YSM Issue 93.2

FOCUS
Ecology
THE ROLE OF
IN PLANT
How does soil organic matter help crop growth?
BY CINDY KUANG
Death, SOM, and Soil
Death often stays in the soil. Over time,
organisms and residues in varying
states of decomposition form a vital
component of the soil: soil organic matter
(SOM). SOM has always been thought of as
an indicator of soil fertility, contributing to
healthier soil and better crop growth. Thus,
building SOM, or raising its levels through the
addition of compost or manure, is assumed to
be a cost-effective way of reducing reliance
on external inputs such as fertilization and
irrigation. But how well are the effects of
SOM actually understood? In various studies,
higher SOM has been shown to correlate with
both higher and lower productivity, so until
now, the effects of added SOM on soil fertility
are inconsistent and unclear.
The question is further complicated by
the possibility that this causative pathway is
bidirectional: Does SOM lead to increased
crop productivity, or do increased plant
inputs lead to higher SOM levels? In order
to figure out this relationship between SOM,
agricultural inputs, and productivity, we
need to be able to isolate and investigate
SOM’s effect on plant growth. Emily Oldfield,
a postdoctoral fellow at the Yale School of
Forestry and Environmental Studies, works
in the Bradford lab to study SOM’s effects. “A
lot of the policies that are being put forth by
organizations like the USDA, the Food and
Agriculture organization rest on the premise
of the more organic matter, the better, but
there’s really no hard quantification of how
much more and how much better,” Oldfield
said. “The goal of my research was to try to
put some numbers behind it.” She described
this greenhouse study as an effort to use
a controlled environment to establish a
causative pathway between SOM and crop
productivity.
What is soil?
Soil itself is a complex mixture of elements,
consisting of around forty-five percent
minerals, (including sand, silt, clay) and fifty
percent air and water. In particular, plants
require nitrogen more than any other nutrient,
but they can only take up mineral forms of it,
including nitrate and ammonia, which only
make up two percent of the nitrogen in soil.
The other ninety-eight percent of nitrogen is
organic and inaccessible to plants, meaning
many farmers rely on mineral N-fertilizer to
facilitate crop growth.
The remaining five percent of soil
composition is soil organic matter—anything
that was once living. Though SOM is a very
small percentage by volume, its influence is
disproportionately large. SOM dictates the
structure of the soil, increasing aeration and
water-holding capacity, and acts as a habitat
for other soil organisms. SOM also powers
the cycling, retention and release of various
nutrients essential to productivity.
The Experimental Setup
Oldfield was determined to quantify the
effect that SOM, fertilization, and irrigation
have on crop productivity, both used
separately and in tandem. She designed an
experiment with four target levels of SOM
(1%, 2.5%, 5.5%, 8.5%) crossed with two
different fertilization treatments (none
versus 100 kg N/ha as urea) and further
crossed with two irrigation treatments
(optimum versus half of optimum). To
verify reliability of results, each treatment
was replicated 10 times—for a grand total of
160 experimental pots.
A dilution approach of organic-rich A
horizon soil (obtained from the Yale Farm in
New Haven, Connecticut) was used to create
these varying SOM levels. By mixing the soil
with an external mineral component (sand
and clay) in different ratios, Oldfield was able
to create a wide gradient of organic matter
concentrations without having to artificially
manipulate SOM (which can lead to other
experimental issues).
This greenhouse experiment was
conducted from May to July, and automatic
ventilation ensured that the pots of
spring wheat (Triticum aestivum, L.), the
experimental crop, never exceeded a daily
temperature of 30 degrees Celsius. A drip
irrigation system was calibrated to emit 0.25
gallons to each pot per hour, though this was
later modified to create different treatments
for various pots. “Optimum irrigation” was
determined to be around 127.2 mL of water
each day and “suboptimum irrigation” was
63.6 mL. At the end of the growing period,
all plants were cut at soil level at the same
time, dried at 65 degrees Celsius and then
weighed in aboveground biomass. Soils were
then passed through a sieve and measured
in terms of SOM content, water-retaining
capacity, pH, microbial biomass, and rates of
net mineralization and nitrification.
16 Yale Scientific Magazine September 2020 www.yalescientific.org