Humified organic matter

Oxides, non-crystalline minerals, and humified organic matter can also develop charges at their surfaces by reactions with the soil solution. In this case, the surface can have positive (CEC)... 

Particulate organic matter is transitoiy pool between fresh plant residues and humified organic matter (Gregorich and Janzen 1996). Often the term light fraction is used as a synonym for POM (von Lutzow et al. 2007). While POM and LF are similar, they are not equivalent fraction and have different C-, N-, O-alkyl contents (Gregorich et al. 2006). 

The Rothamsted Carbon Model (RothC) uses a five pool structure, decomposable plant material (DPM), resistant plant materials (RPM), microbial biomass, humified organic matter, and inert organic matter to assess carbon turnover (Coleman and Jenkinson 1996 Guo et al. 2007). The first four pools decompose by first-order kinetics. The decay rate constants are modified by temperature, soil moisture, and indirectly by clay content. RothC does not include a plant growth sub-module, and therefore NHC inputs must be known, estimated, or calculated by inverse modeling. Skjemstad et al. (2004) tested an approach for populating the different pools based on measured values. 

Mahieu N, OUt DC, Randall EW. 2000a. Accumulation of heterocyclic nitrogen in humified organic matter a N-15-NMR study of lowland rice soils. European Journal of Soil Science 51 379-389. 

Blanchet, R. M. (1958). The direct and indirect effect of humified, organic matter on the nutrition of vascular plants. Ann. Agron. 9,499-532. 

As described above, chemical, biochemical, and microbiological conditions in the rhizosphere differ widely from those of bulk soil this can lead to changes in the dynamics and structure of humified organic matter. On the other hand, little is known about the molecular structure and degree of aggregation of humic molecules in the rhizosphere. 

Fractions of higher molecular mass, which are mostly insoluble, can withhold large amounts of metals, especially in alkaline environments. Metals are thus subtracted from precipitation and subsequent crystallization, processes that would decrease their availability (Schwertmann, 1966), and a reserve of micro-nutrients is created which is in equilibrium with complexing molecules. On the other hand, under conditions of high metal concentrations, complexation by humified organic matter may limit the amount of metals in solution under these conditions, interchain bonds may form, with possible precipitation of humic molecules. This process can be important for toxic elements, the activity of which can thus be reduced to nontoxic levels (Gerke, 1992). 

Soluble humified organic matter of soil (Chen, 1996) may increase metal transport by diffusion to the roots (Pandeya et al., 1998) and favor micro-nutrient uptake by the plants. 

Knicker, H. (2001). Incorporation of inorganic nitrogen into humified organic matter as revealed by solid-state 2-D 1SN BC NMR spectroscopy. Abstr. Pap. Am. Chem. Soc. 221, U517-U517. 

As triazines age in the field they form bound residues (Table 21.9). The most stable bound residues are associated with humified organic matter - especially when that humified organic matter is associated with the coarse clay size fraction (0.2-2 pm). The largest proportion of total bound atrazine residues in whole soil was in the coarse clay size fraction, which also contained 50% of the total organic carbon (Barriuso and Koskinen, 1996). The ratio of bound residues to organic carbon content decreased with particle size and was highest in the fraction >50 pm, which is rich in nonhumified organic matter. 

Generally for sedimentary rocks substitution in silicate lattices of primary minerals is of lesser importance. As the primary minerals decompose during weathering the alkaline earths and alkali metals tend to remain in solution and some of the metallic micronutrients pass into the lattices of the secondary or clay minerals. Others become adsorbed onto clay sized particles, are incorporated into humified organic matter by complexation or separate as precipitates following changes in redox potential. 

Layers dominated by organic matter. State of decomposition determines type highly (Oa), moderately (Oe), or slightly (Oi) decomposed Mineral horizons that have formed at the surface of the mineral portion of the soil or below an O horizon. Show one of the following (i) an accumulation of humified organic matter closely mixed with minerals or (ii) properties resulting from cultivation, pasturing, or other human-caused disturbance (Ap)... 

Tilth—The physical structure of soil, closely associated with the concentration of humified organic matter. Tilth is important in water and nutrientholding capacity of soil, and is generally beneficial to plant growth. 

Bloomfield, C., Kelso, W.I. and Pruden, G., 1976. Reactions between metals and humified organic matter. J. Soil Sci., 27 16—31. 

A horizon. Mineral horizons that formed at the surface or below an O horizon. This layer is distinguished by an accumulation of humified organic matter mixed with mineral fraction and is not dominated by properties characteristic of E or B horizons. 

Soil organic carbon and humified organic matter... 

Alkali solutions dissolve protoplasmic and structural components from fresh organic tissues and these become mixed with the humified organic matter. 

CHAMINADE R. 1958. (The effect of humified organic matter on the efficiency of nitrogen). Annales agronomiques, 9, 167-192. 

The incorporation of several phenolic compounds into humified organic matter, and into the microbial biomass of soil, was studied by Kassim et all, They found that substantial proportions of the intact molecules of phenolic compounds, such as ferulic acid and catechol, were stabilised by incorporation into existing humus. Incorporation into the microbial biomass was greatest during the first two to four weeks of incubation, after which the amount declined with a corresponding increase in the proportion evolved as CO2. 

Stevenson and Vance 1989). Other factors affecting complexation and coagulation include the concentration and source of the organic substance. In mineral soils, most of the humified organic matter is bound to mineral surfaces through linkages with polyvalent cations (Stevenson and Vance 1989). 

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