The humus

The determination by a wet process is performed by oxidizing organic carbon with an oxidizing agent (with Mn(VII), Cr(VI), Ce(IV)) in the presence of sulphuric acid. The amount of carbon oxidized is ascertained either from the amount of CO2 obtained (as for the elementary analysis) or from the amount of the oxidizing agent consumed during the analysis. 

The total iron content may be determined directly in the soil by the X-ray fluorescence analysis or by a spectrographic method. In the case of classic methods, the soil sample should be first decomposed either by melting with Na2C03 or by the action of hydrofluoric acid. The melting procedure is more advantageous, since in this case further elements can also be determined simultaneously with iron. When it is only necessary to determine iron, it is advantageous to use the decomposition with hydrofluoric acid, which does not need the separation of silicic acid. In solution, iron is determined most frequently by the photometry, AAS or polarography.  

The total aluminium content is obtained in order to characterize the soil with respect to the soil-forming substrate, weathering, etc. Together with analyses of other components of clay minerals, it may be used for calculating values which may be employed in the identification of clay minerals. To obtain the aluminium solution, the sample is melted with sodium carbonate and the melt is dissolved in concentrated hydrochloric acid. 

The total aluminium in the solution may be determined by a photometric method with aluminon (ammonium salt of 4-hydroxy-5,5 -dimethylfuchso-ne-3,3 -dicarboxylic-2 -sulphonic acid), by a gravimetric method, by atomic absorption spectrometry. X-ray fluorescence, etc. 

Element The whole soil Soil solution (mg 1 ) Plants range (ppm) 

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The following day the acid liquid is filtered with suction through a large Buchner funnel, to remove large quantities of humus which are produced. The humus on the filter is washed with two 350-cc. portions of water and finally with two 300-cc. portions of benzene. The filtered liquid and aqueous washings have a volume of approximately 10 1. The s-methylfurfural is... 

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). 

In soils of non-agricultural ecosystems, above ground biomass (foliar uptake) and metal cycling is considered important (see Figure 8), due to large impact on the metal distribution in the humus layer and mineral soil profile. Especially in soils of Forest ecosystems, it may affect the accumulation in the humus layer, which is considered a very relevant compartment regarding the calculation of a critical load. In these soils, however, a steady-state element cycle is assumed, which implies that mineralization, Minj, equals litterfall, Mjf. 

The humus content in Steppe ecosystem soils reflects the total biomass production and humidity. 

Figure 4.11. The carboxylate anion (an ionized acid) and a phenoxy (the anion of phenol) groups. The R stands for the rest of the humus molecule to which these groups are attached.

Methanol can also be produced from syngas with hydrogen and carbon monoxide in a 2 1 ratio. Coal-derived methanol typically has low sulfur and other impurities. Syngas from coal can be reformed by reacting with water to produce hydrogen. Ammonium sulfate from coal tar by pyrolysis can be converted to ammonia. The humus substances can be recovered from brown coal by alkali extraction. 

Organic matter in the Carboniferous deposits mainly represents the humus type with vitrinite as a main component. The Ro index measured on the authigenic vitrinite increases towards the bottom of the deposits from 0.49 to 1.15% (Grotek 2005). These data point to a maximum palaeotemperature of about 120°C. 

Extraction of Organic Compounds from the Soil. Soil biochemicals that are free or absorbed loosely, but not bound to the humus, were extracted by the following procedures ... 

It must be strongly emphasized that the charge on the humus and mineral particles depends not only on the nature of the surface but on the pH, the negative charge, hence CEC, rising with increase in pH. The CEC can therefore be far higher than it would be in the field. It is therefore necessary to... 

C N (or carbohydrate protein) ratios are important with respect to the relative requirements of plants and microbes. Straw has a C N value of approximately 80 1, whereas after ploughing under to form a humus-rich soil, the value narrows to about 12 1. Albrecht found that the humus fraction of the experimental Sanborn Field, cultivated over 50 years, had a C N ratio of about from 2 1 to 3.4 1, where the lower ratio is similar to that of the microbes themselves. Therefore, if microbes feed on straw that has been incorporated into the soil, they will require an additional source of nitrogen, and happen to be more successful at competing with plants for the same nutrient. In Missouri clay, Albrecht found 1.5% C and 0.15% N, which represents a favourable ratio of 10 1, and is an average value for well-weathered soils (Walters, 1989). 

Initially, at room temperature, the chlorine reacted rapidly with the humus. This reaction was evident by the change in color of the solution from a thick black to a transparent brown immediately after the addition of the chlorine. A drastic drop of pH was observed after 17 h (e.g., a drop from 7.0 to 2.6 while being chlorinated at neutral pH). Any storage after 90 h was at 4 °C for no more than 48 additional hours. The chlorinated samples were lyophilized to dryness and stored in a refrigerator. The acetone-soluble fraction of each sample was subjected to the following analyses. 

The greater the amount of carbon dioxide in soil, the more hydronium ions and so the lower the pH. Soil that has a low pH is referred to as sour. (Recall from Chapter 10 that many acidic foods, such as lemon, are characteristically sour.) Two main sources of soil carbon dioxide are humus and plant roots. The humus releases carbon dioxide as it decays, and plant roots release carbon dioxide as a product of cellular respiration. A healthy soil may have enough carbon dioxide released from these processes to give a pH range from about 4 to 7- If the soil becomes too acidic, a weak base, such as calcium carbonate (known as lime or limestone), can be added. 

Oxisois The predominant soils of the Tropics. Oxisois have experienced the greatest degree of mineral alteration and horizon development of any soil. The humus breakdown is rapid and the soils are usually deep and porous, Oxisois require fertilization to support continued crop production. 

PHOTOS NOTE If the humus and residual soil samples are taken from same area and the organic layer is shown in the soil pit photograph, separate photographs for humus are not required. 

Engstrom, D. R. 1987. Influence of vegetation and hydrology on the humus budgets of Labrador lakes. Canadian Journal of Fisheries and Aquatic Sciences 44 1306-1314. 

A small molecular size to penetrate through the humus matrix... 

In a review of the work of Schreiner and Shorey, Shmuk (1924) considered that their approach tended to divide the humus concept into small groups of peripheral units and overlooked the major reserve of organic substances in the soil. 

Figure 6.10. Accumulation of C in non-steady-state soils of a mature black spruce/moss forest in central Manitoba, Canada. Data shown are (A) for sphagnum moss that has accumulated since the site last burned (-100 yr before sampling), and (B) for the humus and charred layer below the regrowing moss and including the A horizon. The soil is developed on the sediments of a lake that dried up -7000 years ago. The parameters 7 = plant input (kgCuf2yr ) and k = decomposition constant (yr-1). Reprinted from Trumbore and Harden (1997), with permission from the American Geophysical Union.

These observations later stimulated the general belief that prevailed through the earlier decades of the 19th century—namely, that humus is the only or the major soil product supplying nutrients to plants. The direct utilization of humus by plants was fully developed by Thaer (1808,1846), who stated that humus comprises a more or less considerable portion of soil fertility of the soil depends largely upon it since, besides water, humus is the only material that supplies nutrients to plants. This concept was referred to as the humus theory. 

Figure 4.5a Colors of the leachate from the humus soaked in the buffer solutions with pH 11-3 at one-unit intervals (left to right), showing the importance of alkalinity in this color change and the sensitivity of the soil to treatment. The humus was collected from a Pinus-Quercus forest, Ibaraki, in April 1997. (From Yamanaka, T., Ph.D. diss., Kyoto University, 2002. With permission.) (See color insert following p. 178.)...

Organic matter extracted with 50% methanol increases during the EP (Yamanaka 1995a). This may be because of the increased solubility of the humus under the alkaline conditions as previously described. In contrast, water-soluble carbohydrates and phenolics decrease by the treatment. This maybe explained, at least partly, by the damage of plant roots that are known to exude water-soluble carbohydrates to soil (Yamanaka 2002). 

Figure 4.9a Coprinus tuberosus (arrows) fruiting on the ground after decomposition and disappearance of human feces (Appendix 4.2). Note the black color of the humus that indicates the flow-in of ammonia in the past. For scale see pine needles. (See color insert following p. 178.)...

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