improve the physicochemical properties of finished compost.  In collaboration with Dr. Siegner, we will educate farmers, ranchers, composters, and researchers about the process and results from this project through videos, articles and conference presentations.  

Our research will primarily assess how the addition of clay to a compost feedstock alters the nutrient density and carbon retention of finished compost.  We also intend to develop a mechanism to disperse clay at scale into compost feedstocks; to assess clay co-composts (Co-composting is the controlled aerobic degradation of organics, using more than one feedstock) as growing media ingredients on quality and yield of lettuce seedlings; and to demonstrate an easily replicated small aerated compost system. We will utilize diverse forms of outreach with goals of reaching 5,000 producers, agriculture professionals and researchers through professional videos, conference presentations, and articles in respected producer journals. We will conduct on-farm education for local farmers, farm interns, professionals, and local high school students. 


Click linked name(s) to expand/collapse or show everyone's info
  • David Bill - Producer
  • Doug Collins - Technical Advisor
  • Dr. Alana Siegner (Educator)


Materials and methods:

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.

Mixing local clay slurry
Mixing local clay slurry
Mixing clay with feedstocks
Mixing the local clay into the compost feedstocks.

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 (​​ 

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

bulk density
Dropping a bucket 10 times while performing a bulk density test.


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. 

experimental bin
Looking down into an experimental bin. The aeration tube is visible at the bottom.

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.

Blowers on experimental compost bins
Blowers and timers on experimental compost bins.

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.

Replicate 1 Compost Tems
Compost temperatures logged for Replicate 1
Compost temps
Compost temperatures and blower time for Replicate 2
compost temps
Compost Temperatures and blower data for Replicate 3

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.

Making experimental bins
Attaching plastic to experimental bin

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 -

Compost experimental bins
The replicates tucked under cover and protected from rain.
Research results and discussion:

We are still in process and have no results at this point.

Participation Summary

Research Outcomes

Recommendations for sustainable agricultural production and future research:

Our experiments are still in progress and we do not have any test results back at this date, hence have no conclusions to report. 



Education and Outreach

1 Consultations
2 Tours

Participation Summary:

Education and outreach methods and analyses:

Instagram Posts

  • October 19, 2022- Post about first replicate, explaining the project. 79 likes and 9 comments
  • November 18, 2022- Post about second replicate. 68 likes and three comments

Lopez Island Farm Tours

  • October 7, 2022 Presented the experiment as part of our farm tour.  45 participants. 4 farmers

Farm Tour that was part of a Rural Leadership Conference at the University of WA  pre conference fieldtrip

  • November 17, 2022. 40 participants. 2 farm educators
compost tour
A compost tour which highlighted the clay in compost experiment

Lopez Island High School Farm Class

  • November 29, 2022. 20 students
Education and outreach results:

To date the tours that we have conducted and the social media posts have presented our hypothesis, explained our methods and thanked SARE for the support.  

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.