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2023
The Transi琀椀on to
Organic Partnership
Program (TOPP)
Prepared for USDA Organic by Acres U.S.A.
www.acresusa.com | 1
Soil Balancing Basics for Organic Farming
ll farmers, and par琀椀cularly
organic farmers, understand
the importance of the soil
for growing healthy crops and animals. Farmers who overuse the plow
and neglect the necessary prac琀椀ces
for returning fer琀椀lity to the ground
generally don’t remain in business
long. The ques琀椀on, then, is how to
build and maintain that healthy soil.
Simple laboratory soil analysis
methods became available in the early 20th century. This allowed scien琀椀sts and farmers to begin to discern
A
2 | Copyright Acres U.S.A.
exactly how much of which minerals
were in the soil. The predominant
form of agriculture that arose from
this knowledge emphasized the three
primary macronutrients: nitrogen,
phosphorus and potassium. For about
a century — and certainly since synthe琀椀c sources for each became widely and inexpensively available a昀琀er
World War II — tes琀椀ng for N, P and
K and amending based on de昀椀cits of
these three minerals has been the
mainstream approach.
But a separate analy琀椀cal system
has been in use since that same 琀椀me
by a smaller number of farmers that
doesn’t require yearly addi琀椀ons of
commercially sold synthe琀椀cal fer琀椀lizer, nor even on-farm-produced organic ma琀琀er. This method — popularly
known as soil balancing — leads to
more fer琀椀le soils and healthier plants,
and healthier plants are more resistant to insect pests and diseases.
In this paper we’ll provide an introduc琀椀on to soil balancing and how it
can help organic farmers provide robust fer琀椀lity to their plants.
PHOTO COURTESY ACRES U.S.A.
PREREQUISITES: A SOIL TEST AND BIOLOGY
Soil balancing requires a soil test.
There are many di昀昀erent methods of
tes琀椀ng soil today (and of tes琀椀ng irriga琀椀on water, plant sap, soil biology,
etc. — see the AcresUSA/TOPP paper “Soil and Plant Tes琀椀ng Basics for
Organic Farming”), but the test used
for soil balancing — the Mehlich 3 extrac琀椀on method — is widely available
and inexpensive.
The Mehlich 3 method uses a strong
acid to determine both available and
some unavailable nutrients. There
are other extrac琀椀on models — ammonium acetate, Morgan, modi昀椀ed
Morgan, etc. — and each is useful in
the proper circumstances, but the soil
balancing methods developed in the
early 20th century were 昀椀rst based on
the Mehlich 3 extrac琀椀on.
Also, just because we’re talking
about soil chemical balancing, we
should never forget the importance
of soil biology for farming. We’ve
learned much in the past few decades
about the vital importance of microbiology — whether in the human gut,
Ca琀椀on Exchange On Clays
ZIMMER, ADVANCING BIOLOGICAL FARMING
Table of Ca琀椀on Exchange Capaci琀椀es
SAND = 2 to 3 CEC
SILT = 5 to 7 CEC
CLAY = Up to 60 CEC
HUMUS = 250 CEC
SCHRIEFER, AGRICULTURE IN TRANSITION
the soil or plants. Microbes — bacteria, fungi, and others — receive signals from plants, mainly in the form
of root exudates, and exchange those
exudates for forms of minerals that
are available (usable) to the plant.
This will be the focus of a forthcoming
white paper by AcresUSA and TOPP.
In the current paper we will concentrate on ensuring the proper minerals
— whether available or not — are in
the soil. This is chemistry in support of
biology, which in turn supports overall
plant health.
CATION EXCHANGE CAPACITY
Building a balanced soil begins with
understanding Ca琀椀on Exchange Capacity (CEC).
CEC is the soil’s holding tank. A low
CEC is like having a very small gas tank
for your car — you can’t drive very far
without running out; a low CEC means
your soil doesn’t hold very many nutrients. Also, when you try to put too
much gas in a small tank, you make
a mess — excess nutrients in the soil
means that much of what you apply
gets washed away, plus you have an
increased risk of plant disease.
Technically speaking, CEC is the
ability of the soil to hold and release
elements and compounds. Clay has a
high CEC — it is able to hold onto nutrients be琀琀er than sand, which has a
low CEC. Organic ma琀琀er has a very
high CEC and is able to retain and
give away nutrients even be琀琀er than
clay. CEC is called “ca琀椀on” exchange
capacity because it measures the
ability of the nega琀椀vely charged soil
par琀椀cles (clay, silt, sand and organic
ma琀琀er) to hold and release ca琀椀ons
— posi琀椀vely charged nutrients, most
importantly calcium, magnesium,
potassium and sodium.
Nutrients are also in the soil in the
form of rocks and liquids. We generally do not consider these for analysis,
however, because minerals in rocks
are not available to the plant un琀椀l
they’re broken down by microbes
over the course of centuries, and
nutrients in liquid form are likely to
simply leach out of the soil any琀椀me
there’s rain.
www.acresusa.com | 3
The fact that soil par琀椀cles are nega琀椀vely charged explains why posi琀椀vely charged ions are a琀琀racted to it.
But how are these ions “exchanged”?
Plant root exudates and soil microbes
give o昀昀 H+ ions (carbon dioxide, CO2,
actually, but this interacts with water
in the soil and creates carbonic acid,
H2CO3, which contains H+ ions). When
two hydrogen ions get close enough
to a calcium (Ca++) ion, for example,
they together take the place of the
calcium and displace it into the soil for
the plant or the microbe to use.
In the early 20th century — a昀琀er the
advent of laboratory tests capable of
A Well-Balanced Soil
Primary ca琀椀ons (as % of Ca琀椀on Exchange Capacity
Calcium (Ca++)
Potassium (K+)
Magnesium (Mg++)
Sodium (Na+)
60–80%
10–20%
2–5%
1–4%
Primary anions
Phosphorus (P-) Should equal K by weight –
but phosphate (P2O5) should
be twice the potash (K2O)
Sulfur (S–)
Half of K, up to 300 ppm
Secondary elements
Iron (Fe+)
Manganese (Mn+)
Zinc (Zn+)
Copper (Cu+)
Boron (B+++ or B-)
Chlorine (Cl-)
1/3 to 1/2 of K, min 50 ppm
1/3 to 1/2 of Fe, min 25 ppm
1/10 of P, 10