The soil

The soil is one of the most complex components of the ecosystems, and as the Earth crust’s uppermost and fertile layer it plays a determinative role in the distribution of plants. The soil is a limiting factor for each of the terrestrial plants, it keeps plants fixed in the ground and it is an essential source of nutrients and water.

Soil formation is strongly influenced by geological factors (bedrock type), climate and living organisms. Due to the complex interplay of these factors, soils start forming with the fragmentation and weathering of the bedrock, following which plants and animals, soil fungi and micro-organisms of the soil (i.e., the edafon) are all responsible for the further complex transformation processes of the soil.

Soil is a three-phase (solid-liquid-gaseous) polydisperse system, with the following components: (Figure 19):

1. the soil solution

2. the soil colloids: organic-mineral complexes (clay minerals and organic molecules) which constitute colloidal-sized particles in the soil.

3. the air between the soil particles

4. other organic components and the humus

Figure 19: The soil components

Humus and its development

One of the important components of the soil is the humus, which is a collective name for the organic matter in the soil. Humus is made of complex organic macromolecules resulted from the decay of dead plants and animals, soaked in humus acids.

Most of the dead plant and animal residues in the soil are decomposed by microorganisms and fungi, which is followed by the process of mineralization.

The resulting inorganic substances (i.e., nutrients for the plants) are thus re-included in nutrient cycle.

A part of the decomposed organic matter is not mineralized, but becomes the main structural component of the humus (e.g., simple sugars and amino acids, proteins and their products, closed aromatic ring compounds which forms phenols and quinines). These compounds are connected to each other through chemical processes (condensation, polymerization) to form macro-molecules and thus become the constituents of dark humus. Microorganisms are not able to decompose these macromolecules or only do that very slowly, and therefore these humus materials temporarily exit from the ecosystem’s nutrient cycle. Thus the humus acts as the ecosystem’s nutrient storage because its very slow degradation enables a continuous replenishment of soils with nutrients.

Soil properties

- Structure – refers to how loose, aerated or aggregated the soil is. The dimension of soil grains and the volume of pores (i.e., soil porosity) are

important to plants, because they affect the development of the roots, the soil permeability for water and soil aeration. The smaller the pores, the more compact the soils are; this can mean decreased soil permeability for water and less living space for microorganisms (e.g., in clayey soils). In contrast, extremely loose soils have a poor water holding capacity (e.g., sandy soils).

The soil structure is influenced by the ions adsorbed on the surface of the soil colloids. Calcium-rich soils are crumbly, well-aerated, whereas colloids in soils of high sodium content (saline soils) coagulate and soils are poorly aerated.

- Temperature: Has a significant effect on root growth, it affects seed dormancy, seed germination and the soil nutrient supply through influencing the speed of decaying processes and the activity of microorganisms. Extreme soil temperatures represent a stress factor for plants, e.g. in overheated soils during summer, whereas in the winter the frozen water causes water stress in roots.

- PH: Is an important factor because it influences the solubility of soil nutrients.

Plants generally prefer neutral and slightly basic soils which, due to the presence of Ca ions, have a good structure and a favourable humidity. Some plants can also tolerate alkaline and acidic soils, but in order to achieve that they have different adaptive strategies. For example, mycorrhizae can enhance significantly the nutrient absorption.

-Humus content and humus characteristics are important in controlling the nutrient exchange, water balance and soil pH.

- Water content: hygroscopic water, capillary water and gravitational water:

Water in the soil exists in three forms; (i) the colloidal soil particles are associated with an extremely thin film of moisture that adheres to soil particles, which is the hygroscopic water or bound water, (ii) the capillary water that moves between the soil particles, and (iii) the gravitational water that originates from precipitation and is drained downwards by gravity. Only the latter two constitute a source of water for plants.

As seen before, the vegetation composition is strongly influenced by the soil properties. Several plant species are sensitive to the soil properties and can only tolerate a narrow interval of the soil parameters. These plants are soil indicators.

Species that indicate acid soils (i.e., acidophilus plants) are the blueberries Vaccinium myrtillus, V. vitis-idaea, the Wood sorrel (Oxalis acetosella) (Figure 20.), the Heather (Calluna vulgaris), many species of Rhododendron and the Sheep’s sorrel (Rumex acetosella).

Figure 20: Acidic soil indicator species

Bilberry (Vaccinium myrtillus) A., Lingonberry (Vaccinium vitis- idaea) B, Wood sorrel (Oxalis acetosella) C.

Plants which are adapted to a high salt content are indicators of solonetz soils or of soils of very high salt concentration and poor structure – solonchaks. For example, the Sea aster (Aster tripolium), the Sea plantain (Plantago maritima) and late flowering Sea lavender (Limonium gmelini subsp. hungaricum) indicate saline soils (Figure 21).

A B C

Figure 21: Salty soil indicator species.

A. Sea aster (Aster tripolium subsp. pannonicum), B. Sea plantain (Plantago maritima), C. D. Sea lavender (Limonium gmelini subsp. hungaricum)

The high water content of the soil is indicated by species such as the Yellow flag (Iris pseudacorus), species of rushes such as the Soft rush (Juncus effusus) (Figure 22.), sedge species such as Lesser pond sedge (Carex acutiformis). The marshy, oxygen-poor soils are indicated by tree species such as the alders, e.g. black alder (Alnus glutinosa). Alder roots establish symbiotic relationships with actinomycete fungi that enhance the nitrogen uptake from the inorganic, nitrogen poor soils.

A

B C D

Figure 22: High ground water level indicator species: Soft rush (Juncus effusus), A., Yellow flag (Iris pseudacorus) B.

Some plant species are indicators of high metal ion concentrations in the soil.

Most plants avoid such habitats, but some grow well in these ion- saturated soils. The latter are able to accumulate ions in their tissues. For example, the Irish moss (Minuartia verna) is well-known as an indicator of elevated lead concentrations, and the field horsetail (Equisetum arvense) is an indicator of increased zinc content.

There are some plant species that are able to indicate the elevated nitrogen content in the soil by an increased population density and tall-growing individuals that can encroach the habitats. The "neck length" Stinging nettles by the landfills, the Common hop and elderberries grown above animal carcasses are well-known “indicator” situations.

Soil types

Based on the factors that influence soil formation, soils can be:

A B

1. Zonal soils: reflect the influence of the macroclimate. They cover large areas, follow the climate zones across the globe, and have very well developed structure. Zonal soils include e.g. tropical laterite soils and brown forest soils of the temperate zones.

2. Intrazonal soils: usually occupy restricted areas and are associated with zonal soils. The evolution of these soils is determined by local factors, mainly by the influence of the dominant bedrock. For example rendzina soils are characteristic of limestone.

3. Azonal soils: their development is driven by factors other than the climate and bedrock, which may be local factors such as e.g. water.

The structure is usually incomplete and there is no advanced soil profile.

Marshy soil, sediment soils and saline soils belong to this group.

In Hungary, beside the typical temperate zonal soil types many other soil types develop intrazonally or azonally due to edaphic factors.

Figure 23: Soil map of Hungary with the major soil types .