Humates in Hydroponics

Commercial Humates are sourced from peat and coal formed over thousands or millions of years. They are formed during the coalification process from the degradation of organic material by microbial, chemical and geological action. Put simply, humates are various organic molecules of ancient compost.

‘Fulvic acid’ (FA) is the most important and active humate extract where hydroponics is concerned. It is water soluble and is chemically active and readily available for uptake by the plant.

Fulvic acid increases the absorption capacity of plant roots, aids the cell building process and enhances the passage of poorly transported ions into and throughout the plant’s cells by acting as an efficient organic chelator/complexing agent in hydroponic solutions.

Fulvic Acid Benefits

Enhances cell growth

Increases nutrient uptake

Increases nutrient transportation

Increases silica absorption

Stimulates plant immune system

Stimulates cell division

Enhances the permeability of cell membranes

Fulvic Acid and Chelation

Micronutrients are crucial partners (cofactors) of enzymes in every metabolic function of the plant. Respiration, photosynthesis, protein synthesis, energy transfer, cell division and cell elongation are all dependent on an adequate supply of calcium, iron, copper, manganese, magnesium, zinc and other micronutrients.

Many microelements in their basic form are unavailable to the plant. This is largely due to the fact that metal microelements such as iron, copper and zinc are positively charged (cations) while the pores (openings) on the plants' leaves and roots are negatively charged. As a result the positive charges on microelements are repelled by the negative charged plant pores. As a result, there is a problem with the fixation of positively charged minerals at the negatively charged pores (the element can't enter the plant due to the difference in charges).

Another reason that some microelements require chelating is due to their stability in solution. For instance, iron (Fe) is a reactive metal. In concentrated solution it will react with other fertilizer elements, particularly phosphorus (P) to form an insoluble compound. It forms the compound iron(III) phosphate which is a solid precipitate in water, so it falls out of solution. To prevent this happening Fe used in fertilizers is usually provided in the chelated form.

Well-formulated hydroponic nutrients ensure there is a high level of nutrient availability in the correct forms and ratios.  Nutrition that offers a diverse range of highly bioavailable elements will prove more effective than nutrition that has less diversity, particularly where the micronutrients are concerned. For this reason combinations of organic and synthetic chelates in hydroponic formulations are demonstrated to benefit yields.

Studies show that fulvic acid provides for excellent translocation of microelements, such as iron, throughout the plant. When added to an iron chelate in one study it stimulated more growth and better utilization of the iron than with the synthetically chelated iron (Fe EDTA) alone (Chen and Stevenson (1986) Soil organic matter interactions with trace elements).

FA use under conditions where adequate mineral nutrition exists consistently shows stimulation of plant growth when added to hydroponic nutrient solutions. Similar plant growth enhancements have been observed when FAs are applied to the foliage of plants grown in complete nutrient solutions. The degree of stimulation varies depending on the concentration of fulvic acid and on the quality/source of the fulvic acid (Plant Growth Stimulation by Fulvic Acids. K Day et al)

The common types of chelates used in hydroponic nutrients are the synthetic chelates, EDTA (ethylenediaminetetraacetic acid) and to a lesser extent DTPA (Diethylene triamine pentaacetic acid). Chelates such as EDTA and DTPA have a high affinity for e.g. iron and generally form stable complexes with the metal across a pH range from 4 to 7.

Chelates have several points of attachment with which they "grasp" the trace element. EDTA has four connecting points while DTPA has five. Higher numbers of connection points isn’t always an advantage. In some cases the four connection points may hold the element too tightly, while in different situations these may not hold it tight enough. For this reason, various chelates will prove more effective than others based on the ion that is chelated and the conditions in which the chelate is present. 

For instance, the effectiveness of a chelating agent can depend on pH. In the case of iron Fe EDTA is best suited to slightly lower than neutral pH levels while Fe DTPA is most effective at higher pH values. DTPA is more costly than EDTA and less soluble and is usually found in higher quality fertilizers. DTPA is stable up to a pH of 7.5 while EDTA is stable up to a pH of approximately 6.5.

The most effective of the synthetic chelating agents is ethylenediaminedihydroxy-phenylaceticacid (EDDHA). However, it is important to note that EDDHA can only chelate with iron and not with other essential microelements such as Cu, Zn, Mn. Fe EDDHA is the most stable of all the commonly available iron chelates. This synthetic chelate is held in a bond up to 100 times tighter than DTPA because it has six molecular bonds rather than five bonds. Typically EDDHA is only found in premium fertilizers because of its higher cost. EDDHA is stable up to pH 9.0 (pH range = 4- 9) but is not suitable for foliar applications due to EDDHA only being absorbed through the roots of the plant.

As with the synthetic chelators, fulvic acid (FA) enhances the uptake of micronutrients due to its properties as an organic chelator.

Fulvic acid forms four-point bonds with the elements it chelates and can be absorbed into the plant. This adds to the mobility of nutrients. The nutrients chelated by fulvic acid can move more freely which prevents a number conditions like localized calcium deficiency that can occur due to low mobility of nutrients.

FA is a short chain molecule, which has a low molecular weight and soluble in both acid and alkali solutions/soils.

FA is effective when the growing environment in the root zone is above or below optimal levels. For this reason fulvic acid, more so than the synthetic chelators (EDTA, DTPA), retains its effectiveness under a range of pH conditions.

Chelated Minerals versus Complexed Minerals

Some nutrient elements only have the ability to be partially surrounded by a chelator/chelating agent and are referred to as a "complexes", while those that are capable of being completely surrounded are termed chelates.

Confusion and often contradictory information exists surrounding chelated and complexed minerals. Terms such as amino acid complexes, amino acid chelates, polysaccharide complexes, lignosulfonate complexes, and amino proteinates abound. In some cases chelates are referred to as “complexed” or “chelate complexes” while in other cases fertilizer producers wrongly advertise complexed minerals as “chelated”. This only serves to confuse further. However, the difference between “chelated” and “complexed” can be understood via some basic principles.

ALL CHELATES ARE COMPLEXES - NOT ALL COMPLEXES ARE CHELATES

In order for a compound to be called a true chelating agent, it must have certain chemical characteristics. This chelating compound must consist of at least two sites capable of donating electrons (coordinate covalent bond) to the metal it chelates. For true chelation to occur the donating atom(s) must also be in a position within the chelating molecule so that a formation of a ring with the metal ion can occur.

The term “complexed” originates from combinations of minerals and organic compounds that do not meet the guidelines of a true chelate.

The key difference between a chelated mineral and complexed mineral is that chelates are relatively more stable under adverse conditions while complexes are less thermostable and release the atom quickly under adverse conditions.

Not all nutrients can be chelated. The positively charged cations iron, zinc, copper, cobalt, nickel, manganese, calcium , magnesium, and potassium can be chelated while the negatively charged anions such as  phosphorous cannot.

While the negatively charged anions cannot be chelated they can be complexed via the use of donor atoms such as e.g. oxygen, nitrogen or sulphur. Amino acids such as glycine and/or lysergine are often used to complex the anions. Similarly, FA can be used to complex the negatively charged anions.

Both chelated and complexed minerals are more bioavailable than non-chelated and non-complexed minerals. This makes the use of additional organic (e.g. amino acid, fulvic acid, organic acids) and inorganic chelators/complexers highly beneficial in hydroponic formulations.

Humic Science

Definitions

A polymer is a large molecule (macromolecule) composed of repeating structural units (monomers). These units are typically connected by covalent chemical bonds. Although the term polymer is sometimes taken to refer to plastics, it encompasses a large range of natural and synthetic materials with a wide variety of properties.

Because of the extraordinary range of properties of polymers, they play an essential and diverse role in everyday life.This role ranges from familiar synthetic plastics to natural biopolymers such as nucleic acids and proteins that are essential for life.

Aliphatic Compound

Relating to organic compounds where carbon atoms are linked in open chains, either straight or branched, rather than containing an aromatic ring. Alkanes, alkenes, and alkynes are all aliphatic compounds.

Aromatic Compound

Although the term aromatic originally concerned odour, today its use in organic chemistry is restricted to compounds that have carbon rings with particular electronic, structural, or chemical properties. Aromaticity results from particular bonding arrangements that cause certain electrons within a molecule to be strongly held.

Humic Substances

Humic substances (humic matter) are a complex group of naturally occurring heterogeneous (diverse) carbon based molecules produced from the degradation and decomposition (chemical & biological) of plant and animal organic matter.

This decomposition process that results in the formation of humic substances is known as humification. They are dynamic molecules forming polymers and constantly involved in biochemical reactions changing in physical, chemical and biological function. Because they undergo polymerization they form polymers (made of monomers or single sub units) of different lengths and combined with varying elemental compositions’ molecular weight varies greatly. Humic substances rapidly rearrange their molecular structure as the surrounding conditions change (Tombacz & Rice, 1999).

Humic substances possess both aromatic and aliphatic structural characteristics. The dominant functional groups are phenolic & carboxyl groups that contribute to the surface charge and reactivity of humic substances (Stevenson, 1994).

The first recognized study of humic substances was the work done by Achard (Achard, 1786), who extracted a dark, amorphous precipitate through a method of alkali extraction (solubilisation) and acidification. This work lead to the observation that more humic material could be extracted from the lower more decomposed (humified) layers of peat. Importantly it showed that humic matter was formed by the decomposition of organic material.

Decomposing organic material can be defined into two distinct groups. (1) Organic matter which is at various stages of decomposition where the morphology of starting organic material is still distinguishable, and; (2) the organic material has completely decomposed to the stage where none of the structure of the material which they formed from is visible. These two groups along with the fraction of non-decomposed dead and living organic material defines what is termed soil organic matter (SOM) or total organic matter.   This second group of completely decomposed organic material is referred to as humus. The name “humus” was introduced by De Saussure (De Saussure, 1804) to describe the organic dark coloured material found in soil.

Humis is the most important fraction found in soils and has been studied considerably. The interest is because of its pronounced effect on the physical, chemical and biological conditions of the soil (Russell & Russell 1950; Tan, 2000).

Humic matter and humus has become a very confusing subject because many scientists use terms like humic material, soil organic matter (SOM), humic substances and the like interchangeably. Current standards has humus defined into a humified and non-humified fraction (Stevenson, 1994). The non-humified fraction is made of many organic substances with definite chemical characteristics. The most common of these compounds are lignin, proteins (amino acids), carbohydrates, polysaccharides, waxes, melanin, cutin, nucleic acids, lipids and all of the biochemical compounds that have been synthesized by plants and soil microorganisms. These organic compounds are the sources for synthesis of humic substances (humified fraction) which are formed from further degradation and decomposition reactions in the process of humification.

It is important to note that soil organic matter (terrestrial) is not the only source of humic substances. Interest in humic substances has also lead to vast amounts of research on aquatic humic substances found in the waters of streams, oceans, lakes and their sediments. With humic matter making up a large proportion of deposits of peat, brown coal (lignite, leonardite), coal and shale. It is believed to be the most widely distributed source of organic carbon material on the surface of the earth.

As stated, humic substances are varied in molecular weight and elemental composition, dependent on the degree of polymerization, the starting organic material, environmental conditions and the chemical and biological processes of formation. The molecular weight is generally measured in Daltons (Da) which is an accepted alternative to the atomic mass unit. The unit of Daltons is mainly used in the life sciences as a measurement of molecular weight of polymers like proteins. Molecular weights of naturally occurring elements ranges from 1-238 g/mol, simple chemical compounds 10-1,000 g/mol and for polymers, proteins, DNA fragments 1,000-5,000,000 g/mol (1 Da = 1 g/mol). Although humic substances do form polymers they are reportedly held together by weak bonding. Humic material is a supramolecular structure of relatively small bio-organic molecules (having molecular mass <1000 Da). They are self-assembled mainly by weak dispersive forces such as Van der Waals force, π-π, and CH-π bonds into only apparently large molecular sizes (Piccolo, 2002).