Healthy soil and sustainable growing

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Source: Ronald F. Korcak, History of the Organic Movement, 1991.
Early Roots of the Organic Movement: A Plant Nutrition Perspective
http://www.hort.purdue.edu/newcrop/history/lecture31/r_31-2.html

Lord Walter Northbourne first used the term "organic farming" in 1940 as a
chapter heading in his book Look to the Land (Northbourne, 1940). That same
year, coincidentally, was also the 100th anniversary of Justus von Liebig's
monograph Organic Chemistry in its Application to Agriculture and
Physiology. The fundamental tenet of Liebig's doctrine was the development
of the mineral nutrient theory of plant nutrition. About 100 years later,
the primary American proponent of organic farming, J.I. Rodale, cited
Liebig as the founder of the fertilizer industry (Rodale, 1945), which he
probably was, and the N-P-K mentality in agriculture. The basis for these
remarks was that "up to that time the humus theory had been the guiding
basis for agriculture" (Rodale, 1945). What exactly was the humus theory?
How did it evolve? Was it a viable theory? What was the role played by the
pivotal player, Liebig? This review follows the evolution of plant
nutritional theories from the early use of crop rotations in China, to the
development of the humus theory, and finally to the time and work of Liebig
and his influence on theories of plant nutrition.
Introduction

Plant growth has been both a curiosity and a source of much documentation
and experimentation since the beginnings of agriculture. The "substance" of
plants has been explored and debated since the time of the early Greek
philosophers. The current dichotomy over the source of plant nutrients
between the organic vs. inorganic fertilizer camps is not new. Democritus
of Abdera (460-360 BC) proposed what would currently be considered as an
atomic philosophy of matter:

    Mother earth when fructified by rain gives birth to crops for the
nourishment of man and beast. But that which came from earth must return to
earth and that which came from air to air. Death, however, does not destroy
matter but only breaks up the union of its elements which are then
recombined into other forms. (Browne, 1943)

This atomic, cyclic, and nonconvertible chain of elements through the
soil-plant-animal system was opposed by Aristotle's (384-322 BC) mutual
convertibility of the 4 elements: earth, water, fire, and air. Since,
according to Aristotle, the material constituents of the world were formed
from unions of these 4 elements, plants assimilated minute organic matter
particles through their roots which were preformed miniatures (Browne,
1943). This concept of "organic" nutrition of plants, evolving into the
humus theory of plant nutrition, held for more than 2000 years, until the
time of Justus von Liebig.

The course of experimentation and thinking on the subject of plant
nutrition, from the post-Greek philosopher period until the time of
Liebig's 1840 monograph, is the concern of this review. Liebig's monograph
laid the foundation of a plant mineral nutrient theory that was a precursor
for the fertilizer N-P-K mentality of crop fertility. The use and/or abuse
of synthetic mineral fertilizers subsequently became the bane of the
organic fan movement.
Early investigations

The beneficial responses observed from using green manures and animal
manures and the use of crop rotations on crop growth have directly
influenced the development of plant nutrition theories. The earliest record
of the benefits of green manures dates back to the Chou dynasty (100 BC) in
China (Pieters, 1927). Later (500 BC), Tsi gave the following advice: "They
(green manures) are broadcast in the 5th or 6th month, and plowed under in
the 7th or 8th month... Their fertilizing value is as good as silkworm
excrement and well-rotted farm manure" (Pieters, 1927).

King (1911) concluded from travels in the Orient, notably China, that the
practice of deliberately adding organic matter to the soil dates back at
least 4000 years and summarized his observations as: "This is a remarkable
practice in that it is very old, intensive application of an important
fundamental principle only recently understood and added to the science of
agriculture, namely, the power of organic matter, decaying in contact with
soil, to liberate from it plant food."

Early Roman compilations of agricultural practices that enumerated the use
of organic manures and crop rotations were accumulated by the prolific
agricultural observer Cais Plinius Secundus (AD 23-79), better known as
Pliny the Elder (Browne, 1943). He enumerated the advantages and
disadvantages of most animal manures and recommended the use of green
manures (Brown, 1943). "It is universally agreed by all writers that there
is nothing more beneficial than to turn up a crop of lupines, before they
have podded, either with the plough or the fork, or else to cut them and
bury them in heaps at the roots of trees and vines."

Though Pliny and subsequent writers over the centuries extolled the
benefits of manuring from a scientific viewpoint, little advance was made
on the reasons for these benefits. Generally, the Aristotelian concept of
the 4 elements held sway into the Middle Ages. The Middle Ages, generally,
represent a quiescent period devoid of any advances in science and
technology-no less in the understanding of plant mineral nutrition. Some
notable exceptions to this void would have profound influences on the
development of a theory of plant nutrition near the end of the Middle Ages.
Post-Middle Ages

Philippus Theophrastus Paracelus (1493-1541) gained fame as the first
scientist to lecture in German, in lieu of the traditional Latin, allowing
for understanding and involvement in science among lay persons, and
recognition of the importance of experimentation in chemistry (Browne,
1943). More germane to plant nutrition, he initiated a new concept of plant
nutrition, which was not aligned to the Aristotelian 4 elements. He stated:

    ...So also every vegetable of the earth must give nutriment to the 3
things of which they consist. If they fail to do that the prima Condita
(first substance) perish and die in their 3 species. These nutriments are
earth and rain, that is the Liquor, each of the 3 parts of which nourishes
its own kind-sulfur for sulfur, mercury for mercury, and salt for salt, for
Nature contains these, one with the others. (Browne, 1943)

A modern-day interpretation of this passage indicates that the nourishment
of plants requires 3 principles: organic constituents (sulfur), water
(mercury), and mineral matter (salt). There is even a hint at the "law of
the minimum" concept of plant nutrition. Although the "three-principle"
theory of Paracelus differs only slightly from that of Aristotle's 4
elements, it represents, as Browne (1943) points out, a "break with an
outworn tradition."

The search for the true "substance" of plants and a continuance of the
break with the 4 elements of Aristotelian theory was furthered by Bernard
Palissy (1510-1589). Considered by some to be the founder of agricultural
chemistry (Browne, 1943), Palissy relied on observation and experience in
making practical theories for manuring: ÎManure is carried to the field
for the purpose of restoring to the latter a part of what had been
removed... Proceeding thus you will restore to the soil the same substances
that have been removed by previous crops and which following crops will
regain to their advantage."

The idea that the same substances will be reused by other crops would
become central to the development of the humus theory of plant nutrition
200 years later. Palissy also helped to overthrow the concept of heat as
important in the growth of plants. Through the ages, the heat given off by
a pile of composting manure probably drew the attention of many observers.
This readily noticeable trait coupled with the fire element of Aristotle
was thought to be the "substance" from which plants benefited from
manuring. Palissy had observed that the benefits of man (as a soil
amendment) were long term and, therefore, not explicable in terms of heat.

No chronology of the early development of plant nutrition would be complete
without mention of Jan Baptista van Helmont (1577-1644) and his infamous
potted tree experiment, which lasted 5 years (Browne, 1943). The
experiment, in brief, consisted of growing a 5-lb willow tree in a capped,
earthen vessel containing 200 lb of oven-dried soil. After 5 years, with
the addition of only water, the tree weighed 169 lb and the redried soil
just 2 oz less than 200 lb. The conclusion was that the 164-lb gain in tree
weight was derived from water. Unfortunately, no mention is ever made it
van Helmont weighed the water applied during the experiment's 5 years. If
he had, his conclusion, obviously, would have been different. Van Helmont
had performed prior experiments with combusting charcoal and found 1 lb of
ash from 62 lb of charcoal. The other 61 lb consisted of the "spirit of the
wood," which he coined "gas" (Browne, 1943). Had he measured the water
applied and applied his spirit of the wood concept, plant nutritional
theory may have been advanced by 100 years.

Eighty years later, the classic water culture experiment of John Woodward
(1665-1728) dispelled the concept of water as the sole substance of plants
(Russell, 1926). Woodward, in 1699, noted better spearmint growth in water
containing garden soil than when grown in rain water, or impure water (from
the Thames River).

Although van Helmont believed that the growth substance of plants was
water, it was likewise known that salts were an important plant
constituent. This idea was generated from the many early decomposition and
distillation experiments of plant materials. Two 17th century chemists,
Johann Glauber (1604-1668) and Gabriel Plattes (ca. 1600-1655), exemplify
the early knowledge of the importance of salts and their relation to both
manures and plant nutrition (Browne, 1943). In response to the lack of
manures brought about by the havoc of the Thirty Years War, Glauber
invented what may be considered the first manure substitute, or chemical
fertilizer, to fill the void. He called the material "philosophic dung" or
"fattening salt" and noted:

    Of this salt, which we may use instead of dung, there is great
diversity, for it is prepared of Wood-ashes, of Stones burnt to Lime, and
of other bodies putrefied by length of time. But the Chief of all these is
Salt-petre, being the salt of Vegetables, Animals and Minerals putrefied,
especially because it is endowed with a certain occult and sweet Fire.
(Browne, 1943)

Glauber did not realize that this organically based "fertilizer" added
N-P-K as well as lime to soils. Saltpeter (potassium nitrate) was thought
to be a constituent of plants, not a nitrate source; therefore, by addition
to the soil, one was adding an intrinsic pan of the plant and thereby
maintaining the "fatness" of the soil (Russell, 1926). The term "fatness"
was coined by Plattes as the crop sustaining ingredient of soils (Browne,
1943). He considered both the air and soil as valuable in plant nutrition:
"All fruits are compounded of a double substance, the one terrestriall and
the other aethereall, and for the most pan, the want of the terrestriall
pan causeth ill successe" (Browne, 1943).

The fatness of the soil, akin to its organic component, could be removed
either by crops or carried away by erosion. Rudiments of this concept of a
soil's fatness can still be found today in the adage "living off the fat of
the land."

Coincident with the work of Glauber and Plattes was a change in the English
system of agriculture from manorial to closed farms. Not only was
individual initiative greater, but newer crops allowed for more farm
animals and, thus, more manure. New rotation systems, such as the Norfolk
rotation (Porteous, 1960), eliminated the fallow year but included a legume
crop for feed or for green manure. Green manuring with legumes became the
mainstay of rotation systems in continental Europe as well. By the end of
the 17th century, much debate in Germany centered on whether or not the
legume crop should be harvested or turned under (Pieters, 1927).

In the late 17th century, John Mayow (1643-1679) and Nehemiah Grew
(1641-1712) recognized the importance of air in plant growth, and in the
New World, John Winthrop (1606-1676) promoted the importance of salt and
the manufacture of fertilizer saltpeter (Browne, 1943). Grew conceptualized
the root as a mouth into which entered a watery nutriment solution from the
soil, along with air. The "principles" of plant growth were preformed,
carried into the plant, and then filtered among the various plant parts.
John Woodward, noted earlier for his classic experiment refuting van
Helmont's water-only theory of plant nutrition, continued Grew's argument
for preformed entry of nutriment into the plant. He further elaborated that
different plants require different "corpuscles" (the preformed substances
of plants) and "...that every kind of vegetable requires a peculiar and
specifick matter for its formation and nourishment... It therefore the
soil, wherein any vegetable or seed is planted contains all or most of
these ingredients ('corpuscles'), and those in due quantity, "twill grow
and thrive there"; otherwise "twill not grow" (Browne, 1943).

According to Woodward, the soil could be regenerated only with a "new Fund
of matter" by fallowing, which enabled rain to supply anew stock, or by
manuring, particularly with vegetable manures, since they would serve "for
the formation of other like bodies" (Browne, 1943). Woodward's concepts of
the importance of earth in the nutrition of plants were advanced by his
contemporary, Jethro Tull (1674-1740). Although best known for his ideas
concerning tillage, Tull believed that fine particles of soil entered the
root; therefore, the finer the soil particles, via tillage, the better the
growth. "And Earth is surely the Food of all Plants, that with the proper
share of the other elements, which each Species of Plant requires, I do not
find but that any common Earth will nourish any Plant" (Browne, 1943).

In regards to the use and nutritional value of manures, Tull ascribes their
benefit to the enhanced mechanical and physical properties of the soil:

    All sorts of Dung and Compost contain some Matter, which, when mixt
with the Soil, ferments therein; and by such Ferment dissolves, crumbles,
and divides the Earth very much; This is the chief, and almost only Use of
Dung... This proves, that its (manure) use is not to nourish, but to
dissolve, i.e., Divide the Terrestrial Matter, which affords nourishment to
the Mouths of vegetable roots. (Browne, 1943)

The phlogiston period

The doctrine of the phlogiston school of early agricultural chemistry was
that all substances that are changed by ignition contain a common
combustible matter (Browne, 1943). The most important influence of this
doctrine on plant nutrition was the general assumption that plants
generated alkalis (Russell, 1961).

Advances of a plant nutrition theory during this period resulted primarily
from the work of Francis Home (1719-1813). He not only recognized the
importance of pot studies and plant analysis (Russell, 1961), but also
added fire and oil to the list of important factors (air, water, earth, and
salt) in plant nourishment:

    I join, in some measure, with all these; and assert that plants are
nourished by these bodies, united with two others, oil and fire in a fixed
state. These six principles together, in my opinion, constitute the
vegetable nourishment (Browne, 1943)

Oil was considered one of the "natural principles" that was introduced to
earth in rain, and fire was found "in all bodies." Home's work marks one of
the cornerstones in plant nutrition theory, i.e., a multitude of factors
are considered to explain the nourishment of plants.

Like Home, Johann Wallerius (1709-1785) considered plant nutrition a
multifaceted science. Regarding plant growth in general, Wallerius
believed, "Plants derive no growth from any mineral earths ... The
substances that promote plant growth must be (1) identical or analogous
with substances preexisting in the plant, or (2) capable of being
transmuted and combined into a nature that belongs to plants" (Browne,
1943). Therefore, humus was the "nutritiva" or source of plant food while
all other soil constituents were the "instrumentalia" that assisted in
making this food available (Russell, 1961).

Other notable discoveries during the phlogiston period would have profound
influence on the development of plant nutritional theories. These include
the discovery of O, by Joseph Priestly (1733-1804) and his work on the
purification of air by plants and the discovery by Jan Ingen-Housz
(1730-1799) that plants give off CO2. Air, or more properly "fixed air,"
became the important principle of plant nutrition and, as Priestly stated,
"the principle is phlogiston" (Browne, 1943).
THE HUMUS THEORY OF PLANT NUTRITION

The beginning of the 19th century coincided with the chemical revolution in
agricultural science, However, remnants remained of those who believed in
the Aristotelian 4 principles, phlogiston adherents, and transmutation
power of plants. Before advancing through this period of achievement in
agricultural chemistry and plant nutrition to the time of Liebig, it is
worth exploring what the humus theory of plant nutrition was and how the
pre Liebig scientific community accepted or rejected this theory.

The idea that increasing soil organic matter, either by plant residues or
animal manures, increased soil fertility and hence crop yields had been
realized, as we have seen, for centuries. This readily verifiable role of
organic matter, or humus, combined with the later question concerning the
source of C for plant growth, led to the humus theory of plant nutrition.
As noted earlier, Wallerius in 1761 was the first to allude to the idea
that humus was the food of plants (Kononova, 1961). A half century later,
Albrecht Thaer (1752-1828) is credited with formulating the theory of the
humus nutrition of plants (Waksman, 1942).

Humus was considered the sole and direct source of plant nutrients. Waksman
(1942) summarizes the humus theory, which had gained the support of early
chemists, including Theodore de Saussure (1767-1845) and Sir Humphrey Davy
(1778-1829):

    According to this theory, plants feed upon substances which are similar
to them in nature. The organic matter of the soil, or the soil humus, was
regarded as the chief nutrient for plants and the major source of soil
fertility. The roots of the plants were believed to extract the humus from
the soil and to transform it into plant substance, by combining it with
water. Plant nutrition was thus considered as similar to animal nutrition,
both plants and animals feeding upon complex organic bodies. As regards the
function of minerals in plants, some of the protagonists of the humus
theory believed that these were not essential for growth; they were
believed to act as stimulants rather than as nutrients. Others looked upon
minerals as mere accidental plant constituents, or as the skeleton
substances of plants similar to the bones of animals.

The Modern Period

Theodore de Saussure, the eminent Swiss chemist, in his Chemical Researches
on Plants (1804), overthrew many of the transmutation and "principle"
concepts of his predecessors. As Russell (1961) stated, de Saussure's
"concise and logical arguments" are refreshing compared to the "lengthy and
often wearisome works of earlier writers." Among the accomplishments
accorded de Saussure are the elucidation of plant respiration; the
recognition that soil, not air, was the supplier of N; the realization of
the active role of the root as an absorber of water and salts, not as a
mere filter; the realization that ash constituents of plants all occur in
humus; and debunking the idea that plants generate potash (the salt
principle of his predecessors) (Russell, 1961). About 36 years later,
Liebig erroneously argued that air was the source of plant N (ammonia) and
that this supply limited growth.

De Saussure, however, was a defender of the humus theory. His general
conclusions of plant nutrition included:

    That fertile soil contains a mixture of soluble and insoluble organic
substances and that the entrance of the former into the plant through the
roots is a most important aid for the nourishment which they derive from
air and water.

    That plants obtain their nitrogen almost wholly by absorption of the
soluble organic substances: direct experiment shows that they do not
assimilate it to an appreciable extent in the gaseous condition... (Browne,
1943)

Contradictory evidence for and against the concept that plants had the
power of transmutation to produce the principle of growth, and controversy
over the importance of organic vs. inorganic nutrition of plants, reached
an interesting stage by 1838. A prize was offered in Germany for the most
satisfactory answers to the questions: Do the so called inorganic elements,
which are found in the ashes of plants, occur in these plants when the
exterior sources of these elements are eliminated? Are these inorganic
elementary constituents so essential that the vegetable organisms have
constant need of them for their complete development (Browne, 1943)? The
prize was awarded to A.F. Wiegmann (1771-1853) and L. Polstroff for their
conclusions from an experiment comparing plant growth in a synthetic soil
vs. sand alone. They concluded, in part, "The inorganic constituents of
plants can in no respect be regarded as products of their vital activity
either as formations from unknown elements or as peculiar derivations of
the 4 elements known to make up organic substances" (Browne, 1943).

In regards to inorganic nutrition and manuring, Wiegmann noted that "...the
soil has been so robbed by the previously harvested crop of the inorganic
materials which are necessary for plant development that another crop of
the same kind (even when the ground is plowed and newly fertilized with an
animal manure deficient in the necessary mineral element) is unable to find
the requisite amount of plant food that is necessary for its complete
development" (Browne, 1943).

Opposition to the humus theory was to come from many sides. Experiments
with humus extracts led Wiegmann and Polstrof to conclude that humus plays
an insignificant role in plant nutrition (Browne, 1943). Carl Sprengel
(1787-1859) also ascribed little nutritive value to organic matter:

    The conviction should have been reached long ago that humus is not such
an important substance as we have been led to believe and that the current
doctrine of humus (the humus theory) is exceedingly full of
contradictions... I have come more and more to the conviction that plants
can entirely dispense with it (humus). (Browne, 1943)

Sprengel supported de Saussure's opposition to the old theory of
transmutation of mineral elements by plants and suggested that the mineral
elements of plants are derived from without. The supply of mineral elements
depends on the chemical composition of the soil. Much work by Sprengel
involved classifying fertilizing materials (Browne, 1943). He separated
organic fertilizers from mineral materials such as lime, clay, and a host
of salts. These and other details of Sprengel's writings, such as those on
the effects of minimum and maximum factors on the growth of crops, led
Browne (1943) to conclude that "...Sprengel should always be remembered as
the one who paved the way for Liebig..."

Among the last eminent pre-Liebig scientists was Jean-Baptiste Boussingault
(1802-1887). Boussingault's most significant impact on plant nutrition was
his initiation of the first series of field experiments (Russell, 1961).
Boussingault and his contemporary Gerardus Mulder (1802-1880) were both
proponents of the humus theory of plant nutrition.

In 1840, the Chemical Section of the British Association for the
Advancement of Science requested a report on the state of organic chemistry
(Bradfield, 1942). The invitee was Justus von Liebig. The opening paragraph
of his Organic Chemistry In Its Application to Agriculture and Physiology
would define organic chemistry, and, according to many, the rest of the
text would have dramatic implications on the practice of agriculture and
plant nutrition. The opening paragraph states, "The object of organic
chemistry is to discover the chemical conditions essential to the life and
perfect development of animals and vegetables, and generally to investigate
all those processes of organic nature which are due to the operations of
chemical laws" (Bradfield, 1942).
Justus von Liebig
Fig. 1. Justus von Liebig

Justus von Liebig (1803-1873) was recognized as one of the most
distinguished chemists of his day (Fig. 1). His combined activities as
investigator, editor, teacher, and writer were unequaled during his career
(Browne, 1942). His theory of the mineral nutrition of plants has dominated
the thinking of most students of soils and plant nutrition. Moore (l 947)
perhaps presents the best condensed interpretation of Liebig's theory: "The
soil was a sort of reservoir from which man could take out no more than he
put in."

Two important plant nutrition-related controversies were broached by
Liebig: the controversy concerning the source of C and that concerning the
source of N in plant nutrition.

The humus theory held that humus was the source of C. Liebig argued, in a
typical scathing attack on his contemporaries and predecessors, that prior
experiments on humus as the C supplier "...are considered by them as
convincing proofs, whilst they are fitted only to awake pity" (Russell,
1961). Liebig claimed that plants had an inexhaustible supply of carbonic
acid in the air. According to Liebig, humus was primarily a source of CO2
from decomposition, but the role of this CO, was to aid in the solubility
of inorganic soil constituents (Kononova, 1961). An additional benefit of
this humus-derived CO2 was the increased absorption by leaves as it
diffused from the soil (Waksman, 1938).

Later, however, Liebig does attribute one role of humus as a C supplier to
plants, but in a nonhumus-theory form, e.g., humus in an altered state.
"Humus does not nourish plants, by being taken up and assimilated in its
unaltered State, but by presenting a slow and lasting source of carbonic
acid which is absorbed by the roots, and is the principle nutriment of
young plants at a time when, being destitute of leaves, they are unable to
extract food from the atmosphere" (Waksman, 1942).

Liebig seems to waver as to the role of humus as the source of plant C. His
opinion that humus per se was not assimilated by plants was "proof" that
the humus theory was not valid.

Similar vacillations by Liebig can be found on the topic of the source of
plant N. Liebig maintained that atmospheric ammonia, not humus-derived N,
was the source of plant N. However, Liebig's stance on this important
question is of interest. In his original monograph, Liebig states,
"Cultivated plants receive the same quantity of nitrogen from the
atmosphere as trees, shrubs, and other wild plants; but this is not
sufficient for the purposes of agriculture" (Browne, 1943).

In subsequent editions, the last portion of Liebig's statement was changed
to read "...and this is quite sufficient for the purposes of agriculture a
change that was to have a profound impact (Browne, 1943). What caused
Liebig to alter this most important sentence is not known. Liebig was
intrigued by the problem facing the colonists in Virginia, who, after a
century of wheat and tobacco production without the addition of manure,
were now abandoning their lands. He ascribed this development to the
century-long drain on the mineral elements in the soil, not to insufficient
N or a reduction in soil humus. This conclusion was consistent with
Liebig's concept of an atmospheric source of plant N.

Liebig did recognize, however, that animal manures were important sources
of N (Bradfield, 1961). He had experimented with cow manure (low in N) and
human urine (high in N) and reported more favorable gluten production from
wheat grown from the latter amendment. Three pages later in his monograph,
Liebig notes the- following, which probably could be found among the pages
of many present-day organic magazines: "It should be the care of the
agriculturalist so to employ all the substances containing a large
proportion of nitrogen, which his farm affords in the form of animal
excrements, that they should serve as nutriment to his own plants"
(Bradfield, 1942).

Liebig's theories on the atmospheric source of ammonia-nitrogen for plant
growth led John Lawes in 1843 to establish the now infamous Broadbalk Field
wheat experiment at Rothamsted (Russell, 1926). These experiments showed
the value of phosphates and alkali salts, which Liebig had emphasized, but
they also vividly showed Liebig's mistake in relying solely on an
atmospheric source of N. The initial studies led Lawes to formulate a
"patent manure" consisting of a mixture of super phosphate, phosphate of
ammonia, and silicate of potash (Russell, 1926). A similar artificial
manure was patented by Liebig (1845), but since it was based on the ash
composition of plants, its use was shoo-lived.

Besides these observations and opinions on the current controversies of his
day, Liebig also noted the relationship of plant ashes to crop requirements
and that water extracts of humus yielded little or no residue upon
evaporation (Howard, 1940). He espoused the concept that ash analysis would
foretell which salt would need to be applied to obtain a full crop.
Acceptance of this concept led to the downfall of the humus theory, since
the 2 concepts could not coexist. However, the ash concept often is cited
as one of Liebig's false conclusions in relation to the value of organic
manures.
Conclusions

The evolution of plant nutrition theory has been outlined from the
fundamental observations of ancient writers, who believed that manures
produced better crops, to the time of Justus von Liebig, when visual
observations were merged with the chemistry of the day. The mysterious heat
given off from the composting of manures and the resulting brownish-black
liquor provoked the curiosity of many through the ages. If we ascribe this
progression in plant nutrition theory to the "scientific advancement"
achieved in all fields, one also realizes that this progression was not
independent of the society then in existence. Thus, Glauber's salt, one of
the early saltpeter mineral fertilizers, was not only the product of his
scientific endeavor but also a reaction to the devastation of the Thirty
Years War. Similarly, in Liebig's time, according to Sykes (1949) in
England, the establishment of artificial manures was not only due to
Liebig's theories but also helped "...by the decline in agricultural
technique; by the necessity to reduce agricultural costs; and by the
influences, everywhere in evidence, which were compelling the farmer to get
more out of the land, to put less into it, and to diminish costs at every
turn."

Nor does the progression of a theory necessarily produce a perfect
correlation with truth. The abstract notion that plants contained a "force"
or "creative power" to transmute substances was finally toppled by Liebig.
However, although Liebig knew of the benefits of manures from a N
standpoint, he opted, erroneously, for a purely atmospheric origin for
plant N. The fact that Liebig was a "compiler and summarizer" of the
importance of mineral nutrients placed his mineral element theory on an
unsound foundation (Marschner, 1986). However, this became the primary
reason for the work of Lawes and many other scientists to follow.

Twenty years after the publication of Liebig's monograph, the science of
microbiology began to flourish (Waksman, 1938); after another 20 years,
Darwin published his study on the effects of earthworms (Darwin, 1976).
Studies in soils and plant nutrition would no longer be considered in the
context of an abiotic system. The dynamic nature of soil and the role of
humus as a microbial media were not and could not be envisioned in Liebig's
day.
Epilogue

Given the state of knowledge during Liebig's career, one would be
hard-pressed to accept his summations as the harbinger of doom for the
world's production of food and fiber via the route of the chemical
fertilizer industry. Current plant nutrition theory recognizes the ionic
uptake of nutrients from the soil through the root. Selectivity exists and
varies between plant species as to which elements are taken up and to what
extent. However, this occurs independent of the source of the nutrients,
either synthetic or organic fertilizers.

What then is the controversy between synthetic chemical fertilizer and
organic fertilizer users? Ultimately, if the plant uses N from dried blood
or ammonium nitrate in a similar fashion, why should one be a better
approach? The arguments, pro or con, run the gamut from a concept of a
"living earth" to economics and human health. Many of the contentions
between the organic and chemical fertilizer proponents are not germane to
plant nutrition per se. However, there are several points that have direct
bearing on plant nutrition. Included among these are nutrient concentration
and release rate, a dead vs. a living system, and economics. Numerous
citations on the abuses and the benefits (albeit short term) of chemical
fertilizers can be made. Research on slow-release fertilizers, fertilizer
placement, split applications, and the like are examples of a desire to
maximize nutrient uptake with a concurrent increase in yield. However,
concepts such as using fertilizer as .cheap crop insurance" on high-value
crops are abusive.

According to Balfour (1947), Liebig's theory of mineral plant foods was a
"rather naive theory" since it considered only mineral salts. As noted
earlier, a concept of a dynamic, living soil was beyond the scope of Liebig
or any of his contemporaries. It is, perhaps, more unfortunate that, even
today, this view is still held by many.

The driving force behind the acceptance or rejection of the mode of
providing nutrients to the plant was, as with the advent of Glauber's salt,
and is economics. Turning the economic wheel in either direction may not be
the role of those wearing the Gap of plant nutritionist, but we can hope
future roads may be paved that lead to an enhanced appreciation of the
dynamics underlying the nutrition of plants.

Literature Cited

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Marschner, H, 1986. Mineral Nutrition of Higher Plants. Academic, New York.

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