Chemical characteristics of humic substances

Humic Acid Chemical characteristics

 humic acid Chemical characteristics , having a variety of components including quinone, phenol, catechol and sugar moieties.

A typical humic substance is a mixture of many molecules, some of which are based on a motif of aromatic nuclei with phenolic and carboxylic substituents, linked together, the illustration shows a typical structure. The functional groups that contribute most to surface charge and reactivity of humic substances are phenolic and carboxylic groups.

Humic acids behave as mixtures of dibasic acids, with a pK1 value around 4 for protonation of carboxyl groups and around 8 for protonation of phenolate groups.

There is considerable overall similarities among individual humic acids. For this reason, measured pK values for a given sample are average values relating to the constituent species. The other important characteristic is charge density. The molecules may form a supramolecular structure held together by non-covalent forces, such as Van der Waals force, π-π, and CH-π bonds.

The presence of carboxylate and phenolate groups gives the humic acids the ability to form complexes with ions such as Mg2+, Ca2+, Fe2+ and Fe3+. Many humic acids have two or more of these groups arranged so as to enable the formation of chelate complexes.The formation of (chelate) complexes is an important aspect of the biological role of humic acids in regulating bioavailability of metal ions.

Two properties of humic acid that may have some benefit in turfgrass culture are its cation exchange capacity to form chelates with the metallic micronutrients, iron, copper, zinc and manganese. The cation exchange capacity (CEC) of commercially produced humic acid is in the range of 500 to 600 milliequivalents (me) per 100 grams. This is about five times greater than the CEC of good quality peat moss and twice as high as the CEC of soil humus.

To gain some perspective on the possibility of effectively making use of the high CEC of humic acid, we can examine the recommendations of one manufacturer that call for the addition of 2 lbs per cubic yard of 80:20 sand-peat rootzone mix or substitution of 3 lbs humate for the peat moss. By my calculations, assuming the pH of the rootzone mix and sand are near 7.0, 2 lbs of humate would contribute about 0.37 me CEC/100 g of the 80:20 mix. This would be in addition to the approximately 2.9 me of CEC provided by the peat moss. That turns out to be a rather expensive 13 percent increase in the CEC of the rootzone mix. When you substituted for the peat moss, you wind up with a rootzone mix with a CEC of about 0.55 me/100 g. Consider the fact the potassium leaches readily from sand-peat mixes with five times more CEC than in the sand-humate combination, this doesn抰 seem like a wise substitution.