There are still other holistic standards to measure soil
productivity. With more than adequate justification the great
Russian soil microbiologist N.S. Krasilnikov judged fertility by
counting the numbers of microbes present. He said,
". . soil fertility is determined by biological factors, mainly by
microorganisms. The development of life in soil endows it with the
property of fertility. The notion of soil is inseparable from the
notion of the development of living organisms in it. Soil is created
by microorganisms. Were this life dead or stopped, the former soil
would become an object of geology [not biology]."
Louise Howard, Sir Albert's second wife, made a very similar
judgment in her book, _Sir Albert Howard in India._
"A fertile soil, that is, a soil teeming with healthy life in the
shape of abundant microflora and microfauna, will bear healthy
plants, and these, when consumed by animals and man, will confer
health on animals and man. But an infertile soil, that is, one
lacking in sufficient microbial, fungous, and other life, will pass
on some form of deficiency to the plants, and such plant, in turn,
who pass on some form of deficiency to animal and man."
Although the two quotes substantively agree, Krasilnikov had a
broader understanding. The early writers of the organic movement
focused intently on mycorrhizal associations between soil fungi and
plant roots as _the_ hidden secret of plant health. Krasilnikov,
whose later writings benefited from massive Soviet research did not
deny the significance of mycorrhizal associations but stressed
plant-bacterial associations. Both views contain much truth.
Krasilnikov may well have been the greatest soil microbiologist of
his era, and Russians in general seem far ahead of us in this field.
It is worth taking a moment to ask why that is so. American
agricultural science is motivated by agribusiness, either by direct
subsidy or indirectly through government because our government is
often strongly influenced by major economic interests. American
agricultural research also exists in a relatively free market where
at this moment in history, large quantities of manufactured
materials are reliably and cheaply available. Western agricultural
science thus tends to seek solutions involving manufactured inputs.
After all, what good is a problem if you can't solve it by
profitably selling something.
But any Soviet agricultural researcher who solved problems by using
factory products would be dooming their farmers to failure because
the U.S.S.R.'s economic system was incapable of regularly supplying
such items. So logically, Soviet agronomy focused on more holistic,
low-tech approaches such as manipulating the soil microecology. For
example, Americans scientifically increase soil nitrogen by
spreading industrial chemicals; the Russians found low-tech ways to
brew bacterial soups that inoculated a field with slightly more
efficient nitrogen-fixing microorgamsms.
Soil microbiology is also a relatively inexpensive line of research
that rewards mental cleverness over massive investment. Multimillion
dollar laboratories with high-tech equipment did not yield big
answers when the study was new. Perhaps in this biotech era,
recombinant genetics will find high-tech ways to tailor make
improved microorganisms and we'll surpass the Russians.
Soil microorganism populations are incredibly high. In productive
soils there may be billions to the gram. (One gram of fluffy soil
might fill 1/2 teaspoon.) Krasilnikov found great variations in
bacterial counts. Light-colored nonproductive earths of the North
growing skimpy conifer trees or poor crops don't contain very many
microorganisms. The rich, black, grain-producing soils of the
Ukraine (like our midwestern corn belt) carry very large microbial
populations.
One must be clever to study soil microbes and fungi. Their life
processes and ecological interactions can't be easily observed
directly in the soil with a microscope. Usually, scientists study
microorganisms by finding an artificial medium on which they grow
well and observe the activities of a large colony or pure culture--a
very restricted view. There probably are more species of
microorganisms than all other living things combined, yet we often
can't identify one species from another similar one by their
appearance. We can generally classify bacteria by shape: round ones,
rod-shaped ones, spiral ones, etc. We differentiate them by which
antibiotic kills them and by which variety of artificial material
they prefer to grow on. Pathogens are recognized by their prey.
Still, most microbial activities remain a great mystery.
Krasilnikov's great contribution to science was discovering how soil
microorganisms assist the growth of higher plants. Bacteria are very
fussy about the substrate they'll grow on. In the laboratory, one
species grows on protein gel, another on seaweed. One thrives on
beet pulp while another only grows on a certain cereal extract.
Plants "understand" this and manipulate their soil environment to
enhance the reproduction of certain bacteria they find desirable
while suppressing others. This is accomplished by root exudates.
For every 100 grams of above-ground biomass, a plant will excrete
about 25 grams of root exudates, creating a chemically different
zone (rhizosphere) close to the root that functions much like the
culture medium in a laboratory. Certain bacteria find this region
highly favorable and multiply prolifically, others are suppressed.
Bacterial counts adjacent to roots will be in hundreds of millions
to billions per gram of soil. A fraction of an inch away beyond the
influence of the exudates, the count drops greatly.
Why do plants expend energy culturing bacteria? Because there is an
exchange, a _quid pro quo._ These same bacteria assist the plant in
numerous ways. Certain types of microbes are predators. Instead of
consuming dead organic matter they attack living plants. However,
other species, especially actinomycetes, give off antibiotics that
suppress pathogens. The multiplication of actinomycetes can be
enhanced by root exudates.
Perhaps the most important benefit plants receive from soil bacteria
are what Krasilnikov dubbed "phytamins," a word play on vitamins
plus _phyta_ or "plant" in Greek. Helpful bacteria exude complex
water-soluble organic molecules that plants uptake through their
roots and use much like humans need certain vitamins. When plants
are deprived of phytamins they are less than optimally healthy, have
lowered disease resistance, and may not grow as large because some
phytamins act as growth hormones.
Keep in mind that beneficial microorganisms clustering around plant
roots do not primarily eat root exudates; exudates merely optimize
environmental conditions to encourage certain species. The main food
of these soil organisms is decaying organic matter and humus.
Deficiencies in organic matter or soil pH outside a comfortable
range of 5.75-7.5 greatly inhibit beneficial microorganisms.
For a long time it has been standard "chemical" ag science to deride
the notion that plant roots can absorb anything larger than simple,
inorganic molecules in water solution. This insupportable view is no
longer politically correct even among adherents of chemical usage.
However, if you should ever encounter an "expert" still trying to
intimidate others with these old arguments merely ask them, since
plant roots cannot assimilate large organic molecules, why do people
succeed using systemic chemical pesticides? Systemics are large,
complex poisonous organic molecules that plants uptake through their
roots and that then make the above-ground plant material toxic to
predators. Ornamentals, like roses, are frequently protected by
systemic chemical pesticides mixed into chemical fertilizer and fed
through the soil.
Root exudates have numerous functions beyond affecting
microorganisms. One is to suppress or encourage the growth of
surrounding plants Gardeners experience this as plant companions and
antagonists. Walnut tree root exudates are very antagonistic to many
other species. And members of the onion family prevent beans from
growing well if their root systems are intermixed.
Many crop rotational schemes exist because the effects of root
exudates seem to persist for one or even two years after the
original plant grew That's why onions grow very well when they are
planted where potatoes grew the year before. And why farmers grow a
three year rotation of hay, potatoes and onions. That is also why
onions don't grow nearly as well following cabbage or squash.
Farmers have a much easier time managing successions. They can grow
40 acres of one crop followed by 40 acres of another. But squash
from 100 square feet may overwhelm the kitchen while carrots from
the same 100 square feet the next year may not be enough. Unless you
keep detailed records, it is hard to remember exactly where
everything grew as long as two years ago in a vegetable garden and
to correlate that data with this year's results. But when I see half
a planting on a raised bed grow well and the adjacent half grow
poorly, I assume the difficulty was caused by exudate remains from
whatever grew there one, or even, two years ago.
In 1990, half of crop "F" grew well, half poorly. this was due to
the presence of crop "D" in 1989. The gardener might remember that
"D" was there last year. But in 1991, half of crop "G" grew well,
half poorly. This was also due to the presence of crop "D" two years
ago. Few can make this association.
These effects were one reason that Sir Albert Howard thought it was
very foolish to grow a vegetable garden in one spot for too many
years. He recommended growing "healing grass" for about five years
following several years of vegetable gardening to erase all the
exudate effects and restore the soil ecology to normal.
Mycorrhizal association is another beneficial relationship that
should exist between soil organisms and many higher plants. This
symbiotic relationship involves fungi and plant roots. Fungi can be
pathogenic, consuming living plants. But most of them are harmless
and eat only dead, decaying organic matter. Most fungi are soil
dwellers though some eat downed or even standing trees.
Most people do not realize that plant roots adsorb water and
water-soluble nutrients only through the tiny hairs and actively
growing tips near the very end of the root. The ability for any new
root to absorb nutrition only lasts a short time, then the hairs
slough off and the root develops a sort of hard bark. If root system
growth slows or stops, the plant's ability to obtain nourishment is
greatly reduced. Roots cannot make oxygen out of carbon dioxide as
do the leaves. That's why it is so important to maintain a good
supply of soil air and for the soil to remain loose enough to allow
rapid root expansion.
When roots are cramped, top growth slows or ceases, health and
disease resistance drops, and plants may become stressed despite
applications of nutrients or watering. Other plants that do not seem
to be competing for light above ground may have ramified (filled
with roots) far wider expanses soil than a person might think. Once
soil is saturated with the roots and the exudates from one plant,
the same space may be closed off to the roots of another. Gardeners
who use close plantings and intensive raised beds often unknowingly
bump up against this limiting factor and are disappointed at the
small size of their vegetables despite heavy fertilization, despite
loosening the earth two feet deep with double digging, and despite
regular watering. Thought about in this way, it should be obvious
why double digging improves growth on crowded beds by increasing the
depth to which plants can root.
The roots of plants have no way to aggressively breakdown rock
particles or organic matter, nor to sort out one nutrient from
another. They uptake everything that is in solution, no more, no
less while replacing water evaporated from their leaves. However,
soil fungi are able to aggressively attack organic matter and even
mineral rock particles and extract the nutrition they want. Fungi
live in soil as long, complexly interconnected hair-like threads
usually only one cell thick. The threads are called "hyphae." Food
circulates throughout the hyphae much like blood in a human body.
Sometimes, individual fungi can grow to enormous sizes; there are
mushroom circles hundreds of feet in diameter that essentially are
one single very old organism. The mushrooms we think of when we
think "fungus" are actually not the organism, but the transitory
fruit of a large, below ground network.
Certain types of fungi are able to form a symbiosis with specific
plant species. They insert a hyphae into the gap between individual
plant cells in a root hair or just behind the growing root tip. Then
the hyphae "drinks" from the vascular system of the plant, robbing
it of a bit of its life's blood. However, this is not harmful
predation because as the root grows, a bark develops around the
hyphae. The bark pinches off the hyphae and it rapidly decays inside
the plant, making a contribution of nutrients that the plant
couldn't otherwise obtain. Hyphae breakdown products may be in the
form of complex organic molecules that function as phytamins for the
plant.
Not all plants are capable of forming mycorrhizal associations.
Members of the cabbage family, for example, do not. However, if the
species can benefit from such an association and does not have one,
then despite fertilization the plant will not be as healthy as it
could be, nor grow as well. This phenomenon is commonly seen in
conifer tree nurseries where seedling beds are first completely
sterilized with harsh chemicals and then tree seeds sown. Although
thoroughly fertilized, the tiny trees grow slowly for a year or so.
Then, as spores of mycorrhizal fungi begin falling on the bed and
their hyphae become established, scattered trees begin to develop
the necessary symbiosis and their growth takes off. On a bed of
two-year-old seedlings, many individual trees are head and shoulders
above the others. This is not due to superior genetics or erratic
soil fertility. These are the individuals with a mycorrhizal
association.
Like other beneficial microorganisms, micorrhizal fungi do not
primarily eat plant vascular fluid, their food is decaying organic
matter. Here's yet another reason to contend that soil productivity
can be measured by humus content.