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

Project Overview

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

Information Products


Not commodity specific


  • Soil Management: composting, organic matter, soil quality/health

    Proposal summary:

    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 from proposal:

    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. 

    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.