[lifesaviors] Nutrient Quantity Vs Quality - SRI
- From: "Lion Kuntz" <lionkuntz@xxxxxxxxx>
- To: lifesaviors@xxxxxxxxxxxxx
- Date: Thu, 05 Sep 2002 04:24:10 +0800
http://1lion.bravepages.com/New_Visual_Pages/Agro-Ecology/Agro_Ecology_1.htm
ppathw3.cals.cornell.edu/mba_project/moist/Roland.pdf
http://www.echonet.org/tropicalag/ednissues/text_eng/edn_74t.htm
Nutrient Quantity or Nutrient Access??A New Understanding of How to Maintain
Soil Fertility in the Tropics
Nutrient Quantity or Nutrient Access??A New Understanding of How to Maintain
Soil Fertility in the Tropics
By Roland Bunch
Introduction
In order to achieve high levels of agricultural productivity in the tropics at
the lowest possible economic and ecological costs, we need to properly
understand the relationship between nutrients in the soil and crop
productivity.?For this to happen, the current understanding needs to
change.?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.?There is no need to
use the huge quantities of chemical fertilizers that are so often
recommended.?In fact, often times the use of such fertilizers is unnecessary,
expensive and harmful to the environment, especially because farmers often stop
using organic matter when they use chemical fertilizers.?
?uch of the theory described here was originally developed by Drs. Artur and
Ana Primavesi.?For a much more in-depth analysis of the chemical and biological
issues described in this article, the best book at present is Ana Primavesi's
The Ecological Management of the Soil (unfortunately this book is currently
available only in Spanish and Portuguese).?This article will discuss the
conventional concept of soil fertility and some of its shortcomings; a new
conception of soil fertility; and how the new theory can be put into practice.?
?onventional Concept of Soil Fertility
?oil fertility is more than the soil's content of available nutrients.?For the
purposes of this article, 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."?
?he traditional concept of soil fertility to a large extent sees fertility as a
reflection of the overall quantities or concentration of nutrients in the
soil.?According to this concept, as long as enough nutrients are present, soil
pH is within a certain range, and cation exchange capacity (CEC) is high enough
to hold nutrients, there is good soil fertility.?The basic idea is that the
soil operates like a bank: add nutrients repeatedly, over a long period of
time, and they will gradually build up like a savings account, increasing the
soil's fertility and therefore crop productivity.?We will refer to this idea of
soil fertility as the Nutrient Quantity Concept (NQC).
?n most books on soil properties and management (the majority of them written
by proponents of the NQC), little attention is paid to organic matter or soil
biology.?Rather, sources and quantities of nitrogen (N), phosphorus (P) and
potassium (K) dominate the discussion.?As a result, most recommendations to
restore soil fertility and improve food production in the tropics rely on the
application of chemical fertilizers.?
?irst, 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 actually 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 that is the limiting factor for the plant to achieve maximum
growth.?According to this theory, 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)
?he Inadequacies of the Nutrient Quantity Concept
?heoretical Inadequacies of the Concept
1. The Nutrient Quantity Concept is oversimplified.?Other factors are far
more important for productivity than the total quantity of any single nutrient
or group of nutrients.?These other factors include:?the chemical form in which
the nutrient 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.?These factors also influence
each other, so that the microenvironments within the soil are constantly
changing.?At some times a plant may be able to access most of a given nutrient
in the soil, while at other times it may only be able to access less than 1% of
the total store of that same nutrient.?The key is the bioavailability of the
nutrient, that is, how much of the nutrient is actually available to the plant.?
?he factors listed above are recognized by proponents of the Nutrient Quantity
Concept.?However, their thinking is dominated by one particular fact:?that in a
uniform soil environment, if more of a certain nutrient is present in the soil,
more will be in an available form.?They do not take into account the fact that
soil is non-uniform and that often the relationship does not exist, especially
in the tropics.?For example, soil phosphorus can be as much as fifty times more
available in an organic environment than in an infertile acid soil
environment-and yet most soil scientists still advise adding phosphorus to acid
soil, rather than applying the phosphorus to a mulch, for instance.
2. The Nutrient Quantity Concept seems to assume that nutrients are
relatively stable in the soil.?They really are not, especially where CEC of the
soil is low and/or where erosion occurs.?Nitrogen and potassium in particular
do not remain in the soil for long, and phosphorus is less stable in tropical
soils than has long been assumed.?"Money" is constantly leaking out of the
"bank."?And the more money there is in the bank, the more will leak out.
?hemical fertilizers do not maintain levels of most micronutrients in soil and
they reduce soil pH.?This means farmers may need to use expensive lime or
alkaline fertilizers because of low soil pH due to use of chemical
fertilizers.?Admittedly, organic matter also fails to increase soil nutrient
quantities dramatically over the long term in the tropics.?The use of either
chemical fertilizers exclusively or organic matter exclusively will fail to
achieve long-term improvement in nutrient quantities.?It is not that chemical
fertilizers are completely bad; replacement of some chemical elements in the
soil is acceptable and often even desirable.?
3. Proponents of the Nutrient Quantity Concept have largely avoided taking
into account the tremendous impact in tropical soils of such factors as their
macro and microbiology, organic matter content, microenvironments and
compaction layers.??
Inadequacies in Practice?
Most conventional soil scientists have concluded that "low external input"
technologies must inevitably lead to "low output" results; that "ecological
agriculture" is inevitably unproductive and has virtually no future; and 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.?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.
?hus, the Nutrient Quantity Concept is failing us.?It fails to lead us to
proper conclusions about agricultural priorities.?It fails to predict what will
happen if we apply a whole range of agricultural technologies that are now
being tried in the tropics, and it fails 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.
?et's look a little more closely at these failings.?
1. The traditional Nutrient Quantity Concept in tropical environments has
caused many scientists to dismiss ecological agriculture out of hand.?According
to the Nutrient Quantity Concept way of thinking, if not much is put into the
bank account, not much can be withdrawn.?Because of this thinking, promising
technologies like ecological agriculture and agroecology have been largely
ignored (Pretty and Hine).?
2. The Nutrient Quantity Concept leads to the claim that soils with very
low CEC's will never be able to produce large harvests, because these soils
cannot hold very many nutrients over a crop's entire lifespan.?Thus large areas
of the tropics have been written off as "low-potential" areas, where
investments in agricultural development are not seen as worthwhile
(Mosher).?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.
3. The Nutrient Quantity Concept leads almost inevitably to an excessively
high use of chemical fertilizers, which is particularly expensive for
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.?Over time, 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 organic matter.?Then the response to
chemical fertilizers is reduced until eventually there is no more economic
advantage to using them.?The recent increase in petroleum prices (from $12.00 a
barrel to somewhere between $19.00 and $32.00 a barrel) will increase the cost
of fertilizers because of higher production and transport costs.?Let's learn
about technological possibilities that will let farmers be productive without
relying so much on chemical fertilizers!
4. The Nutrient Quantity Concept lacks predictive abilities.?Very high
productivity is being achieved on soils that could never produce such yields
according to traditional thinking, using only one-half to one-tenth of the
amount of nutrients recommended by the Nutrient Quantity Concept.?We can
pinpoint several specific cases in which the Nutrient Quantity Concept has
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 much greater than the conventional Concept
would have predicted.?The "green manure/cover crops" technology 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. in all cases 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.?
Farmers using green manure/cover crops, resulting in increases of only perhaps
100 kg of fixed N and no additional P or K, have often doubled yields of maize
(Buckles; Bunch and Lopez; Pretty and Hine, for example).?Furthermore, in
northern Honduras, 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)
* Using the System of Rice Intensification (SRI) in Madagascar, hundreds
of farmers are achieving 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; see also ECHO
Development Notes Issue 70).?Yet the world's rice experts hold that the
"biological maximum" for the rice plant is less than 10 t/ha.?The attitude that
"low input agriculture is low output agriculture" 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 West Africa, on very old, low CEC soils, women frequently grow
4-mt-tall, 4 t/ha maize on small plots around their homes.?The only addition to
the soil is the grey water and kitchen scraps from the household, which are
applied daily.
* Traditional slash-and-burn or shifting agriculture has been an age-old,
world-wide method of regenerating soils.?The techniques cannot be fully
explained by the dominant interpretation of the Nutrient Quantity
Concept.?Close to half of fields that West African farmers indicated were ready
to be "slashed and burned" had no visible vegetation on them other than
grasses.?If grasses can regenerate soils by themselves, how can the Nutrient
Quantity Concept explain this worldwide phenomenon?
* The biomass productivity of natural rainforests is much higher than its
CEC would allow under the traditional Concept.?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 CECs in the soil
in general.?Yet many deny the possibility that this same phenomenon of the
rapid circulation of nutrients could be the basis of highly productive crop
agriculture under similar conditions.?In other words, 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.?
?utrient pumping (the bringing of nutrients to the soil surface from deeper
layers by trees) might seem to cloud the above issue somewhat.?However, many
rainforests produce large amounts of biomass above subsoils that, even under
extremely efficient nutrient pumping, provide fewer nutrients than those added
artificially under many "low external input" systems.?Besides, areas deep in
the soil from which 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 an extremely low total concentration of nutrients.
* Chemical fertilizer companies have spent millions of dollars to
research "slow-release" forms of chemical fertilizer.?These companies thus
admit through their actions that the overall quantity of nutrients available at
any given time is not the primary issue in productivity.?Rather, the constant
supply of nutrients is more important than the total quantity available at any
particular time.
?iven 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
?o illustrate the Nutrient Access Concept of soil fertility, we start out with
an experiment reported in Ana Primavesi's The Ecological Management of the
Soil. In this experiment, crops were grown in four hydroponic solutions.?The
solutions were as follows:
1. A normal concentration of nutrients for maximum maize plant development
was used, and replenished every 4 days.?
2. Twice the normal concentration was used and replenished every 4 days.?
3. The normal solution was diluted 50 times and also replenished every 4
days.?
4. The normal solution was diluted 50 times, but was replenished every 2
days.
?lant growth (measured in grams of dry weight) was less in the second case than
in the first.?Plant growth in the third case was also less than in the
first.?But in the fourth case, plant growth was slightly better than in the
first.?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.
?rop growth above a certain extremely low concentration does not depend on the
concentration of nutrients.?Instead it depends on the plant roots' constant
access to the nutrients, even when these nutrients exist in very low
concentrations.?What is needed is a constant supply of a small but
well-balanced amount of nutrients over time, and the unobstructed access of
plant roots to these nutrients.
?his experiment shows that the relationship between concentrations or overall
quantities of nutrients and plant growth is, above a certain minimum
concentration, altogether nonexistent. ?s 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).?
?hese results are more relevant to tropical soils and farmers than to
temperate-zone soils and farmers for several reasons:
1. Tropical soils tend to have lower concentrations of nutrients and fewer
cation-exchange sites (lower ability to hold nutrients).
2. The ambient heat of the tropics makes it difficult for plants to create
enough osmotic pressure to absorb nutrients from highly concentrated
solutions.?Limited concentrations of nutrients are often better.
3. Most farmers in the tropics work by hand or animal traction, so they
can micromanage the soil by hand and create different microenvironments.?In
some of these microenvironments, nutrients are more accessible.
4. Resource-poor farmers can't afford to over-fertilize.?And they often
lose more nutrients than temperate-zone farmers because of high rainfall, steep
slopes, or factors of soil chemistry.
Rather than emphasizing the concentration of nutrients in the soil, the new
concept emphasizes the access of plant roots to soil nutrients. We will refer
to this concept as the Nutrient Access Concept of soil fertility.?Here are the
main claims of this concept:
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
3. maximum access of plant roots to these nutrients (e.g. the maintenance
of good soil structure and/or mulches)
The Adequacy of the Nutrient Access Concept
Can the Nutrient Access Concept explain the phenomena mentioned above better
than the Nutrient Quantity Concept could??For one thing, the Nutrient Access
Concept admits that high levels of productivity can be achieved through high
concentrations of nutrients in developed nation agriculture, and even in highly
capitalized plantation agriculture on the best soils of the tropics.?This is
true in many circumstances, especially in cooler climes, when soils are
compacted or optimal soil structure has otherwise been damaged, when CEC is
high and when farmers are well-capitalized.?
However, where soils have very low CEC's, where soil organic matter is or could
be abundant and cheap, where capital is scarce, and/or where temperatures are
high, the Nutrient Access Concept points to agricultural practices of a
radically different kind from those presently used.?
Many farmers, in southern Brazil and scores of other countries, have realized
competitive yields at relatively low cost on very "low potential" soils, with
more positive long-term ecological impact than agriculture done according to
the Nutrient Quantity Concept.´?
Thus the Nutrient Access Concept could reduce significantly the costs of
producing competitive yields in the tropics.?It also confronts the present
unjust discrimination against those farming on so-called "low potential"
soils.?In fact, 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.
The Nutrient Access Concept also calls into question efforts to subsidize huge
quantities of expensive chemical fertilizers to African nations that are
already practically bankrupt.?Such proposals are based on the Nutrient Quantity
Concept.?Adoption of the Nutrient Access Concept would force a major rewrite of
these proposals, gearing them instead toward the goal of increased yields
through higher levels of biomass production, soil structure improvement and
mulch-based systems.
In addition, the Nutrient Access Concept can explain very adequately those
observed phenomena mentioned previously, which the traditional theory cannot
explain:?
* Green manure/cover crops.?Rather than depending on high concentrations
of chemical nutrients, yields in gm/cc and agroforestry systems depend on the
fixation of N and the recycling of large amounts of organic matter which makes
the P and other nutrients in 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.
* SRI yields.?With the SRI methodology, the soil is aerated and plants
grow almost six times more roots per plant.?This means they can access many
more nutrients in the soil.?
* West African kitchen gardens.?The organic matter thrown out of kitchens
daily maintains a small, steady supply of nutrients.?
* Regeneration of tropical soils.?The regrowth of forests or grasslands
maintains or improves soil structure so that on newly cleared land, crops can
more efficiently access the low concentrations of nutrients.?Organic matter on
or near the soil surface (from years of fallow) supplies nutrients in small
quantities.
* Rainforests.?Good soil structure and mulches are maintained, so that
trees can access the small amounts of nutrients that are constantly being
supplied by the breakdown of soil organic matter.?Trees with deep roots and
lots of feeder roots can capture many nutrients even though they are present
only in low concentrations.
* Slow-release chemical fertilizer.?The benefits of slow-release chemical
fertilizer are much more understandable based upon the Nutrient Access Concept
rather than upon the Nutrient Quantity Concept.
Of course these explanations are very simplistic.?Plants' access to nutrients
is a very complicated phenomenon which involves a large number of
factors.?These include soil temperature, soil organic matter levels, pH, soil
chemical properties, the presence of compaction layers, and nutrient
positioning and equilibrium.?All of these factors are in turn affected by the
activity of hundreds of thousands of microorganisms in every teaspoonful of
soil.?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
?he Nutrient Access Concept can most easily be put into practice through the
copious use of organic matter.?Organic matter supplies low to medium
concentrations of nutrients, almost always in well-balanced quantities.?Organic
matter also 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, soil organic matter can gradually improve soil structure,
both directly (through the provision of binding materials to improve
flocculation) and indirectly (by feeding earthworms and other soil organisms
which also improve soil structure) (Minnich).
?he best way to apply organic matter is to apply it either to the soil surface
or, during the period of transition (from fertilizer-based to mulch-based
agriculture), within 20 cm of the surface.´ During the first year or two of a
transition into mulch-based agriculture, soil compaction below the surface is a
serious limiting factor.?After the first year or two, virtually all the organic
matter should be applied to the soil surface.?
?he Nutrient Access Concept does not necessarily support a totally organic
approach.?But it suggests a greatly reduced use of chemical fertilizers in the
short run.?In the long run, it suggests use of chemical fertilizers only to
replace nutrients not supplied by organic matter and nitrogen fixation.
?ased on the Nutrient Access Theory, the following Five Principles of soil
management have begun to be used around the world in small farmer agriculture:
1. Maximize organic matter production.?Organic matter production can be
increased by a) intercropping of crops or gm/cc's with annuals or tree crops;
b) establishing two- to four-story fields and gardens; and c) growing trees or
gm/cc's on wasteland or during the dry season.?Watering/irrigation can help
increase organic matter production in dry areas.?It is best to produce biomass
on site.?2. Keep the soil covered.?Covering the soil will help to reduce
both weed growth and the heating of the soil.?The latter can accelerate soil
organic matter burnout, reduce crop growth rates, and cause the death of
beneficial organisms in the soil.?By maximizing biomass production and keeping
the soil covered, the need to let land lie fallow can often be
eliminated.?Keeping the soil covered reduces the decomposition rate of soil
organic matter, which means the provision of nutrients to the soil will last
longer and be more constant, even if mulches tend to lose a certain amount of N
to volatilization.?
3. Use zero tillage.?In order to be effective, this technique should be
used in the presence of a maximum production of biomass, so that the supply of
nutrients and good soil structure can be maintained.?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 the populations of organisms that naturally till the soil
increase rapidly as soil organic matter levels increase and soil becomes
covered.?(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 rarely, need to use herbicides.
Tillage damages soil structure and increases the rate of soil organic matter
burn-out.?It also 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.?This principle is primarily important 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. ?eed the crops largely through the mulch.?Many humid tropical soils are
not very hospitable environments for crop roots because of their low pH (below
5.0), their aluminum toxicity and compaction layers.?Crops will often grow much
better if they can also access nutrients from a thick litter layer or
mulch.?Most feeder roots will likely spread immediately under or up into a
mulch layer as long as it remains fairly moist.?The impact of chemical
fertilizers can also sometimes be greatly increased by being applied to the
mulch rather than the soil.?
Feeding plants through the mulch helps compensate for poor soil structure or
less than ideal conditions of root growth.?In poor soils, if nutrients are on
the soil surface, plants will have better access to them.?
Small farmers and NGO's have developed a number of simple ways that plants'
access to nutrients can be inexpensively enhanced during the transition
period.?For example, Edwin Asante, of World Vision/Rwanda has developed a small
farmer version of "precision planting" for potatoes.?In this case, an 8-cm ball
of organic matter, 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 very poor 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, El?as S?nchez developed a type of strip tillage or
in-row tillage (locally called "minimum tillage" or "labranza m?nima") which
concentrates the organic matter in the crop row, where it is more accessible.?
?
These Five Principles are the very same principles a humid tropical forest
employs to maintain its high "productivity" for 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.?
?dditional Impacts of the Nutrient Access Concept
?e can expect a few major results of the Nutrient Access Concept of soil
fertility.??
1. An increase in optimism about the plight of resource-poor
farmers.?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 practically
eliminate the comparative advantage provided by tractors.?
2. More sustainable agriculture.?With the use of the aforementioned
practices, 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).?It will also come from the positive impacts
on the environment of increased biomass production, soil cover, soil organic
matter and biodiversity, and the decrease of farmer dependency on increasingly
expensive fossil fuels.
?ull article with full references (20 pages total) available on request.
?elected References
?hn, Peter Martin (1993) Tropical Soils and Fertilizer Use, Essex, Longman
Group UK Ltd.
?uckles, 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).
?unch, 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.
?unch, Roland and Gabino L?pez (1995) Soil Recuperation in Central America,
Sustaining Innovation after Intervention, Gatekeeper Series No. 55, London,
International Institute for Environment and Development (IIED).
?resser, Malcolm, et al. (1993) Soil Chemistry and its Applications, Cambridge,
UK, Cambridge University Press.
?innich, Jerry (1977) The Earthworm Book, How to Raise and Use Earthworms for
Your Farm and Garden, Emmaus, Pennsylvania, Rodale Press.
?osher, A. T. (1971) To Create a Modern Agriculture, Organization and Planning,
New York, Agricultural Development Council, Inc.
?retty, Jules and Rachel Hine (2000) Feeding the World with Sustainable
Agriculture, A Summary of New Evidence, Colchester, UK, University of Essex.
?rimavesi, Ana (1982) Manejo Ecol?gico del Suelo, La Agricultura en Regiones
Tropicales, Quinta Edici?n, Buenos Aires, Librer?a "El Ateneo" Editorial.
?phoff, 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.
?oung, Anthony (1989) Agroforestry for Soil Conservation, Oxon, UK, C.A.B
International.
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