Colloidal humus

The most important chemical characteristics of a soil are (l) its content of essential nutrients and their availability to plants (2) the exchange capacity (3) the buffering capacity (4) acidity or alkalinity and (5) content of inorganic and organic colloids (humus). Perhaps the state of oxidation or reduction of the soil should be mentioned this is ordinarily not of major importance but may be if a waterlogged or poorly-drained soil is under consideration. 

CHAMINADE R. and BLANCHET R. 1951. (Action of colloidal humus on the nutrition of plants). Comptes Rendus hebdomadaire de seances de I Academie des sciences, 233, 1486-1488. 

Products of this type seem to protect the humus from rapid incorporation into new biological processes. Additional factors that appear to be associated with the accumulation of organic matter in Mollisols are high exchange capacities, saturation with calcium, an abundance of mineral colloids and a high content of minerals of the smectite group (Fenton, 1983). 

As pointed out by Sposito (1984) this equation initiated the surface chemistry of naturally occurring solids. Maarten van Bemmelen published this equation (now referred to as the Freundlich isotherm) more than 100 years ago and distilled from his results, that the adsorptive power of ordinary soils depends on the colloidal silicates, humus, silica, and iron oxides they contain. 

Humus in Soils. Humus in solid form (colloids, or coatings on mineral surfaces) can immobilize many pollutants. 

Phytoplankton, biological debris, humus colloids (colloidal humic acid), fibrils ... 

Discussion. The colloidal clay and humus soil fractions are negatively charged and therefore attract and adsorb positive ions (cations) on to exchange sites. These may be the so-called basic cations defined above, or the acidic cations H+ and Al +. These cations are not soluble in water when in the adsorbed state, but can exchange with H+ which is present in the acidic vicinity of the plant root system. They are now in solution and able to be absorbed into the plant. The extent to which the exchange sites are saturated with cations, together with the ratios of the cations to each other, indicates the nutrient supplying power of the soil. 

Albrecht campaigned against the concept of an acid soil causing poorer crop growth rather, it is the calcium deficiency that needs to be remedied. The acid soil solution dissolves rock particles, such as rock phosphate and limestone, to release beneficial nutrients such as phosphate and calcium respectively. It also mobilizes the other adsorbed ions off the clay-humus colloid. He estimated the optimum ratios of calcium to magnesium and calcium to potassium. These were approximately from 4 1 to 7.5 1, and from 15 1 to 38 1 respectively. The higher the Ca K ratio, the more proteinaceous... 

Ftihr, F., and Sauerbeck, D. (1967a). The uptake of colloidal organic substances by plant roots as shown by experiments with 14C-labelled humus compounds. In Report FAO/IAEA Meeting, Vienna, Pergamon, Oxford, pp. 73-82. 

Wershaw, R. L., Llaguno, E. C., and Leenheer, J. A. (1996b). Mechanism of formation of humus coatings on mineral surfaces. 3. Composition of adsorbed organic acids from compost leachate on alumina by solid-state l3C NMR. Colloid Surface A 108, 213-223. 

Animals. 93% is eliminated within 72 hr, predominantly in the urine Plants. Rapidly metabolized. Metabolites do not accumulate Soil. Strongly adsorbed by soil colloids and humus with very little migration or leaching. DTJ0 c. 3-6 months... 

Humic substances occurring in natural waters in the form of stable negatively charged sols exhibit shielding aetion with respect to colloidal solutions of Si02, Fe(OH)j and Al(OH)3 (Kul skiy, 1960). Colloids shielded by humus do not coagulate and, remaining in the state of sols, can be transported by the waters of rivers, seas, and oceans (Kuznetsov, 1964) for considerable distances, and the Schultz-Hardy rule becomes inapplieable to shielded colloids. 

Equation 57 is typically observed for partitioning processes, for example, for the (ab)sorption of nonpolar oiganic substances in a solid matrix containing organic (humus-like) material. Equation 57 corresponds also to the initial linear portion of a Langmuir isotherm or to a Freundlich equation S = KpC, where b = 1. Note that colloid-facilitated transport is excluded in this treatment. 

The exact nature of the organic matter is ill-defined but it is known to behave as a colloid and is often complexed with silica and heavy metal atoms such as iron, aluminium, and manganese. The weak carboxylic acid functionality of humus organics means that much of... 

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