Subject: Nutrient Quantity, of Nutrient ACCESS?

From: Lion Kuntz <[email-masked]>
Date: Thu, 20 Nov 2003 19:52:13 -0800

No disrespect to Joel, but several hundred thousand
3rd world farmers have personal hands-on experiences
only now beginning to receive agronomic research
attention in the USA...

http://ppathw3.cals.cornell.edu/mba_project/moist/Roland.pdf

Nutrient Quantity or Nutrient Access?
A New Understanding of How to Maintain Soil Fertility
in the Tropics
By Roland Bunch

INTRODUCTION

If we are to achieve or maintain high levels of
agricultural productivity in the tropics, it is
crucial
that we properly understand the relationship between
nutrients in the soil and crop productivity.
This understanding is especially important if we wish
to achieve such productivity at the lowest
possible costs, both economic and ecological. The
thesis of this paper is that the conventional
view of the relationship between soil nutrients and
crop productivity in the tropics is leading
to both damaging agricultural policies and inefficient
and damaging farm-level practices. Of
prime importance to small-scale farmers worldwide is
that this conventional concept results in the
consistent recommendation of the use of huge
quantities of chemical fertilizers that are
biologically
unnecessary, economically extravagant and ecologically
damaging. The ecological damage caused
by chemical fertilizer in its own right is perhaps not
that great. Nevertheless, if we include its
indirect impact caused by the fact that most
small-scale farmers reduce their use of organic matter
(o.m.) once they start using chemical fertilizers, the
indirect ecological damage brought about by
chemical fertilizers in developing world agriculture
and ecology is nothing short of breath-taking.
The present paper cannot delve deeply into soil
chemistry and biology. For a much more in-depth
analysis of the chemical and biological issues
described herein, the best book at present
(unfortunately available only in Spanish and
Portuguese) is Ana Primavesi's The Ecological
Management of the Soil. (Primavesi) Nevertheless, the
present paper will describe, in general, a
new conception of the fertility of tropical soils, and
then, again very generally, ways in which that
theory can be put into practice.

It should also be mentioned that much of the theory
described in this paper was originally developed
by Drs. Artur and Ana Primavesi. Furthermore, the
Primavesis' theories have been widely validated
by hundreds of thousands of small farmers in southern
Brazil with the technical support of people
such as Valdemar Hercilio (Salgado) de Freitas,
Claudino Monegat and Ademir Calegari. Those
of us trying to improve small farmers' productivity
owe a deep debt of gratitude to these people.

THE CONVENTIONAL CONCEPT OF SOIL FERTILITY

Soil fertility is not an easy concept to define,
(Cresser) but for the purposes of this paper, we will
use the definition of soil fertility presented in
Anthony Young's book Agroforestry for Soil
Conservation: "soil fertility...is the capacity of
soil to support the growth of plants, on a sustained
basis, under given conditions of climate and other
relevant properties of land." We choose this
definition, rather than the much more limited one that
takes into account only the soil content of
available nutrients, because we agree with Young that
the latter definition "leads to a myopic view
of soil management, to the neglect of physical and
biological properties." (Young)
The traditional concept of soil fertility basically
maintains that fertility is largely a reflection of
the
overall quantities or concentration of nutrients in
the soil. That is, plant nutrition is to be maximized
primarily by ensuring that all the essential nutrients
are present in the soil in significant
concentrations. Therefore, if a farmer applies
sufficiently large amounts of primarily chemical
fertilizer, uses the right balance of NPK, and the
soil has an acceptable pH and sufficiently high
cation exchange capacity (CEC) to hold those nutrients
in place, the farmer will have efficiently
achieved good soil fertility.

The basic idea is that the soil operates like a bank:
add more nutrients repeatedly, over a long period
of time, and these nutrients will gradually build up,
like a savings account, increasing the soil's
fertility and therefore crop productivity. Although
there are, of course, other factors that must be
taken into account, primarily physical and chemical
processes like leaching, soil pH, the soil's CEC
and nutrient fixation, the basic idea is that the soil
acts more or less like a static recipient of
nutrients. Thus, the vast majority of the effort and
experimentation of these traditional theorists have
had to do with overall nutrient levels and their
physical movement, mineralization and leaching
within the soil profile. For the purposes of this
paper, this idea of soil fertility will be called the
Nutrient Quantity Concept.

The priorities of the adherents of this Concept are
seen very clearly in the books they write on the
subject of soil properties and management. A typical
textbook, based on the Nutrient Quantity
Concept--even one dealing with tropical soils--
dedicates three chapters (73 pages) to NPK, while
not dedicating even a single chapter to either organic
matter (o.m.) or soil biology. (Committee on
Tropical Soils) Even a book published as recently as
1997 on soil fertility in Africa dedicates two
long chapters (82 pages) to NP, and only a fraction of
one chapter (8 pages) to specifically organic
inputs. (Buresh) Other factors are mentioned in these
publications, but chemical sources and
quantities of NPK dominate heavily the entire
discussion.
A common conclusion drawn from this conception by its
proponents as late as 1997 is that in Africa
"mineral fertilizers should be at the core of
strategies to restore soil fertility and raise crop
production." (Quiñones) Another major proponent of
this conception wrote in 1994: "Some people
say that Africa's food problems can be solved without
the application of chemical fertilizers.
They're dreaming." (Borlaug) The Financial Times goes
on to write that the same author said that,
"Sub-Saharan Africa had the lowest use of fertilizer
in the world and soil nutrients were so low that
other efforts to raise crop productivity would not be
successful until fertility was improved."
(Borlaug) By "fertility" being "improved," he is
referring specifically to the application of chemical
fertilizers.

First, allow me to include a short explanation for the
benefit of the layman. Crops are capable of
absorbing some nutrients that exist in the soil at
levels of less than 0.2 parts per million, while other
nutrients are often difficult to absorb at 100 times
that concentration. (Ahn) Thus there is, in fact,
little relationship between a plant's physical ability
to absorb a nutrient and the nutrient's
concentration in the soil. Also, plants do not absorb
the various nutrients primarily according to the
levels present in the soils, but rather in accordance
with the plants' own needs, and in ratios between
the nutrients that are relatively stable for each
species or variety of plant, regardless of the supply
of the nutrient in the soil. Thus, the Nutrient
Quantity Concept is really saying that, other
conditions
being adequate, the growth or productivity of any
plant will depend largely on the quantity and
availability of the nutrient in the soil that is
proportionately least adequate in relation to the
total
amount of that nutrient necessary for the plant to
achieve maximum growth, and that in practice
maximum crop growth should be achieved by having large
enough reserves of these nutrients in the
soil so that adequate quantities of them will exist in
available forms. (Cresser)

THE INADEQUACIES OF THE NUTRIENT QUANTITY CONCEPT
Theoretical Inadequacies of the Concept

First, the Nutrient Quantity Concept, in its
simplified, "banking" form, is very unrepresentative
of
what actually happens in the soil. Far more important
for productivity than the total quantity of any
single nutrient or group of nutrients is the chemical
form in which it occurs, the depth in the soil at
which it occurs, the kinds and numbers of macro and
microorganisms that exist, the presence of soil
compaction layers, and the equilibrium that exists
between the nutrients, the pH of the soil, its
moisture content, its organic matter content, its
macro and microorganisms, its texture and structure,
etc. Furthermore, these factors constantly impact on
each other, creating a very complex and
constantly changing environment within the soil, or,
more accurately, a constantly changing complex
of varying microenvironments within the soil. These
factors may make it possible for a plant, in a
particular time frame, to access a majority of the
total store of a given nutrient in the soil, or much
less than 1 % of the total store of that same
nutrient.

In other words, the same nutrient that is in the same
total concentration within two different soils
may well be fifty to one hundred times more available
to a plant in one of those soils than in the
other. Obviously, the total amount of a nutrient in
the soil is nowhere near as important in terms of
the soil's fertility as is the availability of that
nutrient to the plants growing on it.
Of course, the body of literature and scientific
conclusions that have grown up around the Nutrient
Quantity Concept, if taken in its total complexity,
admits as much. The data of soil science, as a
whole, take into account fairly well the fact that all
the above-mentioned factors can affect fertility
(although soil science could certainly be a good deal
more aware of certain of these factors,
especially the biological ones). In fact, quite
surprisingly, the science built up around the Nutrient
Quantity Concept in no way contradicts the very
different conception of what is required to achieve
soil fertility that will be described below.

Where the advocates of the Nutrient Quantity Concept
of soil fertility have apparently gone wrong
is in the conclusions they have made about soil
fertility, based almost entirely upon only a certain,
specific selection of these scientific findings. That
is, the experiments, although heavily lop-sided
in the dominance of their preoccupation with chemical
fertilizers, do not necessarily support the
Nutrient Quantity Concept any more than they support
the concept presented below. What has
apparently happened is that one particular fact has
come to totally dominate people's thinking about
soil nutrients. This is the fact that, in a uniform
soil environment, the larger the total quantities of
a nutrient that exist in a given soil, the larger will
be the quantity of that nutrient that is in available
forms. Even though there is ample proof that in a
non-uniform soil, and in countless specific cases,
this relationship does not exist at all, many
conventional soil scientists continue to see this
relationship as the most important single logical
basis to soil fertility management. (See, for
example, Cresser) To cite an extreme but very common
example, even when the availability to
plants of soil phosphorus is known to be as many as
fifty times greater in an organic environment
than in an infertile acid soil environment, the
Nutrient Quantity Concept still leads virtually all
conventional soil scientists to recommend applying
additional chemical phosphorus to the acid soil
rather than finding or creating an organic
microenvironment within that soil in which to apply a
much smaller dose of (organic or inorganic)
phosphorus.

Thus, in a very few words, the Nutrient Quantity
Concept does not differ from the concept to be
presented below, in its scientific, experimental
undergirding. Rather, the difference lies in the
somewhat arbitrary decision to emphasize the
relationship between total nutrient quantity and the
quantity of nutrients available to growing crops, even
though scientific evidence shows that this
relationship is in most cases tenuous, at best,
especially in the case of small farmers in the
tropics.
A second major error of the Nutrient Quantity Concept
is that it seems to presuppose that these
nutrients are being more stable in the soil than their
own research shows they actually are, especially
where the CEC of the soil is quite low and/or erosion
is occurring. The idea of building up nutrients

in the soil would seem to presuppose that those
nutrients will remain there over, say, twenty years
or so. Yet virtually all of the N, much of the K, and
even small amounts of the P applied as chemical
fertilizers today, will not be there twenty years from
now, even if no plants were using them. And
on low CEC soils and erodible hillsides, the loss of
even P can be considerable. In short, the
farmers' "money" is constantly leaking out of the
"bank". And, sadly, the more money there is in
the bank, the more will leak out.

This assumption of the persistence of nutrients in the
soil seems to be assumed, also, when
conventional scientists criticize those who propose
using large amounts of o.m. for maintaining soil
fertility. Nutrient Quantity proponents worry at
considerable length that organic inputs cannot
maintain soil nutrient levels over the long term, and
that they do not produce as much long-lasting
humus as often as we all would like. (Buresh) They
seem to forget that chemical fertilizers do not
maintain the levels of most micronutrients, either. Of
more importance economically, chemical
fertilizers, as used by small farmers, do not maintain
soil pH. (And even if farmers could,
theoretically, avoid acidifying their soils with
chemical fertilizers, the costs of liming or using the
more expensive alkaline fertilizers is seldom included
in economic analyses of the costs of chemical
fertilizer use.) Of course, because of the leaking
from the bank, chemical fertilizers, even when used
in recommended quantities, may not achieve the
long-term levels of total nutrients desired.
Thus, organic amendments are little different from
chemical fertilizers in not being able to increase
soil nutrient quantities dramatically over time in the
tropics, especially under small farmer
conditions. This is not to say we should give up on
striving toward at least soil nutrient maintenance
over the long run, but rather to say that the use of
chemical fertilizers exclusively, in the case of
small farmer agriculture, will very likely achieve no
more long-term improvement in nutrient
quantities than will the use of o.m. exclusively. Just
as most small farmers often, but not always,
lack the o.m. to increase stores of P, for instance,
they also usually lack the cash (and often the
profitability of chemical fertilizers on their
depleted lands far from markets) to buy sufficient
chemical fertilizer.

Furthermore, neither this paper nor most of those
people who use low input agriculture oppose all
use of chemical fertilizers. The replacement of some
chemical elements in the soil (mostly P), under
certain conditions, and in moderate amounts, is, for
us, perfectly acceptable, if not desirable.
The third error of the Nutrient Quantity Concept is
that its proponents have largely avoided taking
into account the tremendous impact in tropical soils
of such factors as their macro and microbiology,
o.m. content, microenvironments and compaction layers.

Inadequacies in Practice

Based on these theoretical considerations, most
conventional soil scientists have come to a series
of extremely far-reaching conclusions. Among these,
they have concluded that the best, most
efficient way of improving the soil fertility of the
developing world's resource-poor farmers is to
apply tremendous quantities of NPK along with,
perhaps, a small supplement of o.m. They have
concluded that "low external input" technologies must
inevitably lead to "low output" results. They
have therefore further concluded that "ecological
agriculture" is inevitably unproductive and has
virtually no future. And finally, they have concluded
that soils with very low CEC's, like those of
most of West Africa, have very little potential for
decent crop productivity. None of these
conclusions is based on the scientific understanding
we have of soils in its totality. And concrete
evidence from tens of thousands of farms around the
world, as well as from many scientific
experiments, provides considerable evidence that not
one of these conclusions is, in fact, accurate.
Thus, the Nutrient Quantity Concept is failing us. It
is failing to lead us to proper conclusions about
agricultural priorities. It is failing to predict what
will happen if we apply a whole range of
agricultural technologies that are now being tried in
the tropics, and it is failing to help us understand
a series of both natural and agricultural phenomena
that we are observing. Above all, it is failing
to lead us to promising new technologies that can
provide tremendous benefits at low cost to poorer
farmers within the tropics.

Let's look a little more closely at these failings.
One of the main negative impacts of the traditional
Nutrient Quantity Concept in tropical environments is
that it has caused many scientists to dismiss
ecological agriculture out of hand. It has become
almost a mantra of some of these scientists that
"low input agriculture is low output agriculture."
After all, according to the Nutrient Quantity
Concept way of thinking, if not much is put into the
bank account, not much can be withdrawn.
Thus a whole series of promising technologies for
increasing agricultural productivity–mostly those
which are included in approaches referred to as
ecological agriculture or agroecology--have been
largely ignored by the dominant members of the
scientific community, (Anonymous; Pretty and
Hine) to the tremendous detriment of farmers in the
tropics. In fact, millions of small-scale farmers
in developing countries, representing well over 3 % of
the total, have adopted ecological agriculture
practices in just the last ten years, (Pretty and
Hine) yet these practices are largely ignored by
conventional soil scientists.

Another major negative impact of the application of
this Concept is the claim that because many
tropical soils have very low CEC's, they will never be
able to produce large harvests. Because such
soils cannot hold very many nutrients over a crop's
entire life-span, it is said they will never be able
to support maximum output. This conception of the
dynamics of soil fertility has caused huge areas
of the tropics to be written off as "low-potential"
areas, where significant investments in agricultural
development are therefore not advisable. (Mosher) As a
result, hundreds of millions of people have
been condemned to a not-so-benign neglect and
perpetually low productivity. Since these people
often were already among the poorest of the poor, this
mistaken policy has aggravated already
serious problems of economic injustice and downright
hunger. And all because of a theory of soil
fertility that is questionable at best.

Another factor that becomes extremely important for
those of us who focus our work on the poorer
farmers of the tropics is that the Nutrient Quantity
Concept leads almost inevitably to an excessively
high use of chemical fertilizers. The cost of chemical
fertilizer very frequently represents the largest
single cash cost of resource-poor farmers in the
tropics. Yet experience in nation after nation has
shown that for a much lower total expense, farmers can
achieve the same or even higher yields.
Furthermore, as the years go by, the use of most
chemical fertilizers mine the soil of micronutrients,
acidify the soil even more, and help to erode away,
burn out, or simply fail to replace the soil's o.m.
In time, these factors all work together to reduce the
response that chemical fertilizers achieve in
terms of productivity, to the point that, in many
cases, there remains no net economic advantage to
their use whatsoever.

Furthermore, the recent staggering increase in
petroleum prices (from $ 12.00 a barrel to somewhere
between $ 19.00 and $ 32.00 a barrel) is going to
result in major increases in the real farm gate cost
of fertilizers, both because of increased costs of
production and increased transport costs. Thus, tens
of millions of small-scale farmers for whom chemical
fertilizer is presently economically
advantageous will find that in the future that
advantage will have largely or totally disappeared.
Thus, it is extremely important that we all learn as
much as we can about the technological
possibilities that will still allow farmers to
increase and maintain their productivity without
depending so heavily on increasingly expensive
chemical fertilizers.

An additional major problem of the traditional
Nutrient Quantity Concept is that increasing
observation, especially of ecological or low external
input agriculture, is showing that the Concept's
predictive abilities are seriously lacking. Crop
productivity, in most such cases, is not basically, or
even approximately, a factor of the overall amount of
nutrients in the soil, or of the CEC of the soil.
Very high levels of productivity are being achieved on
soils that, according to the traditional
Concept, could never produce such yields, and with
applications of nutrients that are anywhere from
one-half to one-tenth the quantities the Nutrient
Quantity Concept would indicate are necessary.
Recent scientific experiments with o.m. and fertilizer
use in Africa, where scientists cannot explain
7
with conventional theories the yields that were
achieved, are typical. (Palm)
In what specific cases has the Nutrient Quantity
Concept failed to predict present phenomena?
- The increases in yields achieved by the use of green
manure/cover crops (gm/cc's) in system after
system are greater than the conventional Concept would
have predicted. With increases of only
perhaps 100 kg of fixed N and no additional P or K,
yields of maize crops have often doubled.
(Buckles; Bunch and Lopez; Pretty and Hine, for
example) Furthermore, yields of 2.5 t/ha have
continued to be produced on relatively poor, humid
tropical soils every year for 40 years, with no
application of chemical NPK. Of course, what is
happening here is due to biological, physical, and
chemical dynamics within the soil, not just those of
soil nutrients. Nevertheless, according to the
Nutrient Quantity Concept, the levels of P, at least,
should have become a major limiting factor years
ago. Yet applications of chemical P on these soils
still, after forty years, give no economic response.
(Buckles)

Perhaps it should be added here that "green
manure/cover crops" is a technology that is
traditional
in many tropical areas, but which have only been
developed over the last fifteen years in Brazil and
Mesoamerica and spread around the world largely by
NGO's in the last five years or so. This
technology, not to be confused with traditional "green
manuring," grows biomass, often leguminous,
intercropped with regular crops, under fruit trees,
during the dry season, during frosty periods or on
degraded soils too poor for cropping (i.e. on land
with little or no opportunity cost), thereby adding
huge net quantities of high-nutrient biomass in situ
to agricultural systems and applying it to the
surface where it is highly accessible to subsequent
crops. (See Bunch 2001) "Dispersed trees" is
another traditional practice around the world which
has only recently been studied and promoted in
Central America, but which apparently has tremendous
potential for increasing biomass production
in much of the lowland tropics.

-In Madagascar, hundreds of farmers are now using a
System of Rice Intensification (SRI) that
frequently achieves yields of 12 to 15 t/ha, and
occasionally 18 t/ha, using only moderate amounts
of compost and no chemical fertilizer on low CEC, acid
soils (a classic case of "low potential soils").
(Uphoff) Yet the world's rice experts hold that the
"biological maximum" for the rice plant is less
than 10 t/ha. As one scientist has written, "yields
for multiple varieties peak out at about 8 t/ha, even
with high N applications, up to 200 kg/ha." (Ladha, et
al.). Furthermore, even with applications of
200 kg/ha of N, N assimilation is considered to be the
limiting factor for rice production. (Uphoff)
In this case, the difference in yields between
conventional rice and SRI rice is probably due to a
whole series of factors, but whatever the reasons may
be, the "low input agriculture is low output
agriculture" attitude cannot come even close to
explaining rice yields of 15 t/ha on these "low
potential" soils with so little N introduced into the
system.

- In the case of West Africa's very old, low CEC
soils, books based on the Nutrient Quantity
Concept claim good yields can only be achieved with
major applications of chemical nutrients.
(Avery) Yet around people's homes (admittedly on very
small plots), African women frequently
grow 4-mt-tall, 4 t/ha maize. How? Through the daily
application of grey water and kitchen scraps
from the household. And the soil is the same soil as
in their fields a few mts away.

- The age-old, world-wide regeneration of soils by
means of traditional slash-and-burn, or shifting
agriculture, techniques cannot be fully explained by
the dominant interpretation of the Nutrient
Quantity Concept, either. Attempts have been made to
somehow help the Concept explain this
regeneration of the soil through the idea of nutrient
pumping. But the amount of nutrients pumped
by trees is frequently not enough to explain the
renewed fertility. In many cases, there are precious
few nutrients in the subsoil that the trees could
pump, even if they were physically capable of
pumping the required quantities. And, of course, the
concentration of nutrients in the soils from
which they are being pumped is only a fraction of what
the Nutrient Quantity Theory would consider
sufficient for adequate for plant growth.

Furthermore, I have personally asked West African
farmers to show me which fields were ready to
be "slashed and burned," and close to half of the
indicated fields had no visible vegetation on them
other than grasses. Certainly farmers whose food
supply has depended for generations on their
ability to accurately identify regenerated soils could
not be mistaken half the time. And if grasses
can regenerate soils by themselves, how can the
Nutrient Quantity Concept explain this worldwide
phenomenon?

- The biomass productivity of natural rainforests is
also much higher than its CEC would allow
under the traditional Concept. Interestingly enough,
in this case, scientists who normally adhere to
the Nutrient Quantity Concept freely admit that the
rapid recycling of nutrients in tropical rainforests
permits tremendous levels of biomass production in the
presence of very low levels of nutrients and
CEC's in the soil in general. Nevertheless, they have
been largely unwilling to entertain the
possibility that this same phenomenon of the rapid
circulation of nutrients could be the basis of
highly productive crop agriculture under similar
conditions. Put another way, Nutrient Quantity
proponents freely admit in the case of rainforests
that "low input forests produce high output
forests," yet they refuse to admit that the same
principle might be applicable to agriculture in the
very same environments. This use of one rationale for
proven rainforest productivity while refusing
to admit to the possibility of the very same process
in agriculture would seem to represent a serious
lack of logical consistency.

Nutrient pumping might seem to cloud the above issue
somewhat. Nevertheless, many rainforests
produce prodigious amounts of biomass above subsoils
whose provision of nutrients, even under
extremely efficient nutrient pumping, would be less
than those nutrients added artificially under
many "low external input" systems. We should also
remember that the areas deep in the soil from
which these nutrients are presumably "pumped,"
virtually always possess much lower
concentrations of nutrients than do the soils above
them. Therefore, even with nutrient pumping,
natural forests provide clear evidence that sufficient
nutrients for very high levels of biomass
production are being extracted from soils with
extremely low total concentrations of nutrients.
Furthermore, low external input agroforestry systems
also pump nutrients. Thus, the logic of the
Nutrient Quantity advocates remains problematic at
best.

-Chemical fertilizer companies are dedicating millions
of dollars to research on "slow-release"
forms of chemical fertilizer. This means, in effect,
that the fertilizer companies themselves (which,
not surprisingly, are among the most vociferous
proponents of the Nutrient Quantity Concept) are
admitting--through their actions, if not their
words--that the overall quantity of nutrients
available
at any given time is not the primary issue in
productivity.

Of course, one could argue that slow-release
fertilizer is being developed because its ecological
benefits will demand a sacrifice in productivity from
farmers. Nevertheless, research on slowrelease
fertilizers has shown no such significant reduction in
yields. Thus, the very development
of this kind of fertilizer is an admission that, at
least where this fertilizer is successful in terms of
productivity, the constant supply of nutrients is more
important than the total quantity available at
any particular time.

Given the apparent inaccuracies and even logical
inconsistencies of the traditional Nutrient Quantity
Concept, it is time to develop a new, more
comprehensive and accurate concept of soil fertility
in
the tropics.

THE NUTRIENT ACCESS CONCEPT OF TROPICAL SOIL FERTILITY
An Illustrative Experiment

To illustrate the new concept, I will first describe
an experiment reported in Ana Primavesi's The
Ecological Management of the Soil. (Primavesi) In this
experiment, crops were grown in four
hydroponic solutions. In the first solution, a normal
concentration of nutrients for maximum maize
plant development was used, and replenished every 4
days. In the second, twice the normal
concentration was used and replenished every 4 days.
In the third, the normal solution was diluted
50 times and also replenished every 4 days. And in the
fourth case, the solution used was diluted
50 times the normal concentration, as in the third
case, but was replenished every 2 days.
Plant growth (measured in grams of dry weight) in the
second case was less than in the first. In the
third case, as either theory would predict, the growth
of the plants was 28% less than in the first.
But in the fourth, quite surprisingly, the growth of
the plants was slightly superior to that in the first
case. That is, even when the nutrient solution was
1/50 what the traditional Nutrient Quantity
Concept would have seen as optimal, the plants grew
equally well, as long as the solution was
replaced frequently enough and the roots could access
the nutrients.
That is, crop growth above a certain extremely low
concentration, does not depend on the
concentration of nutrients. It depends, rather, on the
constant access of plant roots to the nutrients,
even when these nutrients exist in very low
concentrations. The Nutrient Quantity Concept's
remedy of increasing the concentration of nutrients by
applying large amounts of chemical fertilizer
misses the point almost entirely. What is needed is a
constant supply of even a very small but wellbalanced
amount of nutrients over time, and the unobstructed
access of plant roots to these nutrients.
This experiment shows that the relationship between
concentrations or overall quantities of nutrients
and plant growth is, above a certain minimum
concentration, altogether nonexistent. As long as
plants enjoy the right conditions of nutrient balance,
accessibility to nutrients, and a constant
resupply of nutrients, the relationship between the
concentration of nutrients in the soil and its
productivity is either zero (i.e. there is no
relationship) or negative (i.e. more concentrated
nutrients
reduce plant productivity).

So why is this fact more relevant to tropical soils
and farmers than to temperate-zone soils and
farmers? Very simply, tropical soils tend to have
fewer cation-exchange sites, and therefore lower
concentrations of nutrients. Because of this paucity
of CEC, it is also much more difficult and costly
for farmers to raise those concentrations of nutrients
over the medium to long term. Second, the
ambient heat of the tropics makes it difficult or
impossible for plants to create the osmotic pressure
to be able to absorb nutrients from highly
concentrated solutions. (Primavesi) Therefore, plants
in
the lowland tropics often thrive better on more
limited concentrations of nutrients (as was
illustrated
by the second case in the experiment above), as long
as the remaining conditions are fulfilled.

Thirdly, farmers who work by hand or animal traction,
can more easily micromanage their soils,
creating varied microenvironments, in some of which
nutrients are accessible, even if the total soil
environment is deficient in accessible nutrients. And
lastly, while northern farmers can often afford
to fertilize their soils more heavily than need be for
present purposes, resource-poor farmers in the
developing countries just cannot afford to
over-fertilize. Furthermore, in high-rainfall or
steeply
sloped areas, resource-poor farmers would lose more of
those nutrients than do their northern
colleagues.

The Concept

The new concept of soil fertility emphasizes not the
concentration of nutrients in the soil, but rather
the maximization of the access of plant roots to soil
nutrients. Thus I will herein call it the Nutrient
Access Concept of soil fertility. The Nutrient Access
Concept of soil fertility in the tropics posits
the following:

Maximum plant growth can best and most cheaply be
achieved in the tropics by:

1) the constant supply of soil nutrients (most
inexpensively achieved with fairly low
concentrations),

2) a healthy balance between the nutrients, and

3) maximum access of plant roots to these nutrients
(i.e. the maintenance of good soil structure
and/or mulches).

THE ADEQUACY OF THE NUTRIENT ACCESS CONCEPT
Of course, the first question we must ask is whether
the Nutrient Access Concept can explain the
phenomena mentioned above better than the Nutrient
Quantity Concept could.

First of all, the Nutrient Access Concept does not
deny that heavy concentrations of nutrients can
produce high yields in many circumstances, especially
in cooler climes, and when soils are
compacted or optimal soil structure has otherwise been
damaged, CEC is high and farmers are well-
capitalized. It would tend to indicate that high-input
agriculture is probably more expensive than
it need be to achieve maximum productivity, but it
does not deny that high levels of productivity can
be achieved through high concentrations of nutrients
in developed nation agriculture, and even
highly capitalized plantation agriculture on the best
soils of the tropics.

Where the Nutrient Access Concept does point to
agricultural practices of a radically different kind
is where soils have very low CEC’s, where soil o.m. is
or could be abundant and cheap, where
capital is scarce, and/or where temperatures are high.
But in these situations, very common in the
tropics, would the Nutrient Access Concept be any more
useful than the Nutrient Quantity Concept?

Adequacy in Practice

The Nutrient Access Concept has already led, in
southern Brazil (Bunch, 1994) and small pockets
of farmers in country after country, to competitive
yields at relatively low costs, often on what were
previously considered “low potential” soils, and with
a much more positive long-term ecological
impact than that of agriculture according to the
traditional Nutrient Quantity Concept. (Pretty and
Hine)

These experiences, plus the nature of the Nutrient
Access Concept itself, would indicate that there
is a good chance that this Concept could reduce
significantly the costs of producing competitive
yields in the tropics. For small-scale, poor farmers,
especially on impoverished soils with low
CEC’s, the technology resulting from the Nutrient
Access Concept could be a major life-saver, both
literally and figuratively.

The Nutrient Access Concept provides absolutely no
rationale for the present unjust discrimination
against those farming on so-called “low potential”
soils. Instead, it supports the idea that with fairly
small, inexpensive applications of highly accessible
nutrients, these soils can produce harvests
several times their present levels. The “potential” of
the soil depends more on the proper
management of the soil than it does on the addition of
large quantities of very expensive nutrients.
It would also call into question efforts to introduce
and subsidize tremendous quantities of expensive
chemical fertilizers to dozens of African nations
already on the brink of bankruptcy. Presently, there
is a lot of discussion about the possibility of
bringing the “Green Revolution” to Africa. Proposals
center on the idea that the World Bank and other major
financial institutions should make a
concerted effort to introduce and subsidize chemical
fertilizers in Africa because of the perceived
dangers of “soil nutrient depletion”. (See IFPRI,
SSSA)

These proposals, of course, fly in the face of
increasing petroleum prices and a decade of Neoliberal
efforts to reduce artificial governmental subsidies.
But more basically, the perception that reduced
levels of nutrients in Africa’s soils will inevitably
preclude high levels of production is, of course,
based on the Nutrient Quantity Concept. Adoption of
the Nutrient Access Concept would force a
major rewrite of these proposals, orienting them
instead toward the achievement of increased yields
through higher levels of biomass production, soil
structure improvement and mulch-based systems,
rather than through the unsustainable use of billions
of dollars of increasingly expensive chemical
fertilizers.

Of course, even if we believed the Nutrient Quantity
Theory, we have mentioned above that small
farmers’ lack of financial resources and the lack of
their depleted soils’ response to chemical
fertilizers are a major an obstacle to maintaining or
increasing soil nutrient levels with chemical
fertilizers just as their lack of access to sufficient
o.m. prevents them from doing the same with o.m.
And with intercropped or dry season gm/cc’s and/or
dispersed trees, the former limitation may be
much greater than the latter. If this is the case,
someone must explain why, if African soil fertility
is to be subsidized, it would be better to subsidize
chemical fertilizers and rock phosphate to the total
exclusion of subsidies of the dispersed planting of
trees or of the price of green manure/cover crop
grains.

Adequacy in Explaining the Observed Phenomena

The Nutrient Access Concept can explain very
adequately those observed phenomena that the
traditional theory cannot:

- Yields in gm/cc and agroforestry systems do not
depend on high concentrations of nutrients.
Rather, they depend on the fixation of N and the
recycling of large amounts of o.m., which makes
the P and other nutrients in the soils much more
soluble (i.e. chemically available), and places most
of these nutrients near the soil surface, where they
are easily accessible to plant roots. Such systems
can therefore produce good yields over long periods of
time with low or no applications of
additional nutrients (though eventually some
nutrients, notably P, obviously have to be added to
achieve sustainability).

- The SRI yields are achieved on very poor soils with
only limited applications of compost because
the rest of the technological package aerates the soil
and makes the plants grow close to six times
more roots per plant than does the conventional rice
system, thereby allowing the plants to access
many more of the limited nutrients in the soil.
(Uphoff)

- West Africa’s extremely poor soils with very low
CEC’s produce well near people’s compounds
because the o.m. thrown out of the kitchens maintains
a small, steady supply of nutrients, and
therefore low CEC’s are sufficient to hold the
nutrients necessary for a few hours or days until more
nutrients are released from the next day’s discarded
o.m.

- The regeneration of tropical soils through fallowing
does not occur primarily because large
quantities of nutrients are brought to the surface and
kept there. Rather, the large amounts of
biomass deposited on the soil by the regrowth of
forests or grasslands either maintain or, gradually
and over a number of years, improve the soil
structure, so that the newly cleared land allows crops
to access much more efficiently the low concentrations
of nutrients that exist in the upper horizons
of the profile. Meanwhile, the organic matter on the
soil surface or near it, produced during years
of fallow, continues to supply nutrients in small
quantities that can maintain reasonably high levels
of productivity, at least for a year or two.

- Likewise, the same process maintains the impressive
productivity of rainforests for millennia.
Optimal soil structure and mulches are maintained,
maximizing the access of the forest’s trees to the
few nutrients that are constantly being supplied
through soil o.m. mineralization. And, to some
extent, deeper tree roots, having a tremendous number
of feeder roots like the rice under SRI, can
capture large amounts of nutrients even in soil
horizons with extremely low concentrations of
nutrients.

In this last case, of course, the Nutrient Quantity
people would largely agree with this analysis.
What they cannot explain is how forests can grow so
well on the basis of these very low
concentrations of nutrients when, according to their
Concept, low nutrient levels (i.e.. “low inputs”),
in whatever part of the soil profile and of whatever
origin, should result in the low productivity (i.e.
“low outputs”) of the entire system.

- Lastly, it is quite obvious that the research on,
and benefits of, slow-release chemical fertilizer is
much more understandable based upon the Nutrient
Access Concept than upon the Nutrient Quantity
Concept.

The above explanations are, of course, quite
simplistic. Much more is happening in the soil than
these explanations would indicate. Plants’ access to
nutrients, though certainly helped by applying
the nutrients to the soil surface or together with
o.m., or very near the seed, or by plants’ growing
in well-flocculated soils free of compaction layers,
is a very complicated phenomenon which
involves a large number of factors. These would
include those of soil temperature, soil o.m. levels,
pH, soil chemical properties, the presence of
compaction layers, and nutrient positioning and
equilibrium, all of which are in turn affected by the
activity of hundreds of thousands of
microorganisms in every teaspoonful of soil. Thus,
nutrient access involves a whole series of
dynamics which we only somewhat dimly understand as
yet. Nevertheless, the Nutrient Access
Concept seems to come much closer to explaining the
overall sum or average of all these varied
and mysterious processes than does the Nutrient
Quantity Concept.

PUTTING THE NUTRIENT ACCESS CONCEPT INTO PRACTICE

It will be immediately evident to many people that the
Nutrient Access Concept can most easily be
put into practice through the copious use of o.m. O.m.
can and does supply low to medium
concentrations of nutrients, and almost always in
well-balanced quantities. Furthermore, o.m., by
its very nature, has a slow-release mechanism,
allowing the nutrients to become available to plants
over a period of several months or years. And lastly,
though this mechanism is somewhat slower
and sometimes problematic in improving the structure
of heavily compacted soils, soil o.m. does
serve to gradually improve soil structure. Soil o.m.
does so both directly, through the provision of
binding materials to improve flocculation, and
indirectly, by feeding earthworms and other soil
organisms, both macro and micro, which also improve
soil structure. (Minnich)

Experience shows that the best way to apply o.m. in
order to improve soil dynamics in these three
respects, as well as to reduce costs, is to apply it
either to the soil surface or, during the period of
transition, within 20 cm of the surface. Although
there is still some argument as to how the o.m.
should be applied to the soil the first year or two of
a transition into mulch-based agriculture (when
soil compaction below the surface is a serious
limiting factor), experience tells us that after the
first
year or two, virtually all the o.m. should be applied
to the soil surface. (See, for instance, Primavesi;
MAG/DGP)

It should be mentioned here that the Nutrient Access
Concept does not necessarily support the total
discontinuation of the use of chemical fertilizer.
While organic agriculture proponents may agree
with this Concept, the Concept does not necessarily
support a totally organic approach. What the
Concept does do is open the door to a greatly reduced
use of chemical fertilizers in the short run, and
the gauging of their use in the long run more
according to the replacement levels of net losses of
nutrients for the purpose of sustainability (minus
those nutrients supplied by o.m. and N fixation),
rather than the much higher levels of use presently
thought to be the only way to significantly
increase productivity.

The Five Principles of Agriculture for the Humid
Tropics

Based on the Nutrient Access Theory, an increasing
number of institutions involved around the
world in small farmer agriculture have begun to use
some or all of the following Five Principles of
soil management (Bunch, 1995):

1) Maximize o.m. production. Frequently, small-scale
farmers can increase dramatically the
amount of o.m. their fields produce while maintaining
yields and only increasing costs slightly, if
at all. In fact, many gm/cc and agroforestry systems
reduce the amount of labor needed for
controlling weeds, thereby increasing overall o.m.
production while at the same time decreasing
costs.

Increases in o.m. production can be achieved by using
the intercropping of either various crops or
gm/cc’s with annuals or tree crops (as in dispersed
tree systems), by establishing two- to four-story
fields and gardens, and by growing trees or gm/cc’s on
wasteland or during the dry season. In
droughty areas, an increased provision of water in
whatever form can also result in greater levels
of biomass production.

Obviously, the more biomass we produce in situ, the
more we will have for applying to the soil,
thereby making the provision of nutrients to the soil
greater and more constant. If animals are
present in the farming systems, they will also be more
numerous and/or produce more manure per
animal if they have more biomass to consume.

2) Keep the soil covered. Soil exposed to the tropical
sun produces more weeds (which are another
form of biomass, but may compete with crops and/or
occasion a good deal of work). Unprotected
soil also becomes very hot, causing a series of
problems, including the more rapid rate of soil o.m.
burn-out, the reduction in crop growth rates and the
death of beneficial macro and microorganisms.
Shifting agriculture has been motivated in most places
in the world by either declining soil fertility
or increasing noxious weed growth. Once we maximize
biomass production and keep the soil
shaded, both of these problems are largely eliminated.
The need to let the land lie fallow for years
is thus also eliminated, as has been shown in country
after country with the use of gm/cc’s.
It is interesting to note that virtually all the
systems of improved fallows or gm/cc’s that farmers
have developed on their own, have increased both soil
cover and the overall production of biomass,
as compared to the previous shifting agriculture
systems.
Keeping the soil covered is obviously consistent with
the Nutrient Access Concept because it
reduces the decomposition rate of soil o.m., thereby
making sure the provision of nutrients to the
soil lasts longer and is more constant, even if
mulches tend to lose a certain amount of N to
volatilization.

3) Use zero tillage. Many traditional agriculture
systems use zero tillage. However, these systems
are often not very productive over time, because, in
the absence of large amounts of soil o.m.,
nutrients are no longer constantly supplied to the
soil and soil structure deteriorates quite rapidly.
If, however, zero tillage is used in the presence of a
maximum production of biomass, then both the
supply of nutrients and good soil structure can be
maintained. Thus, in contrast to many traditional
zero till systems, those systems with plentiful
biomass production can remain highly productive over
decades, as a whole series of gm/cc and agroforestry
systems have proven.
Often zero tillage cannot be practiced the first or
second year of the transition, but as soil o.m. levels
increase and the soil becomes covered, the populations
of organisms that naturally till the soil
increase rapidly, making further tillage by the farmer
unnecessary. (Scientists have shown, for
instance, that earthworms alone can move more
soil/ha/year than is moved with one ploughing using
a tractor-pulled moldboard plough.) (Minnich)

In the conventional textbooks, zero tillage is linked
with a major increase in the use of herbicides.
However, if the soil is kept covered through an
adequate use of gm/cc’s and agroforestry, most
small-scale farmers will find they never, or only very
rarely, need to use herbicides.
Zero tillage has an important relationship to the
Nutrient Access Concept, because tillage both
damages soil structure and increases the rate of soil
o.m. burn-out. Furthermore, tillage exposes the
soil (i.e. violates the principle of keeping the soil
covered) and removes or incorporates the mulch,
which violates the fifth principle below.

4) Maximize biodiversity. Some gm/cc users report
achieving slightly better yields with a mixed
selection of gm/cc’s. Nevertheless, this principle
will find its primary importance not in the short
run, but rather in maintaining the systems’ long-term
sustainability. It can also be very important
in maintaining the balance of nutrients required by
the Nutrient Access Concept. (Primavesi)
5) Feed the crops largely through the mulch. Many
humid tropical soils, with their pH’s below 5.0,
their aluminum toxicity and compaction layers, are not
very hospitable environments for crop roots.
Thus, crops will often grow much better if they can
also access nutrients from a thick litter layer
or mulch. In fact most, if not all, crops that grow in
the humid tropics will spread the vast majority
of their feeder roots immediately under or even up
into a mulch layer as long as it remains fairly
moist. That is, they will feed much more readily from
inside and immediately below the litter layer
than from the soil itself.

Even the impact of chemical fertilizers can sometimes
be greatly increased by being applied to the
mulch rather than the soil. In Costa Rica, edible bean
yields in the “frijol tapado” system, a
traditional slash-mulch system, were not increased
much at all above the traditional 500 kg/ha when
chemical phosphorus was applied to the soil. However,
yields rose two to three times traditional
yields (to above 2 t/ha) when the inorganic P was
applied directly to the mulch. Researchers in
Africa have also noted that fertilizers applied to
mulching materials are more efficient than when
incorporated into the soil. (Thurston)

Basically, feeding plants through the mulch helps
compensate for less than ideal conditions of soil
structure or root growth, providing a supplemental
source of readily available nutrients in small but
constant quantities right at the soil surface, thereby
making it less necessary for crops to develop
huge root systems that extend deep into the soil
profile. Obviously, plants’ access to nutrients will
be better if the nutrients are on the soil surface
than if they are three feet below it, especially in
impoverished, acidic soils with problems of aluminum
toxicity.

These same rules may well apply not only to the humid
tropics, but to the semi-arid tropics, as well.
Reports from some semi-arid areas indicate that this
is the case. Nevertheless, there still exist some
doubts as to what extent crops can survive during, and
recover after, the mulch has dried out
completely due to the frequent droughts in such areas.
Much more experimental evidence is needed
in this case.

Small farmers and NGO’s have developed a number of
other simple ways that plants’ access to
nutrients can be inexpensively enhanced during the
transition period. Edwin Asante, of World
Vision/Rwanda, for instance, has developed a sort of
small farmer version of precision planting for
potatoes. In this case, an 8-cm ball of o.m., lime,
and about one-fourth the normally recommended
amount of chemical fertilizer are placed less than
0.5-cm directly below the seed. Yields in
impoverished soils with a pH of 3.5 have averaged 20
t/ha, as opposed to 9 t/ha without precision
planting. (Personal communication during field visit)
In Honduras, Elías Sánchez developed a type
of strip tillage or in-row tillage (locally called
“minimum tillage” or “labranza mínima”) which
concentrates the o.m. in the crop row, where it is
more accessible. And Dr. Erich Raddatz is
developing mycorrhiza strains that can double fruit
production by increasing plants’ access to
nutrients. (Personal communication)

These Five Principles, apart from having proven
themselves time and time again among small
farmers around the world, are the self-same principles
a humid tropical forest employs to maintain
its high “productivity” during millennia, even on
soils with very low CEC’s. A tropical rainforest
maximizes biomass production and biodiversity, keeps
the soil shaded at all times, and feeds its
plants largely through the litter layer. And, of
course, no human beings have to plough a forest to
keep it growing lush and green, century after century.

Thus, the sustainability of forest ecology over the
millennia provides important evidence that
tropical agriculture following these Five Principles
should also be sustainable over long periods of
time. The small amount of scientific research done on
this issue so far tends to support this
conclusion. (Buckles)

ADDITIONAL IMPACTS OF THE NUTRIENT ACCESS CONCEPT

One major result we could expect of the Nutrient
Access Concept of soil fertility would be an
increase in optimism about the plight of resource-poor
farmers. They are, by the conventional
Concept, in serious, perhaps insurmountable, trouble.
Their soils are increasingly depleted of
nutrients. (Buresh; Henao) Furthermore, they have no
hope of increasing their soils’ nutrient
supplies sufficiently with o.m., and they have far too
little money to invest in chemical fertilizers,
even if the impact of fertilizers on their
impoverished soils were enough to cover the
investment.
To add insult to injury, the world’s rich nations have
now decided that somehow these unfortunate
people are on a level playing field with rich European
and American farmers, and therefore should
have to compete with them without the assistance of
protective trade barriers.

But given the Nutrient Access Concept, even those
farmers with heavily depleted soils should be
able to increase their yields dramatically with very
little investment other than that of more increased
knowledge and the adoption of new agricultural
techniques. Gm/cc’s provide cheaper nitrogen than
fertilizer factories, while zero tillage and cover
crops can eliminate the comparative advantage
provided by tractors. Whether most villager farmers
will ever be able to compete with European or
American farmers is still doubtful, but at least they
should be able to produce enough for their own
consumption to eat well. Although if conventional
farmers in the First World persist in using
increasingly expensive fossil fuels and chemical
fertilizers at the present exorbitant rate, the poor
just might be able to compete, after all.

A second impact of the Nutrient Access Concept would
be that the world’s agriculture will become
a good deal more sustainable. Increased sustainability
will come from the reduced use of chemical
fertilizers (reducing groundwater and stream
pollution, nutrient imbalances and soil
acidification),
from the positive impacts on the environment of
increased biomass production, soil cover, soil o.m.
and biodiversity, and from the decrease of farmer
dependency on increasingly expensive fossil fuels.

NOTES
Ahn, Peter Martin (1993) Tropical Soils and Fertilizer
Use, Essex, Longman Group UK Ltd.
______________ (1999) Alternatives to Conventional
Modern Agriculture for Meeting World Food
Needs in the Next Century, Report of the Cornell
International Institute for Food, Agriculture, and
Development’s (CIIFAD) conference on “Sustainable
Agriculture: Evaluation of New Paradigms
and Old Practices,” at Bellagio, Italy, April 26-30.
18
Borlaug, Norman (1998) as quoted in Dennis Avery,
Saving the Planet with Pesticides and Plastics,
The Environmental Triumph of High-Yield Farming,
Indianapolis, Indiana, the Hudson Institute.
Buckles, Daniel, et al. (1998) Cover Crops in Hillside
Agriculture, Farmer Innovation with Mucuna,
Ottawa, Canada, International Development Research
Centre (IDRC) and International Maize and
Wheat Improvement Center (CIMMYT).
Bunch, Roland (1994) “EPAGRI’s Work in the State of
Santa Catarina, Brazil, Major New
Possibilities for Resource-Poor Farmers,” photocopied.
Bunch, Roland (1995) “An Odyssey of Discovery,
Principles of Agriculture for the Humid Tropics,”
ILEIA Newsletter, Vol. 11, No. 3, October.
Bunch, Roland (2001) “A Proven Technology for
Intensifying Shifting Agriculture, Green
Manure/Cover Crop Experience Around the World,” and
“Achieving the Adoption of Green
Manure/Cover Crops,” both presented at the
International Institute for Rural Reconstructon
(IIRR’s)
Conference on “Best Practices in Shifting Agriculture
and the Conservation of Natural Resources
in Asia,” held August 14-26 at Silang, Cavite, the
Philippines. Both are soon to be published by
IIRR.
Bunch, Roland and Gabino López (1995) Soil
Recuperation in Central America, Sustaining
Innovation after Intervention, Gatekeeper Series No.
55, London, International Institute for
Environment and Development (IIED).
Buresh, Roland J., et al., eds. (1997) Replenishing
Soil Fertility in Africa, SSSA Special Publication
No. 51, Madison, Wisconsin, Soil Science Society of
America (SSSA) and International Center for
Research in Agroforestry (ICRAF).
Committee of Tropical Soils, Agriculture Board, and
National Research Council (1972) Soils of the
Humid Tropics, Washington, D.C., National Academy of
Sciences (NAS).
Cresser, Malcolm, et al. (1993) Soil Chemistry and its
Applications, Cambridge, UK, Cambridge
University Press.
Henao, Julio and Carlos Baanante (1999) “Nutrient
Depletion in the Agricultural Soils of Africa,”
2020 Vision Brief 62, International Food Policy
Research Institute (IFPRI), Washington, D.C.,
October.
Ladha, J. K., et al. (1998) “Opportunities for
Increased Nitrogen-use Efficiency from Improved
Lowland Rice Germplasm,” Field Crops Research, Vol.
56.
Minnich, Jerry (1977) The Earthworm Book, How to Raise
and Use Earthworms for Your Farm and
Garden, Emmaus, Pennsylvania, Rodale Press.
Ministerio de Agricultura y Ganadería (MAG)/Dirección
General de Planificación (DGP) (1995)
Siembra Directa, Primer Encuentro de Productores,
Organizaciones y Técnicos, Asunción, La Rural
Ediciones.
Mosher, A. T. (1971) To Create a Modern Agriculture,
Organization and Planning, New York,
Agricultural Development Council, Inc.
Palm, Cheryl A., et al. (1997) “Combined Use of
Organic and Inorganic Nutrient Sources for Soil
Fertility Maintenance and Replenishment,” in Roland J.
Buresh, et al., eds., Replenishing Soil
Fertility in Africa, SSSA Special Publication No. 51,
Madison, Wisconsin, Soil Science Society of
America (SSSA) and International Center for Research
in Agroforestry (ICRAF).
Pretty, Jules and Rachel Hine (2000) Feeding the World
with Sustainable Agriculture, A Summary
of New Evidence, Colchester, UK, University of Essex.
Primavesi, Ana (1982) Manejo Ecológico del Suelo, La
Agricultura en Regiones Tropicales, Quinta
Edición, Buenos Aires, Librería “El Ateneo” Editorial.
Quiñones, Marco A., et al. (1997) in Roland J. Buresh,
et al., eds., Replenishing Soil Fertility in
Africa, SSSA Special Publication No. 51, Madison,
Wisconsin, Soil Science Society of America
(SSSA) and International Center for Research in
Agroforestry (ICRAF).
Thurston, H. David (1997) Slash/Mulch Systems:
Sustainable Methods for Tropical Agriculture,
Boulder, Colorado, Westview Press.
Uphoff, Norman (2000) “How Can ‘The Biological
Maximum’ for Rice be Exceeded? Possible
Explanations for the High Yields Observed with the
System of Rice Intensification (SRI),” draft
copy, printed paper.
Young, Anthony (1989) Agroforestry for Soil
Conservation, Oxon, UK, C.A.B International.

=====
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Sincerely, Lion Kuntz
Santa Rosa, California, USA
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http://www.ecosyn.us/Interesting/
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