Soil Aggregate Research: Student Science with Limited Resources

​Introduction into Science     
December of 2019, after hours of plant biomass separation, weighing, and data input and analysis, my mentor Alexia and I looked at our first preliminary graph. In this moment, I became hooked on soil science. All the exciting field work and tedious lab work paid off with amazing data showing a trend toward multiple compost benefits. 
My name is Shaun McGrath and I am an undergraduate Environment and Sustainability student at Western Colorado University (WCU) transitioning into the Master in Environmental Management (MEM) graduate program. In September of 2019, I was lucky enough to be offered a paid internship with MEM student, Alexia Cooper, working on her research addressing rangeland resilience through soil health and compost amendments. Throughout my year helping Alexia, I discovered my passion for soil science and the benefits of healthy soils. My experience working with Alexia was a rare one for an undergraduate. Alexia Cooper was awarded the Sustainable Agriculture Research and Education (SARE) grant, which fully funded her project including a stipend for both herself and me. Even though no scientific research can be characterized as ‘easy’, having funds and resources made the process easier.      

Me performing soil pH testing on Alexia’s soil samples.

Tayler Jones (left) and me (right) taking soil core samples from one of the research sites.

​​Initiating My Own Project
With strong momentum at the end of Alexia Cooper’s master project, I was excited to take a soil ecology summer course with Dr. Jennie DeMarco at WCU and initiate my own soil health research. With this excitement, I was ambitious with my goals. 
Carbon sequestration, the storage of atmospheric carbon dioxide in the soil, peaked my interest and made me curious if the compost amendment increased the soil’s ability to mitigate climate change by increasing this sequestration. One indicator of an increase in soil carbon is the stability of aggregates within the soil (Djori, et al. 2019). Soil aggregates are clumped pieces of primary soil particles, soil carbon, and organic matter (Word Press, 2019). These aggregates contribute to soil aeration and water and nutrient cycling. 

Soil Aggregates (Word Press, 2019).

My objectives at the beginning of my project were… 

1. To determine the effects of compost amendments on the soil aggregate stability of grazed rangelands at the Coldharbour Institute. ​
2. Utilize soil aggregate stability as an indicator of soil organic carbon and compare between control and treatment plots.
3. Add to our knowledge of how compost can be used as a soil amendment in managing lands for climate change. 

Challenges Faced: Student Scientists and Land Managers
Early on I discovered many research challenges that I did not have to face while interning for a fully-funded project. Mainly, the time constraint of a summer course proved to make quality scientific research difficult. I would not be able to take accurate data, analyze, and produce a comprehensive write-up within the four weeks I was allotted for this project.

Financial and lab resources were the other challenge I faced when attempting to reach my original project objectives. WCU has a young soils lab, so resources and equipment are often minimal. The COVID-19 pandemic also made access to the soils lab difficult. 

​These challenges are also common for land-managers who do not have the financial ability or access to a lab to perform research. Ranchers are not necessarily resistant to change, but do not have the resources to address their needs or questions. These limitations must be addressed to increase rancher and student participation in land health. 

Science and Land Management with Limited Resources
With so many obstacles preventing my ability to answer my short-term research objective, I altered my project to address future students and ranchers who were in the same position. Research is often halted by a lack of resources, but low-cost and low-time commitment options are available to those students and land managers motivated to follow their curiosity without the assistance of a grant. 

My new project objectives became… 

1. To strengthen our knowledge and relationship with the land even with minimal resources. ​
2. To initiate student and land manager science through an affordable and feasible soil science experience. ​

To address my lack of resources and time, I created my own soil aggregate stability kit designed to perform quick, in-field, qualitative research measuring the class stability of soil introduced by Herrick, et al. (2001). This kit took me just two hours to create. Class 0 soil is the least stable where class 6 is the most stable. This article can be found at: https://doi.org/10.1016/S0341-8162(00)00173-9. The directions for creating the kit can be found at https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_050956.pdf on page 38 and the methods on page 20.

I adapted the following instructions and method from Herrick, et al. (2001).

Required materials:

• About 3 feet of 1-inch diameter PVC pipe.
• Aluminum window screen. About 4’x4’.
• Adhesive. (Epoxy, caulking, or thick glue). 
• ‘Tackle’ box with dividers for at least 18 slots. 

​Instructions:

1. Seal the individual cells of the ‘tackle’ box so you can fill it up with water. 
2. Make 20 marks at 1½-inch intervals on the PVC pipe.
3.  Make a smaller mark ¼ inch to the left of all the large marks.
4. Starting at the left, cut ¾ of the way through the tube at each of the small marks using an oscillating multi-tool.
5. Beginning at the left, use the oscillating multi-tool to make two lengthwise cuts through the tube, leaving a hand ‘tab’ for the sieve. 
6. Using the same tool, cut all the way through the tube at the first large mark.
7. Repeat steps 5-6 for each of the 20 sieves.
8. Cut 20 1¼" x 1¼" squares of aluminum window screen.
9.  Glue a window screen square to the bottom of each sieve.
10. After allowing glue to dry, carefully trim screen to edge of sieves.

Method:

1. Fill the compartments in the box with 2 cm of distilled water at approximately the same temperature as the soil. 
2. Use a trowel to carefully remove some soil from the soil surface and collect aggregates about 1cm in diameter. Collect up to 18 soil fragments.
3. Place a soil fragment on each sieve. 
4. Place one sieve into the water of one cell of the box. 
5. Observe the soil fragment for 5 minutes. If 50% or more of the soil falls through the sieve during this time, refer to the stability class table to determine class (figure 1). If more than 50% remains, move on. 
6. After five minutes, raise the sieve out of the water, then lower it to the bottom five times. (About 2 seconds for one full cycle). This is referred to as a slake test. 
7. Refer to the stability class table to determine class and record findings (figure 1).

My soil aggregate stability kit.
​

Performing the slake test outlined by the USDA. 
​

I followed the methods provided by the USDA to test a total of six soil samples. Three from the control sites with no compost application and three from the treatment sites with two inches of compost top-dressing.  I was able to test these samples in 45 minutes and could easily upscale to more samples, as there is a total of eighteen slots in the soil kit. 
This method proved to be feasible, affordable, and easily replicable for student soil scientists and land-managers seeking to understand the stability of their soil.
​ 
Outcomes
​

A look at how much soil fell through the sieve after the slake test. Control on the left and treatment on the right.

Figure 1: Stability class table (Herrick, et al. 2001).

​My qualitative data revealed that the treatment soils tended to have more stable aggregates as most of the soil remained in the sieve after the slake test. The control samples tended to have slightly less stable aggregates as about 25% of the soil samples fell through the sieve during the test. I characterized the treatment soil as stability class 6 and control as stability class 5 according to the table in figure 1. This result was expected as an input of soil organic matter and an increase in plant biomass production after compost amendments tend to increase soil carbon and therefore increases the stability of soil aggregates. This data can be confirmed using quantitative lab methods in a future project. 
Through offering my experience of a short, self-supplied research project, I hope to encourage current and future students and ranchers to initiate projects to address their questions without the intimidation of limited resources. Even if one does not desire to learn about soil aggregate stability, there are likely many more papers similar to the one I found offering low-cost and low-commitment alternatives to lab equipment. Every curious mind deserves for their questions to be answered without financial ability interrupting their path.   
 
 
Written By: Shaun McGrath
Western Colorado University School of Environment and Sustainability
17 August 2020
Special thanks to Dr. Jennie DeMarco for guiding me through soil ecology and to MJ Pickett for facilitating a beautiful location for student research at The Coldharbour Institute. 
 
 
References
Dorji, T., Field, D.J., Odeh, I.O.A. (2019). Soil aggregate stability and aggregate-associated organic carbon under different land use or land cover types. Soil Use and Management, 36(2). https://doi-org.ezproxy.western.edu/10.1111/sum.12549 ​
Herrick, J.E., Whitford, W.G., Soyza, A.G., Van Zee, J.W., Havstad, K.M., Seybold, C.A., Walton, M. (2001). Field soil aggregate stability kit for soil quality and rangeland health evaluations. CATENA, 44(1), 27-35. https://doi.org/10.1016/S0341-8162(00)00173-9
USDA. Soil Quality Test Kit Guide, Jul. 2001, https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_050956.pdf.
Word Press. "Soils Matter, Get the Scoop!" 15 Jul. 2019. https://soilsmatter.wordpress.com/2019/07/15/what-are-soil-aggregates/. ​