Dr. Paul's Research Perspectives
Feed the soil, not the plant
Organic farming challenges centuries of scientific thinking with regard to plant nutrition—but the proof is in the results.

By Paul Hepperly

Editors' note:
As New Farm Research and Training Manager at The Rodale Institute, Dr. Paul Hepperly has been a regular contributor to NewFarm.org for some time, providing research updates, op-ed pieces, and white papers on topics like carbon sequestration in organic farming systems.

None of those venues do full justice to the range of Paul's experience, however. Paul grew up on a family farm in Illinois and holds a Ph.D. in plant pathology, an M.S. in agronomy and a B.S. in psychology from the University of Illinois at Champaign-Urbana. He has worked for the USDA Agricultural Research Service, in academia, and for a number of private seed companies, including Asgrow, Pioneer, and DeKalb. He has overseen research in Hawaii, Iowa, Puerto Rico, and Chile, and investigated such diverse crops as soybeans, corn, sorghum, sunflowers, ginger, and papaya. He has witnessed the move toward biotech among the traditional plant breeding community and the move toward organics among new wave of upcoming young farmers. Beford coming to the Rodale Institute Paul worked with hill farmers in India to help them overcome problems with ginger root rot in collaboration with Winrock International.

Now we've decided to give Paul his own column, in which he can report on agricultural research from around the world and reflect on its relevance to The Rodale Institute's research program and to the progress of sustainable agriculture more generally in light of his own broad perspective. Enjoy.

Posted November 10, 2005: Ever since the Flemish chemist Jan Baptista van Helmont (1580-1644) planted a willow tree in a pot and weighed the soil in which it grew, people have wondered and argued about what it is exactly that makes plants grow. Van Helmont concluded that water was the principle plant growth substance. Subsequent researchers put more emphasis on soil salts. Justus von Liebig (1803-73) took this idea to the limit, claiming that all plant growth was based on simple salts and that the soil played only a limited role as a nonliving physical support system. This formed the basis for the reigning chemical theory of soil fertility.

More recently, however, organic farmers and researchers have expounded a different view of plant fertility, one which emphasizes the entirety of the soil, rather than simple chemical salts present therein, as the essential life-giving medium for plants. We believe the living soil provides nutrients that are vital for healthy plant growth and productivity. Therefore, we advocate growing systems that feed the soil, not the plant, as the best agricultural strategy.

Started in 1981, The Rodale Institute Farming Systems Trial® (FST) provides an excellent stage for testing the validity and practicality of these dramatically contrary principles, thanks to years of synthetic chemical fertilization in the conventional plots and soil building practices in the organic systems.

Initially, existing FST soil nitrogen levels were unable to meet the needs of corn, but after four years of organic soil management, corn yields reached levels statistically identical to those in the conventionally managed system.

Tracking nitrogen in agricultural systems

In 1987 and 1988, the fate of nitrogen from ammonium sulfate fertilizer in the conventional system, and from red clover in the organic systems, was measured using labeled Nitrogen isotope 15. This isotope tagging allowed the researchers to find: 1) how much nitrogen was absorbed by crops; 2) how much was captured in the soil; and 3) how much nitrogen was lost to the environment.

The results showed that more fertilizer than legume N (40 percent and 17 percent respectively) was directly recovered by corn plants. Since corn yields were equivalent, these data suggest that organically managed soils were able to provide a large proportion of the corn’s N requirements.

A second major finding was that organic legume N was retained in the soil to a much greater extent than fertilizer N (47 percent and 17 percent respectively). The ability of legume N to be stored in soil is an essential element of organic management, since it allows crops to utilize soil resources more efficiently. This explains one mechanism for the soil building that occurs under organic management.

The study also discovered that in the first year after application, 38 percent of the fertilizer N was lost to the environment, compared to 18 percent of the legume N. These data show that the rapid availability of N from chemical sources is a mixed advantage at best, providing rapid plant response but also precipitating serious environmental impacts. This supports the claim that organic systems provide better environmental performance compared to systems based on chemical fertilizers.

Feeding the microbes

Finally, tagged N was recovered and measured in soil microbial biomass. Organically managed soils fertilized with legume N developed 3.6 times as much tagged microbial biomass as soils fertilized with conventional chemical N sources (18 percent and 5 percent respectively).

Click here for graph

But what about the living soil hypothesis? Measurements of microbial activity through respiration rates showed that, in the legume organic system, activity was 2.36 times higher than that of the conventional system (130 micrograms of carbon dioxide per gram compared to about 55 micrograms respectively).

To illustrate the effects of weather on the organic and conventional systems, 2005 provided a pretty severe drought, and 2004 brought the highest rainfall during a crop season in more than 50 years. In both years, the organic systems thrived and were more productive than the conventional system. This is because the healthy, living soil served to retain more nitrogen under heavy rainfall, and helped to capture and retain water more efficiently under scarce rainfall, making both vital elements more available for the crop plants.

With climate change creating environmental “whiplash” throughout the world, it’s time for us to give Liebig’s approach a rest and start looking more seriously at the living soil to support our crops. This strategy works under the toughest conditions and helps to improve agri-environmental conditions to pass along to future generations.


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