February 2020 - 30

GROWER'S POSTCARD
It's important to explore the many faces of fertility
SAM HITCHCOCK
TILTON
rcvcfarm@gmail.com
I have transitioned many times in my
life; from farmhand, to farmer, to graduate
student, to machinery salesman and now
to horticulture instructor. As a teacher, a
fundamental practice is to break things
down into smaller parts and present them
as separate pieces. Oftentimes the putting
back together of the pieces to form a
comprehensive understanding is neglected.
Indeed, the very notion of 'subjects'
presupposes that the world is separated.
Teaching allows me to learn much more
about these particular and segmented
subjects. But one thing that I don't do
enough of in my life of driving, sitting at the
computer, preparing lessons and the other
minutiae of existing in our American Way,
is participating in and observing nature.
While I can understand single subjects by
reading and practicing with my students,
those methods do not make evident the
connections between topics. To continue
learning about the big picture I have lately
been relying on the experience of those
who continually work with all the pieces
of nature and possess a high capacity for
observation and communication.
Although crops grow through natural
processes, economic exigencies and our
human predilection to understand through
division lead us to break up crop production
into the subjects of tillage, seeding, weeding,
irrigating, pathology, harvesting, etc. It is
clear that each one of these subjects affects
all others, and each may be more effectively
practiced when their relationship in the
whole is understood.
I read some transcribed talks of an
experienced gardener. Their exposition
expanded the subject of fertility to
encompass soil respiration, soil moisture,
and soil texture. And it is this bigger picture
of fertility that I'd like to share with you. The
book is " Performance in the Garden, " by
Alan Chadwick.
This gardener posits that the genesis of
tilling soil was the observation by ancient
peoples that plants grew better on the soil
deposited by landslides (I believe a story
does not have to be true for us to learn
something from it). Why did this landslid
soil improve plant growth? Chadwick
posits that the enhanced plant growth was
caused by improved drainage and improved
aeration in the cultivated soil.
It was fun to teach my students that
while leaves combine carbon dioxide with
sunlight to create sugar, roots burn sugar
with oxygen to create energy. Whereas
leaves exude oxygen (thank goodness),
roots and soil organisms are burning oxygen
and sugar and giving off carbon dioxide.
The result is that while the carbon dioxide
content of the atmosphere is about 0.04%
(and rising rapidly due to our lifestyle!), the
carbon dioxide concentration in the soil is
about six times greater.
As this soil carbon dioxide rises into the
atmosphere your plants love it - like adding
supplemental CO2
in a greenhouse. But
roots and microorganisms want oxygen,
not the CO2
that accumulates from in the
soil from their respiration. This is the reason
that we want to promote gas exchange in
our soil, and why we can say that our soil
breathes. Just like we do, roots and soils
inhale oxygen and exhale carbon dioxide in
the rhythm of soil respiration. The more our
soil respires the more biologically active it is.
Everywhere you look nowadays you
hear people talking about soil biology.
And increasingly biological products are
available to improve fertility. Organisms
in the soil contribute to fertility and the
more oxygen their environment has the
more active they will be; soil respiration
promotes soil fertility.
As breathing is a rhythm of inhalation
and exhalation, soil similarly wants a
rhythm in its moisture. We know that soil
should not sit sodden for long periods of
time - as water occupies valuable pore space
and displaces air from the soil, resulting
in a wet soil with little oxygen. And clearly
neither do we want a soil that dries out to a
degree injurious to plants or soil biology.
What soil biology prefers is a rhythm,
oscillating between moist and drier. Both
soil moisture and soil aeration impact soil
biology, and vibrant soil biology contributes
to fertility - but both soil aeration and soil
moisture are influenced by soil texture - oh,
the many connections. As Chadwick put it,
" the texture of a soil is up to two-thirds of
the whole answer of fertility. "
Hyperbolic though that statement may
be, the texture of a soil goes a long way in
determining how the soil breathes and how
it conducts moisture, what I might call the
capillary personality of that soil. We can
think of capillary as the depth of soil which
functions as a single body - drawing water
from below up to the surface. We want a
soil that stores lots of water and later can
redistribute that water from reserves below
up to our plants. We also want the surface
of our soil to remain loose, as crusting will
seal off the surface and reduce both capillary
and the soil's inhalation and exhalation.
With good soil texture we can tend to
" water the soil, not the crop " because with
enough moisture the soil can resupply water
to the plants through capillary over time.
I looked into the scientific literature a bit
and found that soil temperature and soil
moisture are the largest factors influencing
soil respiration. The soil biology wants some
moisture, but too much fills the soil's pores
and displaces the oxygen. The greatest soil
respiration occurred when 50% of the soil's
pores were filled with water, or 50% water
filled pore space (Linn and Doran, 1984).
A soil's texture is also a factor
influencing the amount of oxygen
reaching the soil - with larger soil pore
spaces allowing more oxygen into the
soil. Keep this in mind when your tilling
power resembles milling flour.
We work soil for many reasons
(moisture management, seeding,
incorporating herbicides, etc.), but it is
important to remember that whenever
we cultivate the soil we are sculpting the
soil environment. As a parent hopes to
culture a child with guiding values, so in
tillage one is setting the foundation for a
soil's respiration, moisture, texture and,
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28 | VegetableGrowersNews.com
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February 2020

Table of Contents for the Digital Edition of February 2020
