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
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
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).
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.