Organic vs. Conventional Practices on Soil Quality

Thursday, September 19th, 2019
The organic system in this comparative study benefited beneficial nematodes over harmful ones like this soybean cyst nematode. (Photo: USDA-ARS)

written by Andrea Leiva Soto, Horticulture and Crop Science

Quick Summary
Ohio State researchers compared an organic system to a conventional one, looking at several soil quality indicators such as bulk density, organic matter content, and nematode populations. After four years, the organic system had fewer harmful nematodes, especially during the hay phase of the rotation. Mineral nitrogen was more abundant in the conventional system, while microbial nitrogen prevailed in the organic system. Soil bulk density did not differ between systems, even though intensive tillage was done in the organically managed fields. However, despite the high carbon inputs added to the organic system, organic matter was only slightly higher compared to the conventional system.


Nematodes have a bad reputation for damaging crops and garden plants, but some can be quite important for plant growth. Certain kinds of nematodes eat bacteria and fungi that cause plant diseases. Others decompose organic matter, providing plant nutrients. Studies indicate that nematodes supply 27% of the soil nitrogen that is available to plants. Today, nematodes are increasingly used as an indicator of the status of the soil food web. The soil food web is a complex network with organisms that provide services to the farm ecosystem like regulating pests, nutrient recycling, modifying soil structure, or even breaking down man-made chemicals.

Organic matter additions have been shown to influence nematode populations. Adding green manure cover crops or decomposed animal waste can decrease root-feeding nematodes. Additionally, organic amendments are known to increase soil nitrogen, organic matter and microbial biomass, and reduce soil bulk density, leading to less soil compaction. As a result, roots explore deeper and have more oxygen available leading to more vigorous growth.

However, the intensive tillage practices used to incorporate amendments or control weeds, disrupt the soil ecosystem, affecting the populations of beneficial microbes and nematodes. Synthetic fertilizers, insecticides, and soil compaction can also cause similar undesirable effects.

To better understand these kinds of interactions and develop insights into how best to manage them, a study at the Ohio Agricultural Research and Development Center (OARDC) in Wooster, Ohio, compared conventional and organic farming systems and how soil characteristics, nitrogen cycling, and nematode populations are affected by each system.

The conventional system used chemical fertilizers, herbicides, and reduced tillage in a corn–soybean rotation. The organic system incorporated fresh straw, beef manure, poultry compost, and intensive tillage in a corn–oat–hay rotation. Soil samples were taken in the spring before soil inputs, and in autumn after crop harvest. Samples were taken from between and within the crop rows. Then for each sample, the nematodes were counted and identified, and soil bulk density, organic matter, and nitrogen were measured.

Results: After four years, the organic system had fewer harmful nematodes, especially for the hay phase of the rotation. Mineral nitrogen was more abundant in the conventional system, while microbial nitrogen prevailed in the organic system. Soil bulk density did not differ between systems, even though intensive tillage was done in the organically managed fields. And despite the high carbon inputs added to the organic system, organic matter was only slightly higher compared to the conventional system.

Take Home Messages

  • When you are transitioning to organic, it is important to reduce synthetic inputs gradually. The soil system needs time to build different sources of nutrients to be sustainable in the long-term. It is known that after the transition period, organic farms have more nitrogen in the soil compared to conventional farms, mainly due to a build-up of the microbial nitrogen pool, but these benefits will not be available immediately.
  • Organic amendments and crop rotations can decrease harmful root-feeding nematodes in the soil. And by including hay in the rotation cycle, you can decrease these nematode populations even more.
  • Intensive tillage can reduce the soil-related benefits of organic farming. On the other hand, organic inputs should significantly increase soil organic matter and decrease soil bulk density. In the organic farming system discussed above, the benefits of the large organic inputs were diminished by the intensive tillage routine. Rather than seeing a decrease in compaction level, the soil bulk density remained the same. And there was only a minor boost in soil organic matter. Decreased use of tillage in organic farming would better take advantage of the benefits that an organic system can provide. 

Read more about it:
This study was conducted at Ohio State in the early 2000s. Published results are availabe online.
Briar, Shabeg S.; Grewal, Parwinder S.; Somasekhar, Nethi; Stinner, D.; Miller, Sally A. 2007. Soil nematode community, organic matter, microbial biomass and nitrogen dynamics in field plots transitioning from conventional to organic management. Applied Soil Ecology 37: 256-266.

Read more news and information on organic agriculture research at offer.osu.edu.

Posted In: soils, research, pests, organic transition
Tags:
September 19, 2019 - 5:36pm -- brown.1844

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