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The Role of
Humic Acids
Humic acids
are colloids and behave somewhat like clays,
even though the nomenclature suggests that
they are acids and form true salts. When the
cation exchange sites on the humic molecule
are filled predominantly with hydrogen ions,
the material is considered
to be an acid and is named accordingly.
However, it has no great effect on pH
because the acid is insoluble in water. When
the predominant cation on the exchange sites
is other then hydrogen, the material is
called humate. The humates of monovalent
alkali metals are soluble in water, but the
humates of multivalent metals are insoluble.
Apart from their effect on the solubility of
the materials and their absorption by clays,
the different cations have little effect on
the humic molecules.
The manifold
effect of humic substances on the plant,
shown both in the external medium and in the
biochemical processes occurring in the
plant, has been well demonstrated.
There is a
growing interest in the use of organic
materials as fertilizers or soil amendments.
This may be attributed to: 1) an interest in
the reduction of the use of chemical
fertilizers; 2) public concern for the
potential polluting effects of chemicals in
the environment; and 3) a pressing need for
energy conservation. The research reported
herein was conducted in an effort to explore
humate material as one of the organic
natural resources with the potential for
meeting some of these needs.
Reference: Senn, T. L.
and Alta R. Kingman, 1973, A review of Humus
and Humic Acids. Research Series No. 145, S.
C. Agricultural Experiment Station, Clemson,
South Carolina.
This study
showed significant increases in yield on
potatoes, soybeans and algae cultures on
test plots near Grand Forks, North Dakota.
Other tests in North Dakota documented
yield increases in barley with or
without applied mineral fertilizer (Agvise
Inc., 1977-1979). Tests done by commercial
farms have consistently resulted in a
significant reduction in irrigation water
usage on plots treated with humates, as well
as better seed germination, leaf petiole
growth and crop yields.
Interest in
humates and their value in agriculture is
increasing rapidly. There are numerous
universities and commercial concerns engaged
in testing / documenting the benefits of
humates used in fertilizers, animal feed and
environmental remediation.
Humates
and Humic Acid
Humate materials: their effects and
use as soil amendments
By T.A. Obreza, R. G. Webb and R. H.
Biggs
Humate materials are widely
distributed organic carbon
containing compounds found in soils,
fresh water, and oceans. These
substances are formed from the
biological and chemical breakdown of
animal and plant life, and make up
approximately 75 percent of the
organic matter that exists in most
mineral soils. Humates play a direct
role in determining the production
potential of a soil.
The importance of organic matter in soil is
not a recent discovery. Soil fertility in
early agricultural systems was based on the
recycling of organic wastes, and the
addition of decomposed organic materials
improved plant growth. The rise in
popularity and use of mineral fertilizers
enabled growers to directly supply plant
nutrients to the soil, and rapid growth in
agricultural productivity occurred. As a
consequence, the importance of soil organic
matter was somewhat neglected. In Florida,
organic matter should be considered as very
important due to the sandy nature of the
soil. In soils void of significant
quantities of clay minerals and organic
matter, the addition of humates can have an
impact on soil fertility which may be
noticeable in the form of improved plant
growth.
Effects on Soil Fertility. Native soil humic
substances enhance plant growth both
directly and indirectly. Physically, they
promote good soil structure and increase the
water holding capacity of the soil.
Biologically, they affect the activities of
microorganisms. Chemically, they serve as an
adsorption and retention complex for
inorganic plant nutrients. Nutritionally,
they are sources of nitrogen, phosphorus,
and sulfur for plants and microorganisms.
All of these effects increase the
productivity of the soil.
Commercially-available humic substances
added to the soil do not directly contribute
significant quantities of nutrients to
plants in modern agriculture at the rates
normally applied. However, indirect effects
of these materials on soil fertility can be
significant. Micronutrients, especially
iron, may be made more available to plants
in the presence of humates. Inorganic iron
compounds are very unstable in soil and tend
to become insoluble and unavailable,
especially in calcareous soils. Humate
compounds can incorporate iron into chelated
complexes, maintaining its availability to
plants, although still in insoluble form.
Soil phosphates are often immobilized
through reactions with iron and aluminum,
which in turn may be complexed with organic
matter. Chelating agents can break the iron
or aluminum bonds between the phosphate and
organic matter, releasing phosphate ions
into solution. This dissolution is a process
which occurs in soil in the presence of
naturally-occurring humic substances or
plant root exudates. The addition of humates
may increase the rate of this process,
thereby increasing the availability of
phosphorus to plants.
Applied pesticides substantially interact
with soil humic substances, but the
reactions are complex. Some pesticides may
be immobilized by humates and can
practically disappear from the soil
environment. In this case, humic substances
can be very effective in removing excess
pesticide from sandy soils very low in
organic matter. The most common reaction
between pesticides and humates is
adsorption, followed by a release to the
soil solution at a rate dependent on the
chemical structure of the pesticide.
Degradation of the pesticide will be
determined in part by the rate of release.
Humic substances may be used in this case to
control the concentration of pesticide in
the soil solution, and to avoid toxicity
hazards. A third case involves the mobility
of pesticides by humic material. Some groups
of compounds can form complexes with
humates, which can then be absorbed by plant
roots.
Effects on Plants. Humic acids can have a
direct positive effect on plant growth in a
number of ways. They have been shown to
stimulate seed germination of several
varieties of crops. Both plant root and top
growth have been stimulated by humates, but
the effect is usually more prominent in the
roots. A proliferation in root growth,
resulting in an increased efficiency of the
root system, is a likely cause of higher
plant yields seen in response to humic acid
treatment.
Humic matter has been shown to increase the
uptake of nitrogen by plants, and to
increase soil nitrogen utilization
efficiency. It can also enhance the uptake
of potassium, calcium, magnesium and
phosphorus. Chlorosis in plants has been
prevented or corrected by humate
application, probably the result of the
ability of humate to hold soil iron in a
form which can be assimilated. This
phenomenon can be particularly effective in
alkaline, calcareous soils, which are
normally deficient in available iron and low
in organic matter content.
Effect of Management Practices on Soil
Organic Matter. Cultivation of soils usually
causes a decrease in the organic matter
content. Rather than being completely
destroyed, the organic matter in the soil
tends to reach a new, lower equilibrium
level. For most soils, a high level of
organic matter is maintained only by grass
species. Grass middles between citrus tree
rows can help maintain higher organic matter
in the portion of the citrus tree root zone
that extends into them. However, the
establishment of clean herbicide bands
within three rows to facilitate harvesting
and other operations may decrease the
organic matter content in what is normally
the major area of tree root concentration
and fertilizer application.
Conventional sources of applied organic
matter such as farm manures or crop residues
are not normally used in a citrus grove
situation due to lack of availability or
prohibitive cost. The leaf and dead wood
litter that is generated is not sufficient
to maintain an organic matter content under
the trees which is comparable to that under
grass middles. Efforts to increase citrus
grove soil organic matter content have been
made by growing cover crops using species of
Crotalaria or hairy indigo, but
success was poor because the crops could not
be sufficiently incorporated into the soil
without damaging the tree root system.
Non-conventional Sources of Organic Matter:
Humic substances. Humate products for
agricultural use are produced through
mineral sand mining and recovery operations.
The end product contains a majority of
organic material (concentrated humic acid)
mixed with smaller amounts of mineral
matter. It can be applied to soil to improve
its fertility, especially in the zone of
highest root activity. Humate concentrates
provide many of the advantages of
conventional organic matter sources over a
long period with less handling problems,
especially in situations where there is no
feasible alternative to purchasing
additional supplies of humus. They have been
demonstrated to have favorable effects on
tissue nutrient balance, fertilizer uptake,
top and root growth, and crop yield and
quality for a large variety of field,
horticultural and ornamental plants. They
have been most effective in soils with less
than two percent organic matter.
The plant characteristic that the addition
of humic substances has consistently
enhanced more than any other is root length,
especially on sandy soils. A preliminary
study with the citrus trees potted in sand
showed that after a period of one year, the
root dry weight was increased when a humic
acid material was added at the rate of one
lb. per cubic yard of soil as compared to an
untreated treatment. Tree top growth, vigor,
and trunk cross-sectional area also
increased in response to humate addition.
A field study with young citrus trees is
currently underway to determine if the
addition of humic acid can increase fruit
yield. In this trial, the trunk
cross-sectional area increase of
newly-planted trees was greater for the
first year of growth where 0.5-1.0 lb. of
humate material per tree was applied at
planting. These data are not conclusive, as
much more research is needed to determine
the long-term effects of humic acid addition
to citrus trees, especially as they come
into bearing.
REPRINTED FROM THE CITRUS INDUSTRY - OCTOBER
1989
The authors are Assistant Professor, (Soil
Scientist), Southwest Florida Research and
Education Center, Immokalee; former Research
Scientist, and Professor, Fruit Crops Dept.,
Univ. of Florida Gainesville, respectively.
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