Art by Anasthasia Shilov.
Death, SOM, and Soil
Death often stays in the soil. Over time, organisms and residues in varying states of decomposition form a vital component of the soil: soil organic matter (SOM). SOM has always been thought of as an indicator of soil fertility, contributing to healthier soil and better crop growth. Thus, building SOM, or raising its levels through the addition of compost or manure, is assumed to be a cost-effective way of reducing reliance on external inputs such as fertilization and irrigation. But how well are the effects of SOM actually understood? In various studies, higher SOM has been shown to correlate with both higher and lower productivity, so until now, the effects of added SOM on soil fertility are inconsistent and unclear.
The question is further complicated by the possibility that this causative pathway is bidirectional: Does SOM lead to increased crop productivity, or do increased plant inputs lead to higher SOM levels? In order to figure out this relationship between SOM, agricultural inputs, and productivity, we need to be able to isolate and investigate SOM’s effect on plant growth. Emily Oldfield, a postdoctoral fellow at the Yale School of Forestry and Environmental Studies, works in the Bradford lab to study SOM’s effects. “A lot of the policies that are being put forth by organizations like the USDA, the Food and Agriculture organization rest on the premise of the more organic matter, the better, but there’s really no hard quantification of how much more and how much better,” Oldfield said. “The goal of my research was to try to put some numbers behind it.” She described this greenhouse study as an effort to use a controlled environment to establish a causative pathway between SOM and crop productivity.
What is soil?
Soil itself is a complex mixture of elements, consisting of around forty-five percent minerals, (including sand, silt, clay) and fifty percent air and water. In particular, plants require nitrogen more than any other nutrient, but they can only take up mineral forms of it, including nitrate and ammonia, which only make up two percent of the nitrogen in soil. The other ninety-eight percent of nitrogen is organic and inaccessible to plants, meaning many farmers rely on mineral N-fertilizer to facilitate crop growth.
The remaining five percent of soil composition is soil organic matter—anything that was once living. Though SOM is a very small percentage by volume, its influence is disproportionately large. SOM dictates the structure of the soil, increasing aeration and water-holding capacity, and acts as a habitat for other soil organisms. SOM also powers the cycling, retention and release of various nutrients essential to productivity.
The Experimental Setup
Oldfield was determined to quantify the effect that SOM, fertilization, and irrigation have on crop productivity, both used separately and in tandem. She designed an experiment with four target levels of SOM (1%, 2.5%, 5.5%, 8.5%) crossed with two different fertilization treatments (none versus 100 kg N/ha as urea) and further crossed with two irrigation treatments (optimum versus half of optimum). To verify reliability of results, each treatment was replicated 10 times—for a grand total of 160 experimental pots.
A dilution approach of organic-rich A horizon soil (obtained from the Yale Farm in New Haven, Connecticut) was used to create these varying SOM levels. By mixing the soil with an external mineral component (sand and clay) in different ratios, Oldfield was able to create a wide gradient of organic matter concentrations without having to artificially manipulate SOM (which can lead to other experimental issues).
This greenhouse experiment was conducted from May to July, and automatic ventilation ensured that the pots of spring wheat (Triticum aestivum, L.), the experimental crop, never exceeded a daily temperature of 30 degrees Celsius. A drip irrigation system was calibrated to emit 0.25 gallons to each pot per hour, though this was later modified to create different treatments for various pots. “Optimum irrigation” was determined to be around 127.2 mL of water each day and “suboptimum irrigation” was 63.6 mL. At the end of the growing period, all plants were cut at soil level at the same time, dried at 65 degrees Celsius and then weighed in aboveground biomass. Soils were then passed through a sieve and measured in terms of SOM content, water-retaining capacity, pH, microbial biomass, and rates of net mineralization and nitrification.
Results
To analyze the effect of SOM on growth, the researchers used a statistical method called regression to quantify the impact of each measured variable on plant growth. The regression models showed that aboveground plant growth increased as SOM levels increased until a threshold concentration of around five percent, after which wheat biomass began to decline. For soils with optimum irrigation, this decline started occurring at around six percent SOM.
Across all SOM concentrations, the biggest difference in aboveground biomass was observed between the two experimental extremes: the pots with optimum fertilizer and irrigation versus the pots with no fertilizer and half irrigation. However, this difference was largest at the lowest one percent SOM concentration (pots with optimum treatment produced 3.45 times more aboveground biomass) and became less dramatic when SOM levels were at or greater than give percent (optimum pots produced 1.6 times more biomass). This supports the hypothesis that SOM contribution can, in some cases, compensate for plants that are not receiving any supplemental input and substitute in for mineral N fertilizer. But this raises more questions of cost and reward—will productivity of mineral fertilized soils always outpace that of soils sustained by organic matter alone? And what about the reverse hypothesis: can added mineral nitrogen fertilizer easily compensate for lower SOM levels?
Nitrification
Though SOM levels did not seem to exhibit a strong correlative relationship with net rates of nitrification, they did have an impact on net rates of N mineralization, the process by which organic nitrogen is converted to plant accessible inorganic forms. As SOM levels increased, rates of N mineralization increased. This effect was greater in fertilized soils compared to unfertilized soils. However, after SOM concentrations passed a specific threshold (around seven percent), pots with optimum treatment began experiencing decreases in net rates of nitrification: the plants had less nitrogen accessible to them at eight percent SOM as opposed to five percent SOM.
Oldfield hypothesizes that this eventual decrease in nitrification rate may be related to increased microbial biomass that is correlated with higher SOM concentrations. These microbes themselves need to draw upon specific nutrients in the soil, including nitrogen, phosphorous and sulfur, which may lead to a competitive environment for nutrients and oxygen in the soil. In such an environment, less resources are available for plant use, which could explain why productivity began to decline instead of leveling off at the highest SOM concentrations. “However, it’s very hard to get a holistic picture of the forms of nitrogen. A follow up study would be almost the exact same experimental setup, just with different levels of nitrogen fertilizer,” Oldfield said. This would help determine if these nutritional elements become limiting at high levels of SOM.
Final Conclusions
Returning to the original question, can soil organic matter substitute for agricultural inputs such as insufficient fertilization and irrigation? These results, obtained by the systematic variation of variables, demonstrate an optimistic answer: building up SOM levels in soil will have beneficial impacts on productivity. Though it may not be a perfect replacement for N fertilizer, SOM can still help cut back on costly fertilizer inputs without risking a lowered yield. “We know through other research that’s being done right now that agricultural soils tend to have very low organic matter concentrations as a result of tillage and other conventional practices…You rarely see farm soil that is nine percent organic matter,” said Oldfield when asked whether the SOM threshold of five percent would pose a problem.
Some scientists and agriculturists continue to argue that though productivity may increase with higher SOM concentrations, these benefits will never outpace or outweigh those brought about by additional mineral fertilizer. However, this perspective fails to take into account the cost and availability of fertilizer. “There are potential outcomes that don’t directly translate to yield but are enhancements in other environmental outcomes that we do care about. This could be mitigating agricultural runoff to improve water quality, improving biological activity of microbial communities, and enhancing carbon sequestration,” Oldfield said.
What’s next?
Given that many groups such as the USDA and policy makers rely on the general notion that “more is better” when it pertains to SOM levels in soil, Oldfield is determined to continue delving into the nuances and intricacies of organic matter in soil. She briefly explains how increasing organic matter could pose drawbacks: increased SOM concentrations are related to increases in nitrous oxide emissions, a very potent greenhouse gas. “I’m interested in linking [this research] to other outcomes besides yield,” she says. Her ultimate research goal is to run this experiment on a much larger scale and get the “full farm look,” so she can not only measure crop growth, but also bigger profitability issues such as balancing yield against costs and observing ecosystem outcomes.
About the Author: Cindy is a first-year prospective Neuroscience major in Timothy Dwight College. In addition to writing for YSM, she also participates in Danceworks and the Chinese American Students Association.
Acknowledgements: The author would like to thank Emily Oldfield for her time and enthusiasm about her research.
Extra Reading:
Oldfield, E. E., Wood, S. A., & Bradford, M. A. (2020). Direct evidence using a controlled greenhouse study for threshold effects of soil organic matter on crop growth. Ecological Applications. doi:10.1002/eap.2073
Berry, P.M., R.S. Bradley, L. Philipps, D.J. Hatch, S.P. Cuttle, F.W. Ryans, and P. Gosling. (2006). Is the productivity of organic farms restricted by the supply of available nitrogen? Soil Use and Management. doi.org/10.1111/j.1475-2743.2002.tb00266.x
Loveland, P., and J. Webb. (2003). Is there a critical level of organic matter in the agricultural soils of temperate regions: a review. Soil & Tillage Research. doi.org/10.1016/S0167-1987(02)00139-3