Soil Properties and Their Importance in Agriculture

Properties of Soil

General Properties of Soil

  • The properties of soil arise from:

    • Formation and Composition

    • Described by soil type and soil texture

    • Condition

    • Natural vs. human-influenced aspects

  • Soil properties classified as:

    • Primary Properties

    • Result solely from the soil itself

    • Secondary Properties

    • Describe the usability for human activities, such as agriculture

Soil Type and Properties

  • The relationship between soil type and its properties is crucial in agricultural management.

Composition of Soil

  • Key components include minerals (sand, silt, clay), organic matter, water, and air.

Important Soil Properties

  1. Humus Content

  2. Soil Structure

  3. Porosity

  4. Water Management

  5. Air Management

  6. pH Value and Buffering Capacity

  7. Root Extent

  8. Workability

  9. Stability

  10. Soil Fertility

Humus: Definition and Importance

  • Humus (in a broader sense): Total dead organic substance in soil.

  • Humus (in a narrower sense): Decomposed organic matter in the soil.

Functions and Benefits of Humus

  • Protects against water erosion by allowing rainwater to infiltrate evenly and deeply.

  • Reduces wind erosion.

  • Produces crumb structure through slime and adhesive substances.

  • Nourishes soil organisms, such as earthworms.

  • Mitigates temperature fluctuations:

    • Lowers temperatures in summer

    • Raises temperatures in winter

  • Provides nutrients to plants through gradual release.

  • Increases water retention capacity and reduces evaporation.

  • Buffers chemical changes in the soil with the addition of lime and organic fertilizers.

  • Releases organic acids that:

    • Neutralize alkaline soils.

    • Release minerals.

  • Stores ionic nitrogen compounds in exchangeable and usable forms.

Types of Humus

Classification Based on Macroscopic Features

  • Allows assessment of water and nutrient management as well as biological activity in the soil.

  • Forms of Humus:

    1. Mull

    • Strongly associated with clay mineral substances (Clay-Humus Complex).

    • Not easily washed away or blown away.

    • Contributes significantly to a high void-rich stable crumb structure of basic soil types.

    • Formed from biological processes and differs significantly from mechanically loosened soil.

    • Predominately creatures such as earthworms contribute to its formation.

    • Humus substances are highly polymeric and thus exhibit low mobility.

    1. Moder

    • Typical of very light sandy soils.

    • Humus particles loosely lie between sand grains and are subject to washout.

    • Fewer earthworms; arthropods predominate.

    1. Raw Humus

    • Most unfavorable form of humus.

    • Consists of difficult-to-decompose plant residues that form litter above the mineral soil.

    • Often found in biologically inactive soils, with poor decomposition of plant residues (e.g., conifer needle litter).

Types of Humus Based on Function

  • Classification of humus based on functionality:

    1. Nutrient Humus

    • Easily decomposable organic substances serving as a food source for soil organisms, essential for biological activity.

    • Incorporating this material improves aeration and promotes nutrient turnover.

    1. Permanent Humus

    • Decomposes very slowly, formed from organic substances with the help of soil organisms.

    • Creates the colloidal substance of soil with clay, binding water and nutrients before releasing them to plants.

    • Represents over 90% of the organic substance in soil, also contributes significantly to soil nitrogen.

    • This humus gives the dark color to the humous upper soil layer, facilitating surface warming.

Conversion Processes of Humus

  • Rearrangement of the humus content occurs, emphasizing establishing a
    site-typical humus level where decomposition outpaces formation, leading
    to living organic matter developing into Nutrient Humus and eventually Permanent Humus.

Organic Mass

  • Consists of the deceased remnants of plants, animals, and microorganisms.

  • Nutrient Humus: comprises easily degradable compounds such as carbohydrates, amino acids, and soluble proteins.

  • Permanent Humus: contains hard-to-decompose compounds like lignin, fats, and humic substances.

Humus Content

Influencing Factors

  • Amount of delivered dead organic substance

  • Rate of decomposition process (humus formation and mineralization)

Typical Humus Contents

  • Light sandy soils: 0.5 – 1.5%

  • Chernozem soils: 1.8 – 3.5%

  • Moist grassland soils: 5 – 8%

Relationship between Fine Parts, Precipitation, and Humus Content

Nitrogen Storage in Humus

  • Humus serves as a nitrogen source in soil.

  • Microbial Decomposition: Releases nitrogen from organic binding (mineralization) making it available for plants.

  • Humus quality is assessed based on carbon/nitrogen ratio (C/N).

C/N Ratio

  • Ideal conditions: 8 to 10:1 for agricultural soils.

  • Organic fertilizers and crop residues generally have higher C/N ratios; additional nitrogen is required for humus formation, which can slow the humification process (e.g., straw from grains ranges from 50-120:1).

  • Supplemental mineral nitrogen fertilization can accelerate this process.

Management of Humus

  • Humus Management is crucial on agricultural and forestry lands:

    • Maintaining humus levels prevents:

    • Decrease in nutrient and water retention of soil

    • Increased erosion risk

    • Reduced soil stability (increased compaction risk and difficult tillage)

Methods of Augmenting or Reducing Humus

  • Increasing Humus through:

    • Retaining straw on fields

    • Targeted planting of cover crops

    • Adding organic matter from animal production

  • Decreasing Humus through:

    • Removal of harvest products from fields

    • Increased mineralization from intensive tillage, lowered groundwater, climate changes

    • Erosion and the leaching of humic substances.

Humus Balances

  • Regular testing of humus levels and targeted accounting of humus gain and loss are essential for effective humus management..

  • This practice has been in effect in the former GDR since the 1980s and, in West Germany, since 2004 (Cross-Compliance).

Nitrogen and Humus Balance (in kg N or dt ROS)

Exercise Humus Balancing

  • Objective of the exercise is to train on carrying out a humus balance.

  • Task: Perform a humus balance for the crop rotations presented.

Key Points on Soil Properties

  • Humus Content

  • Soil Structure

  • Porosity

  • Water Management

  • Air Management

  • pH Value and Buffering Capacity

  • Root Extent

  • Workability

  • Stability

  • Soil Fertility

Soil Structure

  • Soil structure refers to the spatial arrangement of solid soil components.

  • The remaining aspect concerns porosity.

  • Soil structure significantly influences:

    • Soil water and air balance

    • Root penetration

    • Nutrient availability

    • Plant physiological properties

    • Stability and workability

  • Generally, the coarser the structure of soil or the denser the individual structural units are packed, the poorer the soil properties.

Structural Types

  1. Primary Structure (Basic Structure)

    • Individual Grain Structure (Elemental Structure): Primary particles without cohesion (sands, silts).

    • Cohesive Structure: Primary particles held together by cohesive forces, forming an undifferentiated mass (clay, silt, loam sediments).

  2. Aggregate Structure (Crumb Structure)

    • Soil particles combined via physical or biological processes into identifiable, heterogeneous structures.

    • Segregate Structure: Formed from drying and shrinkage in fine cohesive structures (clayey subsoil).

Clay-Humus Complex

  • Clay minerals and humic substances are soil colloids.

  • Humic substances can bind individual soil particles into a crumb structure and stabilize the soil structure.

Formation and Significance

  • Clay minerals and humus acid residues from humus particles are negatively charged and bond with multivalent cations like Ca++ and Mg++ to form a neutralizing bridge.

  • Effects include:

    • Stabilizing soil against erosion and slumping.

    • Creating high porosity, aiding water and air management.

    • Storage and even release of nutrients.

    • High elasticity of the formed crumb structure ensuring lasting fertility.

Cation Exchange Capacity (CEC)

  • Definition: The CEC indicates how many cations a soil can hold (mmol / 100 g of soil).

  • Affinity of the participating cations (dependent on pH, charge, and thickness of the hydration layer): Al3+ > Ca2+ > Mg2+ > K+ = H3O+ > Na+.

Factors Affecting CEC

  1. pH of the Soil:

    • The more acidic the soil, the lower the CEC.

    • H+ and Al3+ ions bind more firmly, blocking nutrient cation spots.

  2. Soil Texture:

    • Sand is primarily quartz (SiO2), has a low specific surface area, hence low storage capacity.

    • Clay has a high specific surface (up to 800 m²/g), can bind more cations.

    • Organic matter (humus) boasts a very high specific surface area (800 - 1000 m²/g).

Effective and Potential CEC

  • Potential CEC: Maximum exchangeable amount of cations under optimal conditions in humid climates at a pH of 7 – 7.5.

  • Effective CEC only equals potential CEC under optimal hypotheticals.

  • Base Saturation:

    • The ratio of basic cations to all cations.

    • Over 35% signifies good base saturation.

Porosity

  • Defines the portion of soil volume not constituting solid material (mineral and organic).

  • Essential for water and air management in soil.

  • An optimal ratio of solid material to air to water is generally viewed as 50:25:25.

Types of Pores in Soil

  • Coarse Pores: Medium diameter: >10µm; contain the drainable water in soil, filled with water only in saturated conditions.

  • Medium Pores: Medium pore diameter: 0.2µm - 10µm; filled with air during soil drying.

  • Fine Pores: Medium pore diameter: <0.2µm; contain non-plant available water and dead water.

Water Management

  • Soil Water: All water in the soil excluding crystallized water of soil minerals.

  • Determined gravimetrically by drying the soil at 105°C.

  • Types of water in soil:

    • Freely Movable Water : Drainage water

    • Groundwater: Water retained in pores counter to the force of gravity (adhesion water).

    • Capillary Water: Readily available to plants.

    • Adsorption Water: Not available to plants (10-15 bar).

    • Flood Water: Water retained due to an impediment above 1.3m depth.

Field Capacity

  • Definition: Maximum amount of water a soil can hold against gravity where air is still present in pores.

  • Field capacity is dependent on:

    • Pore size: >50 µm cannot hold water through capillary forces.

    • Humus content and soil type : Determine adsorptive capacity.

Water Balance

  • Water Balance Equation: Total water inflow = Total water outflow + buffering function of the soil.

  • Components of water inflow: precipitation, capillary rise, irrigation.

  • Components of water outflow: evaporation, drainage (surface and subsurface).

Essential Measurements for Water Management

  • Determining active soil moisture crucial for timing irrigation.

Impact on Plant Development

  • Insufficient moisture negatively affects development leading to stress and reduced yields.

  • Appropriate moisture levels promote optimal growth; excessive moisture leads to oxygen deficiency.

Soil Investigation

  • Examination of soil regarding physical, biological, and chemical properties; important for determining fertility.

  • Methods can include laboratory analysis, physical sampling, and advanced sensor techniques.