The urgency to emphasize the importance of humic substances and their value as fertilizer ingredients has never been more important than it is today. All those concerned about the ability of soils to support plant growth need to assist in educating the public. Humic substances are recognized by most soil scientists and agronomists as the most important component of a healthy fertile soil.
To illustrate how humic substances and hormones function, the following summary, based on published scientific data, has been prepared as a guide for an educational program. In addition, by understanding how these carbon containing substances function, professionals will have a solid foundation on which to design environmentally acceptable sustainable agriculture programs.
HUMIC SUBSTANCES AND THEIR INFLUENCE ON SOIL FERTILITY
Humic substances are a good source of energy for beneficial soil organisms. Humic substances and non humic (organic) compounds provide the energy and many of the mineral requirements for soil microorganisms and soil animals. Beneficial soil organisms lack the photosynthetic apparatus to capture energy from the sun thus must survive on residual carbon containing substances on or in the soil.
Energy stored within the carbon bonds functions to provide energy for various metabolic reactions within these organisms. Beneficial soil organisms (algae, yeasts, bacteria, fungi nematodes, mycorrhizae, and small animals) perform many beneficial functions which influence soil fertility and plant health. For example the bacteria release organic acids which aid in the solubility and bioavailability of mineral elements bound in soil. Bacteria also release complex polysaccharides (sugar based compounds) that help create soil crumbs (aggregates). Soil crumbs give soil a desirable structure. Other beneficial soil microorganisms such as the Actinomyces release antibiotics into the soil. These antibiotics are taken up by the plant to protect it against pests. Antibiotics also function to create desirable ecological balances of soil organisms on the root surface (rhizoplane) and in soil near the roots (rhizosphere). Fungi also perform many beneficial functions in soils. For example, mycorrhizae aid plant roots in the uptake of water and trace elements. Other fungi decompose crop residues and vegetative matter releasing bound nutrients for other organisms. Many of the organic compounds released by fungi aid in forming humus and soil crumbs. Beneficial soil animals create tunnel like channels in the soil. These channels allow the soil to breath, and exchange gases with the atmosphere. Soil animals also aid in the formation of humus, and help balance the concentration of soil microorganisms. A healthy fertile soil must contain sufficient carbon containing compounds to sustain the billions of microscopic life forms required for a fertile soil and a healthy plant. A living soil is a fertile healthy soil.
A study on the effects of humic acid on plant growth was conducted at Ohio State University which said in part “humic acids increased plant growth” and that there were “relatively large responses at low application rates”.
Plant growth is influenced indirectly and directly by humic substances. Positive correlations between the humus content of the soil, plant yields and product quality have been published in many different scientific journals. Indirect effects, previously discussed, are those factors which provide energy for the beneficial organisms within the soil, influence the soil’s water holding capacity, influence the soil’s structure, release of plant nutrients from soft minerals, increased availability of trace minerals, and in general improved soil fertility. Direct effects include those changes in plant metabolism that occur following the uptake of organic macromolecules, such as humic acids, fulvic acids. Once these compounds enter plant cells several biochemical changes occur in membranes and various cytoplasmic components of plant cells.
Uptake of major plant nutrients is mediated by humic substances. One stimulative effect of humic substances on plant growth is enhanced uptake of major plant nutrients: nitrogen (N) phosphorus (P), and potassium (K). When adequate humic substances are present within the soil the requirement for N P K fertilizer applications is reduced. As the level of humic substances in soils become depleted the misleading demand for higher concentrations of N P K results. Many growers have over the past several years reported increasing demands for soluble acid fertilizers In order to maintain crop yields. Such observations indicate something is wrong within the soil. Increased leaching of nitrate fertilizer ingredients into the ground water is also a warning of problems to come. Then trends reflect losses in soil humic substances. Growers could reduce their fertilizer requirements and retain the fertilizer ingredients within the plants rooting zone by the application of humate based fertilizers. The application of either dry or liquid humic substances to soils dramatically increases fertilizer efficiency. Other researchers have reported increased uptake of calcium (Ca), and magnesium (Mg) when plants are irrigated with liquid suspensions of humic acids (HAs) or fulvic acids (FAs). Another key mechanism, which maximizes fertilizer efficiency and relates to a function of humic substances, is a reduction in the toxicity and leaching of nitrogen compounds into subsoil water. Humic substances hold these major plant nutrients in a molecular form which reduces their solubility in water. These binding processes reduce leaching nitrogen into the subsoil and help prevent volatilization into the atmosphere.
The absorption of humic substances into seeds has a positive influence on seed germination and seedling development. The application of humic (HA) or fulvic acids (FA) to seeds will increase the seed germination; resulting in higher seed germination rates. Application rates of humic acids (HAs) or fulvic acids (FAs), required for improved seed germination, range from 20 to 100 mg/liter of seed. In order for improved germination to occur the humic substances must be present within the cells of seeds. As the humic substance enter the seed cells, respiration rate increases, and cell division processes are accelerated. These same respiratory processes enhance root meristem development and activate other growing points within the seedlings. Humic substances have been demonstrated to enhance mitotic activity during cell division under carefully controlled experiments. Placement of these humic substances on seeds (seed treatment) or within the seed furrow will significantly improve seed germination and seedling development. Excessive amounts of humic acids (HAs) and/or fulvic acids (FAs) can inhibit seed germination and at high concentrations can kill young seedlings. Therefore follow recommended rates when applying humic substances.
In recent years, humic substances have been shown to increase yields of corn and oats, tobacco roots, soybeans, peanuts, and clover; chicory plants, tropical crops and other crops. More recently, workers have reported increases in the growth of crops grown in planting media amended with humic acids that were extracted from vermicompost. These reports hypothesized that plant growth hormones may become adsorbed on to humic fractions so the plant growth response is a combined hormonal/humic one.
Humic substances have a very pronounced influence on the growth of plant roots. When humic acids (HAs) and/or fulvic acids (FAs) are applied to soil enhancement of root initiation and increased root growth are observed. Thus the common observation that humic acids (HAs) and fulvic adds (FAs) are root simulators. In most experimental studies plant root growth is stimulated to a greater extent compared to stimulation of above ground plant parts. Carefully designed experiments have been conducted under controlled conditions to measure plant response. For example, replicate treatments of plants grown within the greenhouse, with and without humic acid and fulvic acids has illustrated how humic substances influence root growth. In repeated experiments the treated root weights averaged from 20 to 50% heavier compared to the weights of non treated roots. The type of humic substance applied had a significant influence on the percent of increase. Not all humic substances contain a desirable molecular mixture of humins, humic acids (HAs) and fulvic acids (FAs) capable of rapidly stimulation root growth. Some humic substances, because of their large molecular sizes, failed to stimulate plant root development. Root stimulation occurs when the smaller molecular components within fulvic acid (FA) occur at a concentration which ranges from 10 to 100 mg/liter of solution. Growth is further stimulated when fulvic acids (FAs) are used in combination with humic acids (HAs) and other required plant nutrients. Humic substances improve plant nutrition, however they are not complete nutrients by themselves. Excessively high concentrations of humic substances can result
in a reduction in root weight. For optimum plant growth humic acids (HAs) and fulvic acids (FAs) should be applied at relatively low concentrations. Applications of humic substances within a fairly wide range of concentrations are highly beneficial to plant root development.
Humic acids (HAs) and fulvic acids (FAs) are excellent foliar fertilizer carriers and activators. Application of humic acids (HAs) of fulvic acids (FAs) in combination with trace elements and other plant nutrients, as foliar sprays, can improve the growth of plant foliage, roots, and fruits. By increasing plant growth processes within the leaves an increase in carbohydrates content of the leaves and stems occurs. These carbohydrates are then transported down the stems into the roots where they are in part released from the root to provide nutrients for various soil microorganisms on the rhizoplane and in the rhizosphere. The microorganisms then release acids and other organic compounds which increase the availability of plant nutrients. Other microorganisms release other “hormone like” compounds which are taken up by plant roots. The required concentration of humic acids (HAs) and/or fulvic acids (FAs) within the foliar spray should be relatively low, generally less than 50 mg of concentrated dry humic substance per liter of water. Foliar fertilizers containing humic acids (HAs) and fulvic acids (FAs) in combination with nitrogen, potassium, phosphorus and various trace minerals have been demonstrated to be from 100 to 500 % more efficient compared to applications of similar fertilizers to the soil. Foliar fertilizers are also more economical because smaller quantities of fertilizer are required to obtain significant plant response. Plant nutrients within foliar fertilizers are rapidly absorbed by the plant leaves. Within 8 hours after applications of humic substances are applied changes in many different metabolic processes are detected. Enhanced carbohydrate production can be detected within 24 to 48 hours after foliar feeding by use of a refractometer. Enhanced carbohydrate production can result in improved product quality or increased yields.
Young plant roots, leaves, and growing plants are more responsive to applications of humic substances. Actively growing plant tissues are the most responsive to applications of humic substances. Younger tissues have active transport mechanisms that move the required nutrients to sites of metabolic activity. For example, foliar applications of humic substances to young actively growing leaves results in a greater increase in plant growth when compared to foliar applications to older plant leaves. Actively growing plant parts involved in cell divisions and other growth processes, readily integrate various trace minerals and growth regulating compounds into ongoing metabolic processes in contrast older plant parts in which metabolic processes have slowed are unable to efficiently utilize added humic substances and associated nutrients. The concentrations of dry humlc acids (HAs) within the spray solution should range from 5 to 100 mg per liter of water for optimum response. Difference in the active ingredients of a specific substance may require changing these concentrations. At higher concentrations, above 100 mg of dry humic acid (HA) per liter, plant, shoot, and even root growth way be Inhibited, depending on the activity of the substances under test. Plants respond more slowly to soil applications of humic substances because a large percentage of the humic substance is retained within the roots during plant growth.
In most plants less than 30% of the humic substances present within the roots are translocated up the stems into the plant leaves. Foliar applications of relatively small molecular units of humic substances containing trace minerals (on actively growing plants) can be timed to meet the needs of specific plant growth requirements. Applications can be timed to activate vegetative growth, flowering, fruit set, or filling and ripening of fruits.
Energy metabolism is accelerated and the chlorophyll content of plant leaves is enhanced by the presence of humic substances. When Humic acids (HAs) and fulvic acids (FAs) are applied to plant leaves the chlorophyll content of those leaves increases. As the chlorophyll concentration increases there is a correlated increase in the uptake of oxygen. Chlorophyll development within plant leaves is more pronounced when fulvic adds (FAs) are present in the foliar fertilizer. Organic acids [humic acids (HAs) and fulvic acids (FAs)] also increase the concentration of messenger ribonucleic acids (m RNA) In plant cells. Messenger RNA is essential for many biochemical processes within cells. Activation of several biochemical processes results in an increase in enzyme synthesis and an increase in the protein contents of the leaves. During these metabolic changes an increase in the concentration of several important enzymes is detected. Some of the enzymes which are reported to increase are catalase, peroxidases, diphenoloxidase, polyphenoloxidases, and invertase. These enzymes activate the formation of both carrier and structural proteins.