The role of organic in protecting soil health and water quality

This past May, The Organic Center held the first Organic Confluences Summit, aimed at examining the intersection of science and policy to find ways for the two to come together to advance the organic sector. The conference focused on sustainability, with scientific experts, farmers, policymakers, and organic stakeholders gathering in D.C. to discuss how research on organic’s contributions to the environment can be incorporated into government programs to improve the sustainability of U.S. agriculture.

Soil health and water quality—two of the topics covered by scientists at the conference—were especially striking, given the cutting edge research discussed. Dr. Michel Cavigelli of USDA’s Agricultural Research Service (ARS) at Beltsville, MD, spoke on soil health, specifically about organic methods to increase soil organic carbon levels. The improved health of organically managed soil and its increased capacity to hold and store nutrients are also connected to water quality, with organically managed farms experiencing lower nitrogen loss into water than their conventional counterparts. Dr. Cindy Cambardella of USDA’s ARS in Ames, IA, highlighted this role of organic in preserving water quality by discussing her findings from a recently published study.


Soil Health

Several studies have compared conventional and organic farms, finding that organic operations experience increased growth and activity of beneficial soil organisms including microorganisms, beneficial fungi, and earthworms. While this charismatic soil life is critical for healthy soil, it is also important to consider the composition of the soil itself when considering the strength of agricultural soil.

Soil organic carbon is at the heart of most definitions of soil health because it provides structural stability to the soil, which reduces erosion, protects against soil compaction and improves aeration, water infiltration and water-holding capacity. It also serves as a reserve for nutrients essential to plant growth including nitrogen, phosphorus and sulfur, and composes the base of the soil food web, providing a foundation for all soil life. Soils with high organic carbon also hold carbon stores for long periods of time, contributing to global climate change mitigation.

One of the fundamental principles of organic farming is to manage soil organic carbon to increase soil health. Because the use of synthetic fertilizers is prohibited, organic producers increase soil fertility by incorporating manure or compost into the soil, leading to increased soil organic carbon.

The USDA Agricultural Research Service’s Farming Systems Project, established in 1993, has studied levels of soil organic carbon in detail, and provides long-term comparisons of organic cropping systems with conventional systems. Results from this project show that organic carbon in organic systems is consistently higher than in the conventional systems, even when no-till systems were used. This result was likely driven by the fact that carbon in no-till systems largely accumulates at the soil surface where plant residues are deposited, while in organic systems, plant residues are buried, leading to greater soil carbon deeper underground. Soil organic carbon at these depths is more resilient than soil carbon at the surface.

Conventional no-till farming, which relies on petroleum-based glyphosate herbicide, is advocated throughout the United States for its soil quality enhancement. However, the nitrogen mineralization potential in the organic system at the Farming Systems Project was, on average, 34 percent greater than conventional no-till after 14 years. Despite use of tillage in the Farming Systems Project’s organic plots, this pattern of increased organic carbon in organically managed soils was maintained even when the conventional plots incorporated cover crops, suggesting that organic practices can potentially provide greater long-term soil benefits than conventional no-till.


Water Quality

The increase in soil health on organically managed farms can also lead to improved water quality. Agriculture is one of the primary non-point sources of pollutants to U.S. waterways. Nitrate runoff from mineral fertilizers applied in conventional agriculture is of particular concern for water quality because it is a driver of eutrophication, leading to the formation of hypoxic dead zones” devoid of oxygen and unable to support life. Dead zones are present in most major bodies of water including the Great Lakes, Chesapeake Bay and the Gulf of Mexico. However, organic practices such as the use of diverse crop sequences and the use of cover crops and animal manure for fertilization that increase soil organic carbon also increase the soil’s ability to retain water and nutrients. The resulting water quality benefits have been well established in numerous studies, indicating that organic farms experience less water drainage and nutrient loss than their conventional counterparts.

Dr. Cindy Cambardella recently completed one of the most compelling studies evaluating the effects of organic farming systems on water quality. The USDA Agricultural Research Service’s Organic Water Quality (OWQ) experiment, established in 2011, compares transitioning organic and conventional crop rotations and pasture systems on Iowa State University’s Agricultural and Biosystems Engineering and Agronomy Research Farm. Unlike the majority of studies which rely on models or mathematical calculations to estimate nitrate loss, the OWQ experiment rigorously quantifies water flow and nitrate loss using subsurface drainage lines and equipment to collect water samples and monitor subsurface drainage water flow and nutrient loss.

One of the main issues Dr. Cambardella’s team looked at was drainage flow and water nitrate concentrations for conventional and organic grain cropping systems. They found that water flow was similar for all cropping systems, and was related to how much rainfall happened in that region. However, the water nitrogen concentrations were very different between organic and conventional systems. Conventional farms consistently had higher N concentrations in the farm runoff. In fact, the nitrogen load losses for the conventional systems were nearly twice as high as those from the organic systems. Results from this study suggest that organic farming practices, such as the application of composted animal manure and the use of cover crops within extended cropping rotations, can improve water quality. //

 

Dr. Jessica Shade is Director of Science Programs at The Organic Center (www.organic-center.org).

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