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Tree biology and pedology

Module 1

Tree biology and pedology

Table of Contents

Introduction

Trees are an essential part of life on Earth, without them life on our planet could not exist. People use trees for wood, for their fruits or for their aesthetic function.

This chapter provides basic information about the biology of trees and their morphological and anatomical structure. We will also focus on the physiological processes which are constantly taking place in trees and are necessary for their life. We will also focus on  external and internal conditions  affecting these processes.

1.1. Tree biology

Trees are woody plants that can grow more than 5 meters in height. They have the entire stem lignified, the lower unbranched part passing into the branched crown. They are perennial to long-lived and bear fruit many times.

We can divide them into 3 main parts:

  • root,
  • stem,
  • crown.

Cells are the basis of all living organisms, including trees. New cells are created by dividing existing cells. In trees, cell division takes place in meristems. Cells with similar function and structure are grouped into groups called plant tissues. 

Tissues are divided into:

  • dividing tissues (meristems),
  • permanent tissues.

Tissues are grouped into organs. Trees have 5 organs – root, stem, leaf, flower, fruit.

1.2. Tree anatomy

1.2.1. Root

A root is an underground organ of a tree that is hidden in the soil. It has two basic functions:

  • mechanical – its task is to fix the plant in the soil,
  • physiological – its task is to draw water and substances soluble in it from the soil.

On the root, we distinguish the main root, lateral root and root hairs, whose life span is from a few hours to a few days.

Trees can take root:

  • deep – the main root penetrates the soil vertically and is often as deep in the soil as the high above-ground organs, e.g. oak (Quercus), mulberry (Morus),
  • shallow – lateral roots penetrate horizontally and close to the surface of the soil, e.g. spruce (Picea).

We recognize 3 basic types of root system:

  • tap root – the main root is thick and goes vertically into the depth of the substrate, e.g. white fir (Abies alba), oaks, elms, Scots pine (Pinus silvestris),
  • heart-shaped – a larger number of thicker roots that point downwards and obliquely downwards, e.g. linden (Tilia), hornbeam (Carpinus) , birch (Betula),
  • flat – equally thick roots that run parallel to the soil surface and only individual roots penetrate into the deeper layers, e.g. aspen (Populus tremula), ash (Fraxinus), spruce (Picea). This type of root is shallow and not very stable.

1.2.2. Trunk

The trunk is the above-ground main part of the tree. It is a secondarily thickening woody stem that divides into woody branches at the top. On the cross-section we distinguish bark, phloem, cambium and xylem (wood). Each layer of the trunk has a certain function.

Bark – is on the surface of the trunk and branches of trees. The top layer is called bark and is dead. The lower green bark is alive. The deeply furrowed bark mostly develops in the lower part of the trunk. It serves as protection against mechanical damage, water loss or overheating. There are small openings in the bark – lenticels, which allow gas exchange.

Phloem is a conductive tissue that distributes the products of photosynthesis from the leaves to the roots. Phloem consists of living cells. In conifers, these are sieve cells, and in deciduous trees, sieve tube members and companion cells.

Cambium – dividing tissue that produces new xylem and phloem cells. Cambium forms annual rings of xylem during the growing season. One annual ring corresponds to one growing season.

Xylem – has two functions. The first is mechanical and has the ability to keep the trunk in an upright position, and the second is conductive, which conducts water and soluble minerals in the direction from the root to the crown. Storage substances (oil and starch) are also stored in the wood.

The wood of gymnosperms is composed of tracheids and parenchyma cells. The wood of deciduous trees contains vessel elements, fibers and parenchyma cells.

Tracheids are elongated, dead cells with thickened walls. They have a mechanical and conductive function. Fibers provide mechanical strength. Parenchymal cells help in conducting water, store hydrocarbons and are important in defense against decay. Vessels are important conductive elements in deciduous trees.

1.2.3. Crown

The crown is a system of branches that grow from the upper part of the trunk. It consists of:

  • structural (skeletal) branches – they decide on the shape of the crown, they are larger and thicker with a vegetative function,
  • filling branches – weaker, fill the crown, have a vegetative and generative function.

The way the crown is formed is influenced by various factors, namely the species genetics, biotic and abiotic factors. To a large extent, the structure and shape of the crown is influenced by the way its shoots grow.

We distinguish 3 types of shoot growth and branching:

  • monopodial – the main shoot grows from the top terminal bud, on which subdominant lateral shoots grow from lateral buds. They do not grow in the direction of the main stem and do not outgrow it in height. All conifers with a conical crown are branched in this way.
  • sympodial – the main shoot is not formed from the terminal bud, but from a lateral one. This branching pattern is typical for willows (Salix), lindens (Tilia), elms (Ulmus), hornbeams (Carpinus) and black locust (Robinia).

We distinguish between these crown forms:

  • conical (spruce – Picea abies),
  • columnar (Pančić spruce – Picea omorica),
  • ovoid (black alder – Alnus glutinosa),
  • round-ovoid (linden – Tilia),
  • round (wild apple tree – Malus sylvestris),
  • umbrella-shaped (black locust – Robinia pseudoacacia),
  • irregular.
Figure 1.1. Crown shapes
Source: Zubček, V (2019) Atlas drevín. (Accessed 15. 3. 2023)

1.2.4. Leaf

A leaf is a lateral organ that is morphologically differentiated into a leaf blade (the flat part of a leaf), a petiole and a base. The leaves contain the green pigment chlorophyll, which enables the leaves to photosynthesize.

Regarding the attachment of the leaf to the stem, we distinguish between a stalked or a sessile leaf (without a stalk). Regarding the position of the leaves on the stem, we distinguish alternate leaves, opposite leaves and whorled leaves.

Duration and life of leaves

The leaves fall after the end of the growing season. Abscisic acid or in some cases a layer of cork accelerates the shedding process. It causes the leaf to separate and close the wound. In conifers, the leaves fall either in the 1st, 3rd or in the 9th year. This time can be shortened due to unfavorable conditions (acid rain).

The outer structure of the leaf

We can recognize various leaf veins arrangements – venations:

  • pinnate (e.g. cherry – Prunus, linden – Tilia)
  • palmate (e.g. maple – Acer)
  • reticulate (e.g. hellebore – Helleborus)
  • parallel (monocots – grases)

We can recognize various leaf blade shapes:

  1. needle-like (acicular)
  2. linear
  3. lanceolate
  4. spatulate
  5. ovate
  6. inverted ovate (obovate)
  7. elliptic
  8. heart-shaped (cordate)
  9. inverted heart-shaped (obcordate)
  10. round (orbicular)
  11. kidney-shaped (reniform)
  12. diamond-shaped (rhomboidal)
  13. arrow-shaped (sagittate)
  14. spear-shaped (hastate)
  15. shield-shaped (peltate)
Figure 1.2. Leaf by leaf blade
Source: Větivčka,V & Matoušová, V. (1992). In: Stromy a kry. Príroda. Bratislava.

We can recognize leaves by the edge of the leaf blade – leaf margins:

  1. entire
  2. serrate
  3. double serrate
  4. dentate
  5. double dentate
  6. crenate
  7. lobed
Figure 1.3. Leaf according to the edge of the leaf blade
Source: Větivčka,V & Matoušová, V. (1992). In: Stromy a kry. Príroda. Bratislava.

The leaves on the same plant are usually identical in shape, size and position of the leaves on the stem. If there are leaves of different shapes on one plant, we speak of heterophylly which is typical e.g. for ivy (Hedera), maple (Acer), etc.

1.2.5. Bud

Buds are the teardrop-shaped parts of the tree where new growth occurs and are covered with scales to protect them from harsh weather conditions.

We can determine the branching of trees by the position of the buds on the stem. They play an important role in identifying trees in winter, as they are characteristic for most genera and species. 

They are located on the stem at the nodes in various arrangements:

  • alternate,
  • opposite,
  • whorled.

Buds can be:

  • terminal (apical) – located at the end of the shoot, they are the most active,
  • lateral – have the potential to form shoots (either vegetative or reproductive), some are dormant buds which take over if the terminal bud is removed.

1.2.6. Flower

Flowers are formed by the transformation of leaves and are used for sexual reproduction. The flower consists of:

  • flower stem – used to fasten the flower to the stem,
  • flower bed – extended upper part of the flower stem, does not participate directly in reproduction, is differentiated in color and shape into the calyx (green lower part of the flower covers – sepals) and corolla (distinctly colored part – petals),
  • reproductive organs – stamens and pistils.

Stamens are the male reproductive organs that produce pollen grains. It consists of a thread and an anther formed by two pollen sacs. A cluster of stamens in a flower is called an androecium.

A pistil is a female sexual organ that is formed by the fusion of one or more carpels. A set of carpels in a flower is called a gynoecium. The pistil consists of:

  • a stigma – the upper part of the pistil, which captures the pollen grains,
  • a style – the middle part of the pistil,
  • an ovary – the lower expanded part that contains one or more eggs.

Flowers are divided into unisexual and bisexual. Bisexual flowers have both male and female reproductive organs (e.g. apple tree – Malus) and unisexual flowers have only male or female reproductive organs. If there are only male or only female flowers on a plant, we say it is dioecious (e.g. willow – Salix). If there are both male and female flowers on the same plant, we say it is monoecious (e.g. oak – Quercus).

1.2.7. Fruit

The fruit is a multicellular reproductive organ. Its main task is the protection and dissemination of seeds. It consists of seeds and pericarp (fruit wall). Seeds and fruits can be spread by:

  • by wind (anemochoria) – using various flying devices, e.g. membranous wings (birch – Betula, maple – Acer),
  • with the help of animals (zoochory) – fruits or seeds can spread on the surface of the animal’s body (by attaching to the fur) or through the digestive tract,
  • with water (hydrochoria)
  • by one’s own forces (autochoria) – by being thrown into the environment.

Distribution of fruits:

  1. pulpy – pericarp is divided into outer (peel), middle (pulp) and inner part. These include stone fruits (e.g. cherry), pome fruit (e.g. apple) and berries (e.g. gooseberry)
  2. dry – pericarp is hard or leathery. This includes e.g. nuts.

1.3. Tree physiology

The basic life processes of trees include intake, transport and excretion of water. Water is an irreplaceable component, it is a solvent and has an important role in the transport of substances, in metabolic processes, thermoregulation, etc.

WATER INTAKE

Trees absorb water through their root system. Water intake can be affected by:

  • soil temperature – if the soil temperature decreases, water intake also decreases
  • the concentration of the soil nutrients – with a high concentration of osmotically active substances, water intake is prevented
  • intensity of transpiration – if plants release more water, water intake also increases
  • oxygen content in the soil – with more intense transpiration, more water is taken in.

WATER TRANSPORT

The transport of water with dissolved inorganic substances from roots to the leaves is called the xylem flow. It enables:

  • transpirational pull – when water evaporates from above-ground organs, a negative pressure is created in the vessels and water is absorbed by the roots
  • root pressure – pressure that pushes water and substances dissolved in it from the root to the above-ground parts
  • cohesion – cohesion of water molecules
  • adhesion – adherence of water to vessel walls

EXCRETION OF WATER

Excretion of water is ensured by two processes, namely transpiration and guttation. Transpiration is the evaporation of water from above-ground organs, from leaves. No energy input is required for this process. It can be done in two ways:

  • stomatal transpiration – through the stomata
  • cuticular transpiration – through the entire surface of the leaf cuticle.

Guttation is the excretion of water in the form of drops. In this way, water is excreted in high humidity, when transpiration is not possible.

Water excretion is affected by:

  • the water potential (water concentration) in the plant – when there is a lack of water, the stomata close, when there is an excess, they open
  • air temperature – as the temperature rises, so does transpiration, after a certain temperature is reached, the vents close
  • air humidity – as humidity increases, transpiration decreases
  • light – increases transpiration, the stomata open
  • number and location of stomata

1.3.1. Photosynthesis and respiration

Photosynthesis is a process that takes place in the chloroplasts of (primarily) leaf cells containing the green pigment chlorophyll. It is the most important biochemical process on Earth, in which the transformation of inorganic substances into organic matter occurs in the presence of sunlight and water.

Chemical formula of photosynthesis:

12H2O + 6CO2 + chlorophyll, sunlight → C6H12O6 + 6O2 + 6H2O

The intensity of photosynthesis is affected by light, the concentration of carbon dioxide in the air, temperature, water, the amount of chlorophyll, the age of the leaves, mineral nutrition, etc.

Respiration is a reaction that releases energy from organic compounds. This energy is used by the organism for various synthetic processes, intake of nutrients, growth, etc. The intensity of respiration depends on temperature, oxygen concentration, amount of water in the plant, age of the plant, etc.

Photosynthesis takes place in the leaves and needles, but respiration takes place in all parts of the plant – including the trunk and roots.

1.3.2. Growth and development of trees

Growth and development take place simultaneously during the life of trees. Growth is the enlargement of vegetative organs – root, trunk and crown. It is initiated by a tissue division. Growth is influenced by various external and internal factors.

External factors:
  • light
  • temperature – most plants grow in the range of 5-40° C. Each plant has a different temperature optimum at which it grows the fastest
  • water

Internal factors include plant hormones that influence plant growth and development. We distinguish:

  • growth stimulating hormones (stimulators) – auxins, gibberellins, cytokinins
  • growth-retarding hormones (inhibitors) – abscisic acid, ethylene

Development is the changes that take place during the life of a plant from germination to death. Tree development is divided into 3 periods:

  • youth – starts with the germination of the seed and ends when the tree is able to bear fruit. In this phase, the trees are characterized by rapid growth.
  • maturity – in this period, trees begin to bear fruit, the rate of growth decreases and also the ability to adapt to environmental changes
  • aging – growth and fertility decrease and could eventually stop, trees may lose the ability to adapt to environmental changes and may begin to die

The development of plants is affected mainly by temperature and light.

Conclusion

If we want to protect trees and continue to use them for our benefit and our planet, we need to have basic morphological, anatomical and physiological knowledge about them. The life of trees is also affected by various internal and external factors. By studying them we can increase the quality and preserve populations of trees on our planet.

Biology Quiz

Test your knowledge in the following quiz and see if you understood the contents of this section:

2. Pedology

Introduction

We understand the soil as a living and constantly developing three-dimensional natural-historical formation that was created due to the action and interaction of the atmosphere, biosphere, hydrosphere and lithosphere. In modern history, man has become the most important biotic force for soil development. In this chapter we will learn about the  physiological, biological and chemical properties of the soil. We will also focus on the soil  structure.

Pedology (from the Greek pedon – soil, logos – science, science) or soil science is a natural science that deals with the study of soil, its formation, classification, physical, chemical and biological properties. Soil is a diverse system. Together with climate and weather factors, it is an important component of the environment. Soil is formed from parent rock under the influence of various environmental conditions. It is a complex process in which the parent rock breaks down through weathering and turns into soil-forming substrate. From this substrate, soil is created by soil-forming processes as a separate natural formation. The soil-forming process does not end with the formation of soil, but continues.

The problem of land development is:
  • formation of soil (formation of soil by the action of soil-forming agents)
  • soil evolution (gradual change of already formed soil over a certain period of time)
  • soil metamorphosis (soil change as a result of soil-forming factors).

During the weathering of rocks, mechanical (physical), chemical and biological processes take place.

Figure 2.1. Soil composition
Source: Bezděkovský, M. et al. (1997). In: Technológia rastlinnej výroby. Príroda. Bratislava.

2.1. Soil properties

In areas with approximately the same set of soil-forming factors, especially with the same humidity and temperature conditions, it is possible to observe more or less the same nature of the deposition and decomposition of organic substances, the water and air regime, the movement of substances, as well as the same natural fertility. The layers made in this way are called soil horizons. 

Soil type is the basic identification unit of the morphogenetic and agronomic categorization of soils. Soil types are defined by a set of diagnostic horizons whose most important properties have been acquired through long-term development under environmental conditions and cultivation.

The formation of soil horizons is influenced by precipitation, heating and cooling, chemical reactions and biological activities. The soil profile normally consists of five horizons (O, A, E, B, C), sub-layers and transitional layers. These layers are sometimes distinguished from each other by a color that indicates differences in drainage, content of organic substances as well as changes in other properties. The upper part of the profile is formed by a thin layer of decomposing organic material, which is called the organic layer (O). A layer that is located below the organic layer is the horizon (A). This horizon contains absorbent tree roots and is normally rich in organic material. The horizon (E) is the area of mineral weathering. Horizon (B), located at intermediate depth, is composed of fine-textured materials originating from horizon (A) and soil particles of the parent rock. Horizons (A) and (B) can be modified by the environment to such an extent that they can be considered topsoil. The horizon (C) or foundation layer of soil is the lowest layer of soil distinct from the subsoil and is composed of parent rock.

Figure 2.2. Sample of soil: O - organic horizon, A - humus horizon, B - iluvial horizon, C – parent rock
Source: US Department of Agriculture (no date) Ukážka pôdneho profilu.
(Accessed 23. 3. 2023)

The organic layer on the surface of the soil contains leaves, branches, bark and organisms that produce the humus layer. The humus layer is biologically active, causing its slow decomposition. Mineral substances, decomposed biological material and other residues reach the bottom mineral layer of the soil.

Soil types were created by the action of soil-forming factors: black soil (the most fertile soil type), brown soil (the most widespread soil in warm regions), glay soils (heavy soils in wet areas), rendzina (humus-rich shallow soils that are usually formed from carbonate rock), alluvial soils (soil near a watercourse), peat soils (have high permeability), halomorphic soils (form due to soil salination) and undeveloped soils (raw soil, rocky and shallow).

Soils are divided into soil types based on the content of clay particles in the soil:

Soil type Soil Content of clay particles
Heavy
clay
Above 60
silty clay
45 – 60
Medium
sandy clay
30 – 45
sandy clay loam
20 – 30
Light
sandy loam
10 – 20
sand
0 – 10

Table 1.1. Classification of soils into soil types

Grain size:

  • Clay …  < 0,002 mm
  • Silt   …  0,002 – 0,05 mm
  • Sand  …  0,05 – 2 mm

Division of soil types according to the content of granular particles

Figure 2.3: Division of soil types.
Source: Groenendyk, Derek et al. (2015). USDA Soil Texture Triangle. In Groenendyk, Derek et al. (2015). Hydrologic-Process-Based Soil Texture Classifications for Improved Visualization of Landscape Function PLoS ONE 10(6).

Light soils contain coarser grains, they lack colloids. They hold water poorly, plants in them often suffer from drought. They contain a lot of air and organic residues decompose quickly in them. They have a low humus content. They are easy to process both dry and wet.

Medium soils are characterized by an adequate amount of finer and larger particles. With their properties, they create a transition from light to heavy soils.

Heavy soils contain predominantly the finest clay particles and few capillary pores (holes that bind and attract water and air) and non-capillary pores (holes that do not attract water and air). They pass water poorly and are poorly airy with low biological activity. They are characterized by high cohesion and adhesion, which makes them difficult to process. When dry, they harden, crack, and a coating forms on their surface. They stick when wet.

2.1.1. Physical properties

They affect the workability of the soil, the time and method of carrying out individual work operations. They represent a set of properties that are conditioned by the dispersion (scattering) of elementary particles and the mutual relationship between the solid phase, soil solution and air in the soil.

We divide them into:

  • basic physical properties (granularity, structure, specific gravity, bulk density and soil porosity)
  • hydrophysical and aeration properties (moisture, maximum hydroscopicity, water permeability, buoyancy, air capacity and aeration of the soil)
  • temperature properties (heat capacity, thermal conductivity and soil temperature)
  • physical-mechanical properties (cohesion, adhesion, consistency, ductility, plasticity, swelling, settling)

Specific weight represents the ratio between the weight of the soil solid phase without pores and the weight of the same volume of water at 4°C. It is defined as the weight of 1 m3 of solid, non-porous soil, expressed in tons (t/m3)

Soil volumetric weight is the weight of a certain volume of soil (1 m3). It is an important parameter for determining soil properties and conditions for plant growth. Its values influence the growth and development of plant roots, influence the water-air regime of the soil, chemical and biological properties of the soil.

Soil porosity (P) expresses the total percentage of free space between solid soil particles. In the soil, we distinguish between non-capillary pores, which let gravity water through quickly, and capillary pores, which ensure the percolation through the pull of water. Non-capillary pores let water and air pass through the soil quickly, allowing the excess water into the deeper lower layers. Capillary pores help to retain water. Capillary water is subjected to gravity only to a limited extent. Air also moves more slowly in capillary pores.

Soil moisture indicates the current water content in the soil in mass or volume percentages. It has an impact on determining the productive capacity of soils. It varies seasonally, with a limiting impact on crops.

Maximum hygroscopicity is the largest amount of water that the soil can bind in the pores. Its value is used to determine the limit of physiologically inaccessible water (wilting point), which corresponds to the water content in the soil that plants can no longer use and begin to wither.

Permeability of the soil is the ability of the soil to limit the movement of water or to bind it. It is expressed in different forms of water capacity. The greatest water retention is characterized by the maximum (field) water capacity, which is equal to the total porosity value. It can also be defined as the ability to pass water from the surface to the lower layers. It depends on the type and amount of pores. Clay soils are poorly permeable and are waterlogged on the surface. Sandy soils are very permeable and therefore dry out quickly.

The amount of water that is maintained in the capillary pores is expressed by the capillary water capacity.

Air permeability is an important indicator of soil aeration. Air permeability does not occur in aggregates smaller than 0.5 mm. Aggregates larger than 1 mm let air through.

The thermal capacity of the soil expresses the ability of the soil to receive and retain heat.

Soil thermal conductivity is the ability of soil particles to conduct heat from the surface to deeper soil layers.

Cohesion is the ability of the soil to resist external pressure acting on the crushing of aggregates and to resist the penetration of foreign bodies into the soil. It is the force by which soil particles are bound together. The smaller the soil particles, the greater their contact surface and the greater their cohesion. Clay soils have a high cohesion, sandy soils less.

Adhesion is the ability to mutually attract soil particles to a body penetrating the soil. The degree of adhesion depends on the size of the soil particles and their moisture. Cohesion and adhesion have a significant impact on the workability of soils. Soils with high cohesion and adhesion are more difficult to process.

The color of the soil depends on the color of the mineral particles of the soil, granularity, structure, soil moisture and humus content.

Soil temperature depends on the amount of heat captured by the soil surface. The soil in the surface layer absorbs solar radiation and converts it into thermal radiation that radiates back into the atmosphere.

Water capacity is the ability of the soil to hold water. It depends on its porosity and the predominant type of pores.

Maximum water capacity is the amount of water needed to completely saturate the soil – to fill all the pores.

Absolute water capacity is the amount of water that remains in the soil 24 hours after its complete saturation.

Air capacity represents the air content in the soil.

Water vapor represents water loss. A certain amount of water is lost through surface runoff, percolation to greater depths beyond the reach of plant roots, and evaporation into the air. The intensity of evaporation depends on the physical properties of the soil, the vegetation cover and the soil treatment.

Skeletality is evaluated together with grain size. Soil particles larger than 2 mm are considered skeleton. Particles with a size of 2-60 mm are considered gravel and over 60 mm as cobbles.

2.1.2. Chemical properties

They represent a set of individual and at the same time mutually integrated chemical parameters. These include: content and types of nutrients, soil reaction and soil water. The chemical composition and various chemical processes that take place in the soil determine the chemical properties of the soil.

The mineral content in the soil consists of individual elements: oxygen (O) 49%, silicon (Si) 26%, aluminum (Al) 7%, calcium (Ca) 3%, iron (Fe) 4%, potassium (K) 2.5 % phosphorus (P) 0.08%. These elements are found in the soil in the form of various compounds, mainly as silicates, carbonates and phosphates.

The organic component is created by the decomposition of organic substances in the soil. The process produces simpler organic substances, and complete decomposition produces carbon dioxide (CO2), water (H2O) and simple organic substances.

Water acts on the soil component as a solvent. It contains CO2, which is produced by the decomposition of organic matter. This makes it a weak acid with increased dissolving power. Air in the soil enables and conditions oxidation and reduction processes.

Soil colloids are the basis for the humus-clay-like sorption complex. Exchange reactions are still taking place between the sorption complex and the soil solution, which are of great importance in making nutrients available to the plants. These reactions further influence the sorption of individual nutrients and the reaction of the soil.

The content of humus in the soil is expressed in %. The humus content up to 0.5% is considered an extremely small amount, 0.5-1% very small, 1-2% small, 2-3% medium, 3-5% high, over 5% very high. 

The content of nitrogen (N) in the soil is determined by chemical methods in the collected soil sample and is expressed in %. The rating is as follows: very low nitrogen content up to 0.12%, low content 0.12-0.16, medium content 0.16-0.24%, high content up to 0.24%-0.34% and very high content (over 0.34%).

Soil reaction is determined in an aqueous solution or in a solution of potassium chloride (KCl). It can range from 1 to 14 and is expressed in pH values.

Below pH 4 is considered extremely acidic, 4.6-5 strongly acidic 5.1-5.5 acidic 5.6-6.5 weakly acidic 6.6-7.2 neutral and 7.3-7.7 alkaline, above 7 ,7 strongly alkaline.

The pH value has different effects on the ecology and chemistry of the soil. It affects which species and which soil organisms will be present in a given location. The availability of mineral substances is one of the important influences of pH on tree growth. At certain pH levels, basic elements form chemical compounds that are insoluble in water and cannot be taken up by plants, because the roots are only able to take up water-soluble minerals.

In order to reduce the pH, we add sulfur (S) to the soil, to increase the pH we use lime. In the area of trees, the change in pH is difficult to achieve due to the large volume of soil in the root system area of the tree.

Mineral substances necessary for tree growth are dissolved in water. Cation exchange capacity (CEC) is a measure of soil attractiveness and retention.

2.1.3. Biological (Ecological) properties

Many organisms, microorganisms and macro-organisms live in the soil. From the view point of the biological properties of the soils, parameters are considered to be the total amounts of individual species, physiological groups of organisms in the soil, but also the intensity of their specific biological manifestations.

The number of organisms in the soil is evaluated primarily through the soil macroedaphon (soil insects, worms, rodents,…). The numbers of microorganisms (bacteria, fungi, algae, actinomycetes, protozoa) are not considered to be a sufficiently stable component of soil to be significant in terms of parameterization of soil properties.

Bacteria are divided into two basic groups:

  • aerobic that live with access to air
  • anaerobic, which live without access to air.

According to activity, bacteria are divided into:

  • decomposers, decomposing organic nitrogen-free substances
  • nitrifying and denitrifying, decomposing organic nitrogenous substances
  • nitrogen-fixing, which can capture atmospheric nitrogen
  • sulphur, iron and methane transforming bacteria

Mineralization of soil organic matter is the activity of microorganisms measured by the amount of CO2 released from the soil. Nitrogen (N) mineralization expresses the intensity of the biological release of ammonia from soil organic matter. Nitrification is the rate of biological oxidation of ammonia to nitrates in the soil.

Soil is an ecosystem containing billions of organisms. Some soil organisms can damage the roots, they are useful, while others have no direct effect on trees. As the roots penetrate the soil, the root cap and top layer are shed and substances from the roots enter the soil. They are a constant source of organic material for microorganisms. Mycorrhizae – fungal roots, are special roots of most plants on which fungi are found. These fungi live in a symbiotic relationship with the roots. Mushrooms and roots benefit from this coexistence. The roots provide the fungi with living space and food (sugars). Fungi increase the ability of roots to absorb water and provide basic elements, especially phosphorus (P).

Nutrient cycling is especially important in natural plant systems. As the plant grows, the roots absorb essential elements from the soil solution and produce new woody material and leaves. Over time, plants or parts of them die and fall to the soil surface, where soil organisms and weathering processes take over. Gradually, decomposition occurs and nutrients are released into the soil, where they are then available again to the roots.

Animals, e.g. various types of insects and worms that inhabit the soil and the humus layer aerate the soil and accelerate decomposition processes. Other animals feed on roots. Some nematodes can parasitize tree roots and transmit some diseases. Others feed on pathogenic organisms that cause various diseases.

The rhizosphere is the root zone of intense biological activity near the growing roots.

2.2. Soil moisture

Soil moisture is the current water content in the soil. It is expressed as a percentage of the weight of dry soil (w) or as a volume percentage (Θ). Changes depend on weather conditions, desiccation of the root system of plants, soil depth, granularity and structure.

The following methods are used in research soil science practice to determine soil moisture:

  1. methods that analyze samples taken from the soil environment. These include the gravimetric (excretion), alcohol (analysis using alcohol), glycerin (analysis using glycerin C3H8O3), carbide (measurement using a CM device) and pycnometric method (determination of the density of substances)
  2. methods that can analyze in field conditions. These include conductometric (method of measuring specific conductivity), tensometric (method of removal), gammascopic (method of measuring absorption and scattering dependent on density) and neutron method (method of measurement using neutrons).

Soil moisture, together with soil and vegetation characteristics, is responsible for the exchange of water and energy between the soil surface and the atmosphere. Information on soil moisture in the surface and root zone is crucial for achieving sustainable land use and water management.

When the soil is at field capacity, water is absorbed by plant roots or evaporates. Roots draw water from the soil as long as they are able to overcome the adhesive forces holding water in the soil particles. The leaves of the trees wither during the high water demands during the day and regenerate at night when evaporation decreases. Depending on the water holding capacity of the soil, a point is reached where the tree is unable to draw any water from the soil. It is called the permanent wilting point. Unless water is supplied to the soil, the plant (tree) will not recover from wilting.

Trees need both air and water. In the pores of the soil, water and gasses (oxygen, nitrogen and carbon dioxide) are in balance. Tree roots require oxygen and release carbon dioxide in the process of respiration. The exchange of gasses between the soil and the atmosphere generally takes place in the form of diffusion through the soil surface. If the exchange of gasses is insufficient, for example in soaked or compacted soils, there may be an accumulation of carbon dioxide and a deficit of oxygen. Such a situation can reduce the growth and function of the roots, if it persists for a long period, the roots can die.

Soil texture plays a significant role in the water infiltration process. If there is a layer of coarse-grained soil (sand) on top of a layer of fine-grained soil (clay), water accumulates in the upper layer as it infiltrates the lower layer slowly.

The amount and size of pores in the soil and the total surface area of the soil particles determine the amount of water that the soil can hold. Clay soils have greater total pore volume and particle surface area than sandy soils. It follows that clayey soils have a greater water-holding capacity than sandy soils.

If the tree is planted in clay soil with coarse soil added, the planting hole can act as a bowl that holds water. Water trapped for a long time drowns the roots. If drainage is not improved, water continues to remain in the finer textured soil until it reaches saturation.

Optimum soil moisture is one that does not fall below wilting moisture, but is not saturated with water either. When the water content in the soil is greater than the field water capacity, the soil is no longer sufficiently aerated.

Wilting point and field water capacity depend on the type of soil. Light soils (sandy) have a low wilting point and field water capacity. Medium and heavy soils (clay,  loam) have a higher wilting point and field water capacity.

2.3. Tree nutrition

Trees need and absorb 17 essential elements dissolved in water through their roots. Each basic element fulfills a specific role in the plant and cannot be replaced by another element. Trees require certain elements known as macronutrients in relatively large quantities. The most important of these macronutrients is nitrogen (N), because it is the most often limiting element, followed by phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S).

Nitrogen (N) is a component of proteins and chlorophyll, it plays a role in photosynthesis and other processes that take place in plants. Under natural conditions, soil nitrogen comes from organic material and the atmosphere. Soil organisms break down organic matter. Much of the nitrogen in the soil can be lost through leaching or volatilization. Removing fallen leaves and other natural sources of nitrogen can disrupt the cycle of nitrogen uptake into the soil. Nitrogen deficiency is manifested by slowed growth and smaller leaves. Sometimes the new, developing leaves are greener because nitrogen is moved in greater amounts in the plant to the site of the new growth. These symptoms cause a number of other problems that affect root health and nutrient uptake. Fertilisation is usually aimed at providing nitrogen, as it is an element that is usually lacking in trees. In addition to nitrogen, phosphorus, potassium and sulfur are needed. These elements are usually insufficient in urban soils. Supporting nutrients are magnesium and calcium.

Other elements that are often referred to as micronutrients (microelements – boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn), nickel (Ni) ), are also necessary for the tree. The lack of any element can have significant consequences on the health of the trees. The most widespread is chlorosis, which is caused by a lack of iron. Young leaves are small and chlorotic (yellow), with green veins, and older leaves are dark green. A lack of iron can cause the tree to die.

Soil and leaf analysis

The most appropriate way to determine the needs of the tree is a laboratory analysis of the soil and leaves. Soil analysis provides information on the presence of essential elements, pH, organic matter and cation exchange capacity. The pH value and salt content (especially in dry areas) are the most important values. When taking a soil sample, small samples should be taken from the entire root area. These samples should be mixed. The quality of any soil test depends on the representativity of the samples. The required level of individual elements is different for individual types of trees. One way to perform the analysis is to compare healthy and symptomatic trees of the same species. Leaf samples taken from the whole tree, dried and analyzed can help diagnose the deficiency of certain elements. However, soil analysis and leaf analysis alone can be misleading. There may be a situation where some mineral elements are present in small amounts in the leaves, although there are plenty of them in the soil, but they are not available to the tree due to the pH value.

2.4. Urban soils

Soils in urban areas often lack an organic layer. They may be compacted or hardened, have damaged soil profile, altered drainage, increased pH, or may have subsurface barriers due to road foundations or underground utilities. All of these factors can affect root growth and tree health and eventually cause tree death.

In many urban soils, the organic layer is replaced by lawns or built-up areas. Built-up areas can worsen aeration and water infiltration. A reduction in the content of organic material reduces biological activity, prevents the development of soil structure and disrupts the cycle of nutrients. Urban soils usually lack important microorganisms, such as mycorrhizal fungi.

Trees and the soil are in mutual symbiosis. Urban development disrupts the ecological balance and thus creates unfavorable or even antagonistic conditions. The death of trees is caused by stressful conditions in the soil. Trees need to obtain enough oxygen, water and other components from the soil in order to meet their energy requirements.

The construction of modern housing estates is usually preceded by the removal of humus topsoil from the entire area of the construction site, and not only from the built-up area.

Conclusion

We must look at the soil not just as a basic means of production in agriculture. We must consider it as one of the natural, destructible and very difficult to renew natural resources. We drew attention to a wide range of practical applications of knowledge about soil and its practical use, which will lead to the protection of the soil fund and the environment.

Pedology Quiz

Test your knowledge in the following quiz and see if you understood the contents of this section:

Literature

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