Organic matter benefits soil productivity not because it is present,
but because all forms of organic matter in the soil, including its
most stable form--humus--are disappearing. Mycorrhizal fungi and
beneficial bacterial colonies around plant roots can exist only by
consuming soil organic matter. The slimes and gums that cement soil
particles into relatively stable aggregates are formed by
microorganisms as they consume soil organic matter. Scats and casts
that _are_ soil crumbs form only because organic matter is being
consumed. If humus declines, the entire soil ecology runs down and
with it, soil tilth and the health and productivity of plants.
If you want to manage your garden soil wisely, keep foremost in mind
that the rate of humus loss is far more important than the amount of
humus present. However, natural processes remove humus without our
aid or attention while the gardener's task is to add organic matter.
So there is a very understandable tendency to focus on addition, not
subtraction. But, can we add too much? And if so, what happens when
we do?
How Much Humus is Soil Supposed to Have?
If you measured the organic matter contents of various soils around
the United States there would be wide differences. Some variations
on crop land are due to great losses that have been caused by
mismanagement. But even if you could measure virgin soils never used
by humans there still would be great differences. Hans Jenny, a soil
scientist at the University of Missouri during the 1940s, noticed
patterns in soil humus levels and explained how and why this occurs
in a wonderfully readable book, _Factors in Soil Formation._ These
days, academic agricultural scientists conceal the basic simplicity
of their knowledge by unnecessarily expressing their data with
exotic verbiage and higher mathematics. In Jenny's time it was not
considered demeaning if an intelligent layman could read and
understand the writings of a scientist or scholar. Any serious
gardener who wants to understand the wide differences in soil should
become familiar with _Factors in Soil Formation._ About organic
matter in virgin soils, Jenny said:
"Within regions of similar moisture conditions, the organic matter
content of soil . . . decreases from north to south. For each fall
of 10 degree C (18 degree F) in annual temperature the average
organic matter content of soil increases two or three times,
provided that [soil moisture] is kept constant."
Moist soil during the growing season encourages plant growth and
thus organic matter production. Where the soil becomes dry during
the growing season, plant growth slows or stops. So, all things
being equal, wet soils contain more organic matter than dry ones.
All organic matter eventually rots, even in soil too dry to grow
plants. The higher the soil temperature the faster the
decomposition. But chilly (not frozen) soils can still grow a lot of
biomass. So, all things being equal, hot soils have less humus in
them than cold ones. Cool, wet soils will have the highest levels;
hot, dry soils will be lowest in humus.
This model checks out in practice. If we were to measure organic
matter in soils along the Mississippi River where soil moisture
conditions remain pretty similar from south to north, we might find
2 percent in sultry Arkansas, 3 percent in Missouri and over 4
percent in Wisconsin, where soil temperatures are much lower. In
Arizona, unirrigated desert soils have virtually no organic matter.
In central and southern California where skimpy and undependable
winter rains peter out by March, it is hard to find an unirrigated
soil containing as much as 1 percent organic matter while in the
cool Maritime northwest, reliable winter rains keep the soil damp
into June and the more fertile farm pastures or natural prairies may
develop as much as 5 percent organic matter.
Other factors, like the basic mineral content of the soil or its
texture, also influence the amount of organic matter a spot will
create and will somewhat increase or decrease the humus content
compared to neighboring locations experiencing the same climate. But
the most powerfully controlling influences are moisture and
temperature.
On all virgin soils the organic matter content naturally sustains
itself at the highest possible level. And, average annual additions
exactly match the average annual amount of decomposition. Think
about that for a moment. Imagine that we start out with a plot of
finely-ground rock particles containing no life and no organic
matter. As the rock dust is colonized by life forms that gradually
build in numbers it becomes soil. The organic matter created there
increases nutrient availability and accelerates the breakdown of
rock particles, further increasing the creation of organic matter.
Soil humus steadily increases. Eventually a climax is sustained
where there is as much humus in the soil as there can be.
The peak plant and soil ecology that naturally lives on any site is
usually very healthy and is inevitably just as abundant as there is
moisture and soil minerals to support it. To me this suggests how
much organic matter it takes to grow a great vegetable garden. My
theory is that in terms of soil organic matter, vegetables grow
quite well at the humus level that would peak naturally on a virgin
site. In semi-arid areas I'd modify the theory to include an
increase as a result of necessary irrigation. Expressed as a rough
rule of thumb, a mere 2 percent organic matter in hot climates
increasing to 5 percent in cool ones will supply sufficient
biological soil activities to grow healthy vegetables if _the
mineral nutrient levels are high enough too._
Recall my assertion that what is most important about organic matter
is not how much is present, but how much is lost each year through
decomposition. For only by decomposing does organic matter release
the nutrients it contains so plants can uptake them; only by being
consumed does humus support the microecology that so markedly
contributes phytamins to plant nutrition, aggressively breaks down
rock particles and releases the plant nutrients they contain; only
by being eaten does soil organic matter support bacteria and
earthworms that improve productivity and create better tilth.
Here's something I find very interesting. Temperate climates having
seasons and winter, vary greatly in average temperature. Comparing
annual decomposition loss from a hot soil carrying 2 percent humus
with annual decomposition loss from a cooler soil carrying 5
percent, roughly the same amount of organic matter will decay out of
each soil during the growing season. _This means that in temperate
regions we have to replace about the same amount of organic matter
no matter what the location._
Like other substantial colleges of agriculture, the University of
Missouri ran some very valuable long-term studies in soil
management. In 1888, a never-farmed field of native prairie grasses
was converted into test plots. For fifty succeeding years each plot
was managed in a different but consistent manner. The series of
experiments that I find the most helpful recorded what happens to
soil organic matter as a consequence of farming practices. The
virgin prairie had sustained an organic matter content of about 3.5
percent. The lines on the graph show what happened to that organic
matter over time.
Timothy grass is probably a slightly more efficient converter of
solar energy into organic matter than was the original prairie.
After fifty years of feeding the hay cut from the field and
returning all of the livestock's manure, the organic matter in the
soil increased about 1/2 percent. Obviously, green manuring has very
limited ability to increase soil humus above climax levels. Growing
oats and returning enough manure to represent the straw and grain
fed to livestock, the field held its organic matter relatively
constant.
Growing small grain and removing everything but the stubble for
fifty years greatly reduced the organic matter. Keep in mind that
half the biomass production in a field happens below ground as
roots. And keep in mind that the charts don't reveal the sad
appearance the crops probably had once the organic matter declined
significantly. Nor do they show that the seed produced on those
degenerated fields probably would no longer sprout well enough to be
used as seedgrain, so new seed would have been imported into the
system each season, bringing with it new supplies of plant
nutrients. Without importing that bushel or so of wheat seed on each
acre each year, the curves would have been steeper and gone even
lower.
Corn is the hardest of the cereals on soil humus. The reason is,
wheat is closely broadcast in fall and makes a thick grassy
overwintering stand that forms biomass out of most of the solar
energy striking the field from spring until early summer when the
seed forms. Leafy oats create a little more biomass than wheat.
Corn, on the other hand, is frost tender and can't be planted early.
It is also not closely planted but is sown in widely-spaced rows.
Corn takes quite a while before it forms a leaf canopy that uses all
available solar energy. In farming lingo, corn is a "row crop."
Vegetables are also row crops. Many types don't form dense canopies
that soak up all solar energy for the entire growing season like a
virgin prairie. As with corn, the ground is tilled bare, so for much
of the best part of the growing season little or no organic matter
is produced. Of all the crops that a person can grow, vegetables are
the hardest on soil organic matter. There is no way that vegetables
can maintain soil humus, even if all their residues are religiously
composted and returned. Soil organic matter would decline markedly
even in an experiment in which we raised some small animals
exclusively on the vegetables and returned all of their manure and
urine too.
When growing vegetables we have to restore organic matter beyond the
amount the garden itself produces. The curves showing humus decline
at the University of Missouri give us a good hint as to how much
organic matter we are going to lose from vegetable gardening. Let's
make the most pessimistic possible estimate and suppose that
vegetable gardening is twice as hard on soil as was growing corn and
removing everything but the stubble and root systems.
With corn, about 40 percent of the entire organic matter reserve is
depleted in the first ten years. Let's suppose that vegetables might
remove almost _all_ soil humus in ten years, or 10 percent each year
for the first few years. This number is a crude. and for most places
in America, a wildly pessimistic guess.
However, 10 percent loss per year may understate losses in some
places. I have seen old row crop soils in California's central
valley that look like white-colored blowing dust. Nor does a 10
percent per year estimate quite allow for the surprising durability
I observe in the still black and rich-looking old vegetable seed
fields of western Washington State's Skaget Valley. These
cool-climate fields have suffered chemical farming for decades
without having been completely destroyed--yet.
How much loss is 10 percent per year? Let's take my own garden for
example. It started out as an old hay pasture that hadn't seen a
plow for twenty-five or more years and where, for the five years
I've owned the property, the annual grass production is not cut,
baled, and sold but is cut and allowed to lie in place. Each year's
accumulation of minerals and humus contributes to the better growth
of the next year's grass. Initially, my grass had grown a little
higher and a little thicker each year. But the steady increase in
biomass production seems to have tapered off in the last couple of
years. I suppose by now the soil's organic matter content probably
has been restored and is about 5 percent.
I allocate about one acre of that old pasture to garden land. In any
given year my shifting gardens occupy one-third of that acre. The
other two-thirds are being regenerated in healing grass. I measure
my garden in fractions of acres. Most city folks have little concept
of an acre; its about 40,000 square feet, or a plot 200' x 200'.
Give or take some, the plow pan of an acre weighs about two million
pounds. The plow pan is that seven inches of topsoil that is flipped
over by a moldboard plow, the seven inches where most biological
activity occurs, where virtually all of the soil's organic matter
resides. Two million pounds equals one thousand tons of topsoil in
the first seven inches of an acre. Five percent of that one thousand
tons can be organic matter, up to fifty priceless tons of life that
changes 950 tons of dead dust into a fertile, productive acre. If 10
percent of that fifty tons is lost as a consequence of one year's
vegetable gardening, that amounts to five tons per acre per year
lost or about 25 pounds lost per 100 square feet.
Patience, reader. There is a very blunt and soon to be a very
obvious point to all of this arithmetic. Visualize this! Lime is
spread at rates up to four tons per acre. Have you ever spread 1 T/A
or 50 pounds of lime over a garden 33 x 33 feet? Mighty hard to
accomplish! Even 200 pounds of lime would barely whiten the ground
of a 1,000 square-foot garden. It is even harder to spread a mere 5
tons of compost over an acre or only 25 pounds on a 100-square-foot
bed. It seems as though nothing has been accomplished, most of the
soil still shows, there is no _layer _of compost, only a thin
scattering.
But for the purpose of maintaining humus content of vegetable ground
at a healthy level, a thin scattering once a year is a gracious
plenty. Even if I were starting with a totally depleted, dusty,
absolutely humusless, ruined old farm field that had no organic
matter whatsoever and I wanted to convert it to a healthy vegetable
garden, I would only have to make a one-time amendment of 50 tons of
ripe compost per acre or 2,500 pounds per 1,000 square feet. Now
2,500 pounds of humus is a groaning, spring-sagging, long-bed pickup
load of compost heaped up above the cab and dripping off the sides.
Spread on a small garden, that's enough to feel a sense of
accomplishment about. Before I knew better I used to incorporate
that much composted horse manure once or twice a year and when I did
add a half-inch thick layer that's about what I was applying.