Hello to all...
Today in class, we synthesized the last several weeks of readings
into some key agro-ecological principles.
Only some of the principles that we discussed have any connection to
gardening. The principles are listed below for your perusal.
***Nutrient economy principles
The largest biologically active elements in temperate ecosystems is soil
organic matter. Most of the SOM pool is very resistance to biological
activity and serves primarily as a storehouse for nutrients (both
constitutive and electrostatically retained ions) and water.
Cation exchange capacity has a very significant role in the nutrient
dynamics of temperate soils but much less role in tropical soils.
The primary currency of the nutrient and energy economies in ecosystems is
active organic matter. This pool consists primarily of recent plant
residues and is thus quite sensitive to management.
Plant-soil systems with maximum transpiration are least susceptible to
leaching losses.
A soil pH ranging between 6-7 optimizes bioavailability of nutrients.
Synchronicity between microbial activity and plant growth
minimizes loss of nutrients
High C:N ratio residues retained on the soil surface (as compared to
incorporated) minimizes loss of nutrients
A residue layer at the soil surface reduces nutrient
loss by preventing erosion and moderating soil temperature. Soil microbial
activity is more balanced with plant root activity at moderate soil
temperatures.
***Population process principles
Ecosystems with moderate levels of stress and disturbance tend to support
the greatest biodiversity
Agricultural systems with less soil disturbance will have weeds with a
different life history than systems with high soil disturbance.
Two organisms cannot occupy the same niche (Gause's law)
Polycultures of species with complementary niches are likely to
"over-yield".
Polycultures that fill available niches are les suceptible to weed
incursion.
***Agroecosystem energetics principles
Ecosystems rarely capture more than 1% of incident solar energy.
Modern breeding has changed the harvest index of crops but has had no
significant impact on photosynthetic efficiency.
Increases in cultural energy inputs nearly always decrease the energy
efficiency of agriculture but often result in increased crop yields and
labor efficiency.
Reducing use of synthetic N fertilizer and minimizing tillage are the two
biggest opportunities for increased energy efficiency in industrialized
agriculture.
Cheap energy sustains the intensive use of tillage and N fertilizer and
reliance on deep non-renewable water sources.
Joel Gruver
Center for Agriculture, Food and Environment
Tufts University
P.S. I received your letter Henry and your copy of the article is on its
way.
Dear Joel
Your summation of fumdamental principles is profoundly helpful, and puts many
things in their correct perspective. There are a few points that are a bit
unclear to me, and I would very much appreciate your comments on them.
Joel Brooks Gruver wrote:
How are the components which store nutrients broken down, so that the
nutrients then become available? Is it a case where "excess nutrients are
"bound up", but when particular "available nutrients" become low, then the
"stored nutrients" then are amenable to release?
> Cation exchange capacity has a very significant role in the nutrient
> dynamics of temperate soils but much less role in tropical soils.
What are the practises to avoid, to avoid interfering negatively with cation
exchange proceses? How can one tell if cation exchange processes are not
proceeding properly?
> The primary currency of the nutrient and energy economies in ecosystems is
> active organic matter. This pool consists primarily of recent plant
> residues and is thus quite sensitive to management.
I would assume that cellulose is the major constituent in "active organic
matter." Is this correct? If it is correct, then would this not suggest that
it would be better to mix in organic wastes directly with the soil, to have
composting occur "in place", rather than "pre-consuming" the active organic
matter, as as done in normal composting?
> Plant-soil systems with maximum transpiration are least susceptible to
> leaching losses.
Could you please clarify the term "transpiration" in this context?
> A soil pH ranging between 6-7 optimizes bioavailability of nutrients.
I can understand how this would be corect in conventional "chemical
agriculture." However, there was a recent thread about growing acid loving
rhodenderons in compost soils, and the general conclusion was that normal pH
readings were not really meaningful in a highly organic system....iron would
be available to the rhodenderons, and yet the same compost soil could be used
for growing crops which required nutrients that would normally be available at
pH's where the iron is unavailable. Could you please comment on this apparent
paradox?
Is this a "good thing" or a "bad thing?" Does this perhaps suggest that it is
best to employ a "mixed tilling strategy", where one employs "no-till" for
several years, and then "tilling" for one or two years?
> Two organisms cannot occupy the same niche (Gause's law)
> Polycultures of species with complementary niches are likely to
> "over-yield".
Very interesting!! Would you have some general guidelines on how to mix
species to encourage this phenomenon?
> Polycultures that fill available niches are les suceptible to weed
> incursion.
More interesting!!!.....Could the invading weed species be used as "indicator
plants" for selection of a paying crop that would fill the available niche?
> ***Agroecosystem energetics principles
> Ecosystems rarely capture more than 1% of incident solar energy.
I presume that you mean that the energy content of dry matter is only 1% of
the solar energy falling on a given area. Solar energy accompllishes a
"pumping function" by permitting the nutrients to be carried from the root
zone to the leaves, and then disposing of excess water. This is not a loss, or
an inefficiency, but rather a "parasitic energy requirement".
> Modern breeding has changed the harvest index of crops but has had no
> significant impact on photosynthetic efficiency.
This makes sense, in that fundamental growing mechanisms are not changed.
Why is energy efficiency important? Are not cost of production and quality of
the product more important? "Cost of production" is a very deep
concept....more appropriate would be the "long term cost of production." This
leads to inclusion of sustainability factors.
> Cheap energy sustains the intensive use of tillage and N fertilizer and
> reliance on deep non-renewable water sources.
I would suggest that the fundamental problem is subsidies, which guarantee a
distorted use of resources. If there were no subsidies, resources would rise
to their true cost, and the farmers would quickly evolve systems which were
truly more efficient.
I welcome your comments, and I thank you very much for posting these very
helpful and thought provokong principles.
Kindest regards,
Kevin Chisholm
Hello Kevin and all others... I have tried to answer Kevin's questions
below:
*snip*
> How are the components which store nutrients broken down, so that the
> nutrients then become available? Is it a case where "excess nutrients are
> "bound up", but when particular "available nutrients" become low, then the
> "stored nutrients" then are amenable to release?
Lots of mechanisms are involved with the conversion of stored nutrients
into plant available forms. The most readily available pool of nutrients
is the pool that is electrostatically held on charged soil surfaces (most
of which are negative).
My understanding is that this pool is rarely far from equilibrium
with the soil solution... so when plants or microbes take up nutrients
depleting the soil solution, nutrient ions oscillating at charged soil
surfaces quickly move out into the soil solution.
Roots maintain charge neutrality as they take up nutrients, releasing H+
or bicarbonate- ions. These ions can exchange with nutrients on the
charged soil surfaces.
The soil solution is a soup of enzymes, many of which catalyze
reactions which slice off pieces of large organic matter molecules (such
as ancient humus or more recent residues). Nutrients in these little
pieces are either utilized in biosynthesis (immobilized) by soil microbes
or excreted (mineralized).
Microbial grazers with higher C:N rations graze upon microbes with lower
C:N ratios leading to the release of excess N.
(I haven't read Dr. Ingham's newsletter but I suspect she discusses some
of these ideas in the newsletter)
There are three basic mechanisms which protect SOM in soil.
1) Physical protection inside aggregates or pores too small for soil
organisms to enter (protection in micropores is very significant in fine
textured soils)
2) Chemical protection because OM binds to mineral surfaces or
condensation/polymerization reactions that link small OM
molecules into very complex and recalcitrant humic molecules
3) Biological protection within living biomass
Mechanical disruption of soil releases physically protected OM
(e.g. tillage, freezing and thawing, wetting and drying, root extension)
Chemical equilibrium driven reactions such as hydrolysis and
biologically mediated chemical reactions such as enzyme reactions can
release chemically protected OM
Soil biological activity (e.g. grazing of microbes by nematodes and
protozoa) releases biological protected OM.
Depending on your soils clay mineralogy more or less of the surface charge
is pH dependent. All organic matter surface charge is pH dependent. At
higher pHs, pH dependent charge is more negative and thus the soil has
greater CEC. Soil CEC drops as soils acidify, either naturally or as a
result of management. Excessive application of K fertilizer can cause the
inner layers of some clay minerals to collapse, reducing their surface
area and surface charge.
Cellulose is the dominant component of plant biomass and so it would also
be the dominant constituent of fairly fresh residues... but being a
polymer of glucose units (C6H12O6) it does not contain any of the
nutrients that plants obtain from soil...
I think that there are benefits both to composting in a pile and
decomposition in the soil. Growing roots in the soil is one of the best
ways to increase the active fraction of OM.
> Plant-soil systems with maximum transpiration are least susceptible to
> leaching losses.
> Could you please clarify the term "transpiration" in this context?
Transpiration is evaporation from leaf surfaces. Soils with more upper
ward movement of water driven by transpiration have less leaching.
I think that adding lime to soils that have a pH below 5.5 is a
worthwhile practice unless you are growing calcifuge/acid loving plants.
Lime is relatively cheap and it is difficult to overapply if you are
starting with an acid soil. Heavy applications of organic matter can serve
the same role of adding calcium and neutralizing soil acidity/increasing
bioavailability of nutrients as lime...
In long term no-till systems, you will have very few of the summer annual
weeds that are ubiquitous in regularly tilled soils (lambs quarters,
amaranth, ragweed, galonsauga (sp ?)... but in agricultural situations,
perennial weeds such as poison ivy, sumac, brambles. mulberry trees... can
start invading... in a regularly tended no-till garden, these types of
perennial weeds are not likely to be a problem... they move in relatively
slowly and you can yank them out...
About a month ago, I sent a few comments on complementary cover crop
niches to the OGL list... these would also be relavent for crop:crop
combos or crop/cover crop combos
Yesterday, i was at a farm where the japanese millet cover crop had a fair
amount of amaranth/pig weed growing in it. The inclusion of a broadleaf
cover crop such a clover, rape or oil seed radish would have probably
excluded most of the amaranth.
Yes, this is what I meant...
> Why is energy efficiency important? Are not cost of production and quality of
> the product more important? "Cost of production" is a very deep
> concept....more appropriate would be the "long term cost of production." This
> leads to inclusion of sustainability factors.
Energy efficiency is probably not the most important characteristic or
sustainable agricultural systems but it is an important consideration.
If fossil energy were to suddenly become expensive (as occurred during the
70s) then energy efficiency would be much more important. During the 70's
everyone was doing energy efficiency studies... the studies looking at the
energetics of agriculture done in the last 15 years could probably be
counted on one hand...
Well... this took longer than I thought...
Joel
After posing my question, I re-read this post of yours, Joel. I've asked
for some clarification below relevant points. TIA for any light you can
shed. :-)
> ***Nutrient economy principles
(snip)
> Plant-soil systems with maximum transpiration are least susceptible to
> leaching losses.
How then does one maximize transpiration?
> High C:N ratio residues retained on the soil surface (as compared to
> incorporated) minimizes loss of nutrients
By...???
Does this also apply during winter? Is it one reason why high C/N ratios
retained on the soil surface minimize nutrient losses?
Diane Ridout, Instructor, ACP.............................................
Kwantlen University College, "Talk
12666-72 Avenue doesn't
Surrey, BC, Canada V3W 2M8 cook rice," they say.
Tel: (604) 599-2964 Voice mail 9837.......................................
