Henry and Austin

Organic matter is important in soil because it provides structure to the soil, allowing for water absorption. It also provides essential nutrients to plants and other organisms living in the soil.¹ So, how does organic matter get into soil in the first place? Simply put, organisms continually grow, die, and then decompose. The older organic material gets buried beneath the new material, where it continues to get broken down by microbes which release CO₂ into the atmosphere through cellular respiration. On a farm, this natural system is disrupted, decreasing soil nutrients, and, consequently, decreasing the nutrient content of crops. Additionally, when crops are harvested annually, their roots are often very small, so they don’t contribute very much carbon to the soil. Each successive harvest causes the soil’s organic matter to decline, and, eventually, the soil becomes less desirable for farming. In order to work around this problem, farmers often till their land. Tilling breaks up the soil and exposes older organic matter that is deeper down. All of what has developed in healthy soil gets destroyed when that soil is tilled: organic matter, air, water, minerals, and microbes in the soil are all depleted by tilling. Furthermore, tilling is a large producer of the greenhouse gas CO₂ because the carbon present in soil is volatilized–released into the atmosphere–when it's brought to the surface. Losing organic material from tilling also reduces the water retention capacity of soil.

Over the past month, Henry and Austin ventured to Brookwood Community Farm in order to collect soil samples in and around the location. We specifically looked at locations around the property that were distinct from one another in terms of what they were used for and how often they were disturbed, in one form or another, by humans. Working with the lead farmer, we established where they had tilled the land and where it was left untouched. We decided to test five different locations around the property. The first location was a tilled field, used for a multitude of different crops. The second was a covered field that had not been tilled this past year, but it had been tilled in years past. The third field was one that was used to grow asparagus, a crop that does not involve tilling. The fourth location was a natural field off the side of the farm that has not been used in decades. The fifth location was in the woods near the farm, and it contained soil that had not ever been used for farming, at least to our knowledge. We then massed these samples before leaving them in a drying oven at 225^o F for 24 hours. This step was to remove moisture from the soil. The dried samples were massed afterwards, then brought to a kiln to be fired at 925^o F for roughly 10 hours. Finally, each sample was massed again. Using Excel, we calculated the average percent change in soil mass for each location: shown in Figure 2. Excel was also used to find whether or not the fields had a significant difference from one another in terms of percent change in mass. We calculated water mass percentage by dividing the difference in masses after the samples were in the oven by the mass of the sample before it was in the oven. The carbon mass percentage was calculated by dividing the difference in sample masses before and after the kiln by the masses of the samples before they were in the kiln and after they were in the oven.

The relationship between soil tilling and water retention is shown in Figure 2: the forest soil had a significantly higher water mass than that of the tilled field. Even the asparagus field, which does not get tilled, had a significantly lower water mass than the forest, most likely because it is an annually harvested crop. However, the soil in the asparagus field still had better water retention than that of the tilled field. Interestingly, Figure 2 also shows that the asparagus field had less carbon than the tilled field. This finding contradicts our hypothesis because it was assumed that tilling at Brookwood Farm would reduce soil carbon, but our data shows the opposite: the tilled field had significantly more carbon than the asparagus field. Note that it’s also possible there was simply more carbon near the tilled field’s surface due to tilling, and there may have been less near the asparagus field’s surface, due to lack of tilling. Afterall, our soil core sampler only gauges the top 6-12 inches of soil.

The importance of these data reveals that although tilling does not cause a significant difference in soil carbon at Brookwood Farm, it does play a large role in the soil's water retention. These data reveal that there is a significant difference between the amount of water tilled and untilled soil can hold. Furthermore, this disparity is magnified when comparing our soil samples from farmed and unfarmed locations. While our data might not express the negative impacts of tilling land, farmers should consider a non-tilling approach to growing crops, for there is still, at least, the water retention benefit that comes with not tilling soil. This change may reduce the amount of water needed for crops, and it could be especially beneficial in places that experience frequent draughts. Reduced tilling is also a very productive method for carbon sequestration in the midst of a global climate crisis.

Sources:

1. Christina Curell, Michigan State University Extension. “Why Is Soil Water Holding Capacity Important?” MSU Extension, 2 Oct. 2018, www.canr.msu.edu/news/why_is_soil_water_holding_capacity_important.

2. (Photograph) Frank, Jon. “Home.” International Ag Labs, aglabs.com/services/soiltesting.html.

3. “Interpreting Soil Test Results for Gardens and Grounds - Cooperative Extension: Garden & Yard - University of Maine Cooperative Extension.” Cooperative Extension: Garden & Yard, extension.umaine.edu/gardening/manual/soils/interpreting-soil-tests/.

4. Jocelyn Lavallee Research Scientist, and Francesca Cotrufo Professor of Soil and Crop Sciences and Senior Scientist. “Soil Carbon Is a Valuable Resource, but All Soil Carbon Is Not Created Equal.” The Conversation, 2 Mar. 2021, theconversation.com/soil-carbon-is-a-valuable-resource-but-all-soil-carbon-is-not-created-equal-129175.

5. Ontl, Todd A, and Lisa A Schulte. Nature News, Nature Publishing Group, www.nature.com/scitable/knowledge/library/soil-carbon-storage-84223790/.

6. Schlesinger, William H. “Carbon Sequestration in Soils.” ESA, Ecological Society of America, 25 June 1999, www.esa.org/esa/wp-content/uploads/2012/12/carbonsequestrationinsoils.pdf.

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