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
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
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 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).
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.