Soils I - Part 2 - The Solid Phase

By the "fine earth fraction" we mean all soil particles < 2mm. Shape and size of the particles have an influence on their classification. 

In the field the finger test is used to determine the fraction type.

• Clay: good malleability, lubricates; shiny, smooth lubricating surfaces
• Loam (contains clay, silt, sand): malleable, lots of fine substance, pencil-thick rollable
• Silt: not binding, adheres in finger grooves floury, rough lubricating surfaces
• Sand: granular, not malleable, not cohesive, doesn't "fit" into finger grooves

In the laboratory the fraction size is determined by sieving, destroying binding substances and/or by laser diffraction.

1. The sandier, the more permeable, the less water storage, because of the large pores
2. The more clay, the more impermeable, the higher the water storage and the more stagnant water, because of fine pores
3. The more silty, the greater the storage capacity for water available to plants, because it has medium pores

1. The sandier, the less nutritious and the lower the buffer capacity
2. The more clay, the more nutritious and the greater the buffer capacity

• The OM makes up only a very small, but very important, %-age of our soils
• The OM is made up by C to 50%
• Carbon helps in stabilizing the OM and keeps it at equilibrium
• OM acts as a sorbent (for pollution and nutrients)
• The OM is a source for C and energy
• Soil forms important aggregates

1. arable soils (dt. Kulturboden) - humus poor - < 20 g/kg
2. grassland soils - moderate humus - 20-40 g/kg
3. forest soils (mineral soils) - humus rich - 50-100g/kg
4. moors - boggy - > 300g/kg
5. raised bogs - boggy - near 1000g/kg
6. forest soils (top layer) - organic layer - near 1000 g/kg

Organic matter is humus in a broad sense: all dead plant and dead substances in and on the mineral soil and their organic transformation products.

Edaphon (soil biota): living organisms, living roots (does not belong to the soil organic matter)

Complete microbial degradation to inorganic substances → when organic material becomes inorganic.

formation of humus = protection of OM from further decomposition (stabilisation of carbon in the soil)

Decomposition is the breakdown of organic matter into smaller molecules. Transformation of organic residues by heterotrophic (= organisms that need organic matter to survive/ feed on) organisms leads to differentiation of the organic matter (e.g. plant remains, microbial remains, mineral-bound organic substance (charcoal), dissolved organic carbon (highly reactive but stored in water).

The C/N ratio is an indicator for the level of decomposition of the OM and for biological activity. It is based on the finding that through mineralisation CO₂ is realeased while N is incorporated into the microbial biomass (and then the OM). Consequnetly the C/N ratio becomes smaller over time. Typically the smaller an organic matter is, the better it is degradable i.e. Mushroom mycelium (15-19), grass (18-25), spruce wood like pine needles (100-400).

1. Parenchyma tissue (basic tissue): is living green tissue, e.g. leaves, needles and fine roots (greenish mush/ fleshy part between the two outer layers of the leaf). It is made up of cellulose (what we make paper of, strands of glucose), hemicellulose (chain of sugars that are not glucose cell walls) and Proteins. Its C/N ration is < 50.
2. Lignified tissue (very hard material, due to lignin): this is made up of a wood part (xylem), supporting tissue (sclerenchyma) and a lignified cell wall made from cellulose, hemicellulose and lignin. Its C/N ratio is > 100 (much harder to degrade).

Different substances are either directly made inorganic (mineral substances, carbohydrates) or are first turned into humic substances (lignins, N-containing substances, fats, tannins and waxes). These processes usually reach an equilibrium. Humification is stabilized over a certain period of time and then goes back into the atmosphere as CO₂.

1. Preliminary phase of decomposition: Biochemical reactions of substances produced by the plant (hydrolysis and oxidation processes by tissue enzymes). This happens in the plant and not on the soil.

• Degradation of chlorophyll → autumnal discoloration (degraded by cold)
• Starch → Sugar
• Protein → amino acids

2. Initial phase

• Hydrolysis and oxidation of high polymeric compounds
• Leaching of water-soluble components (e.g. sugar, amino acids)
• Strong increase of microorganisms that live from the re- leased substances

3. Crushing phase

• Litter is partly bitten, partly eaten and modified by the macrofauna, and is excreted again in modified form
• Incorporation into the soil by earthworms, enchytraeids and arthropods = accessibility for mesofauna (collem- bola, mites, nematodes)

4. Dismantling and conversion phase

• Enzymatic cleavage of the organic fragments
• Formation of simple inorganic components like CO₂, H₂O, NH⁴⁺, NO3-, PO₄^2- (mineralization)
• Relative enrichment of hardly degradable compounds (e.g. tannins)

Easily degradable polymers like proteins, starch, ribonucleid acids and polar lipids are hydrolized in the initial phase. The more complex substances need to become subject to physicial degradation before they can be broken down by enzymes. 

Internal Factors

• N content of the substance (C/N ratio)
• Lignin content (lignin/ N ratio)
• Tannin content (polyphenol/ N ratio)

External Factors

• Heat/ temperature
• Availability of H2O and O2
• pH (the soil in forests are acidic, which is why the microorganisms cannot work under perfect conditions)
• Inhibitors (bactericides, fungicides)

Stabilization is a general term for processes and mechanisms that slow down the degradation of OM in soil and thus forms humus. In other words it is the accumulation of OM in the soil with slowed degradation (i.e. lower pH in forests and moors). Additionally stabilized OM is older than fresh organic matter.

Stabilization happens through...

... sorption: Once adsorbed to the surface of phyllosili- cates or neutral minerals, it is much harder for microorganisms to extract the nutrients.

... aggregation: Primary particles will be stabilized and shrink together by the organic compounds. These compounds make it harder for microorganisms to get to the organic matter and form CO2.

... physical disconnection (Fig. 63): The spatial location of OM in the soil influences the accessibility for organisms and enzymes, e.g. in aggregates.

... sorption and aggregation: The more molecules and nutrients that are either built into aggregates or bound to the mineral surface, the longer Carbon dioxide is stored in the soils.

... functional diversity:

a) molecular diversity: if only one type of organic substance is available (e.g. monoculture), only one type of microorganism will grows, which will be highly specialized in the degradation of that specific organic substance. If there are many different source of organic matter in the soil and there also add some difficult compound to degrade, numerous types of microorganisms will be in competition with each other and that will reduce the capacity of the microorganisms to release CO2. The more diverse the organic matter input is, the less carbon will be emit- ted.

b) spatial diversity: if everything is situated in the same pore size of the soil, microorganisms will be able to easily access the nutrients. If there is organic matter in different pore sizes, it is much harder for microorganisms to have access to it.

c) temporal diversity (can go in both directions): Too much temporal variability (hot/cold, wet/dry), the microorganisms will have a hard time to adapt and therefore not be able to flourish as much. At the same time it is possible that through temporal variability nutrients are released and made available to microorganisms more easily. Following this, they would be able to flourish more and release more CO2.

Organic matter is so important for soils because it...

• is a nutrient source for plants (N, P and S)
• has exchange properties and can both bind/adsorb cations and attach itself to clay minerals; gradual release of (nutrient) cations
• promotes aggregation of clay particles (clay-humus complexes): stabilizes aggregates, thus reducing erosion
• has a high water-binding capacity (3-5 times its own weight)
• has the capacity to adsorb light (soil temperature)
• it catalyzes organic pollutant decomposition
• functions as a carbon storage
  • Biochar/Terra preta
  • Permafrost carbon
  • Peat and moors