AGRI 103: Agronomy - Soils

AGRI 103: Agronomy - Soils

Importance of Soil

  • Quote by Paul Harvey: "Despite all our accomplishments, we owe our existence to a six-inch layer of topsoil and the fact it rains."

  • Soil provides a vital medium for plant roots to anchor and absorb:

    • Water

    • Oxygen

    • Nutrients essential for plant growth and development.

Composition of Agricultural Soils

  • Productive agricultural soils are typically composed of:

    • Half solids

    • Half pore space: this portion includes animal and plant residues in various decomposition stages.

  • Soils are composed of essential elements required for plant growth.

  • Various physical, chemical, and biological properties of soils affect plant growth.

Soil Texture

  • Definition: Fineness/coarseness of soil, determined by the proportion of different sizes of mineral soil particles (sand, silt, and clay).

  • Soil texture determines relative size of particles:

    • Sand

    • Silt

    • Clay

  • Soil Textural Triangle: Used to classify soil texture based on sand and clay proportions according to the Canadian system of soil classification.

Properties of Soil Particles

  • Sand and Silt:

    • No surface charge.

    • Do not bind to other particles.

    • Have relatively low surface area.

    • Chemically inert.

    • Contribute few nutrients to plants.

  • Clay:

    • Has a negative surface charge.

    • Crystalline and layered structure.

    • Chemically reactive.

    • Provides nutrients for plant growth.

Soil Organic Matter (OM)

  • Definition: Portion of the soil that includes animal and plant residues in various decomposition stages.

  • Varies greatly with climate, as moisture and temperature affect productivity and decomposition rates of residues.

  • Grassland soils tend to have high organic matter content.

  • Soil zones in Saskatchewan are separated by color (Black, Dark Brown, Brown, etc.) based primarily on OM content.

  • OM is chemically reactive, having a negative surface charge that attracts water and nutrients.

Functions of Soil Organic Matter

  • Source of essential nutrients:

    • Nutrients bound to organic matter particles or released during decomposition.

  • Soil aggregation and structure:

    • OM acts as a binding agent, joining mineral portions of soil together, which is vital for soil tilth and aeration.

  • Water storage:

    • OM absorbs water like a sponge, significantly aiding in water retention for plant growth.

  • Carbon sequestration:

    • Carbon is a major component of OM, linking OM to the global carbon cycle.

Soil Structure and Aggregation

  • Productive agricultural soils often exhibit a granular structure:

    • Clusters of soil particles are bound by organic matter and clay.

    • Well-aggregated soils maintain good aeration and tilth, promoting root growth.

  • Compaction:

    • Caused by excessive traffic and tillage, leads to reduced pore space and aggregation, impeding vertical water movement and plant root penetration.

Soil Pore Space

  • Productive soils are characterized by:

    • Not being densely packed, thus containing pore spaces filled with air and water.

  • Soil Saturation:

    • When water fills 100% of pore spaces.

  • Field Capacity:

    • Maximum water retention due to adhesion-cohesion forces.

  • Permanent Wilting Point:

    • The point at which water is tightly bound to particles and unavailable for plant uptake.

  • Gases diffuse from the air into the soil, ensuring aerobic conditions unless saturated with water.

Soil Layers (Horizons)

  • Soil forms in layers due to environmental responses and geological material deposition, known as Pedogenesis.

  • These layers are referred to as soil horizons, differing in:

    • Color

    • Structure

    • Texture

    • Chemical and Physical properties.

Soil Horizons

  • O Horizon: Organic horizon (peat), typically water-saturated, found in bog vegetation.

  • LFH Horizon: Accumulation of organic materials like leaves, twigs, and woody material over mineral soil, common in forest regions.

  • A Horizon: Surface mineral horizon undergoing the most pedogenic processes, often accumulating organic matter, hence darkening the soil.

  • B Horizon: Forms from materials leached from the A horizon or changes in parent material.

  • C Horizon: Contains little to no evidence of pedogenic activity (parent material).

  • Chernozemic Order: A specific classification representing rich organic soils.

Soil Orders

  • Environmental conditions can lead to distinctive soil processes, resulting in unique soil horizons.

  • Soil classification follows the Canadian System of Soil Classification, with levels comprising:

    • Order

    • Great Group

    • Subgroup

  • Podzolic Order: A specific order of soil recognized in Canada representing specific soil horizons and processes.

Water-Holding Capacity of Soil

  • Different soil types exhibit varying water-holding capacities:

    • Soil texture is the largest influencing factor.

    • Sandy soils have larger pore spaces, allowing high saturation points but lower field capacities.

    • Clay soils retain more water at field capacity because of small pore sizes, enabling stronger adhesion-cohesion due to charged particle surfaces.

    • Silty soils can hold the most plant-available water due to weaker binding.

    • Soil organic content also influences water-holding capacity.

Cation Exchange Capacity (CEC)

  • Definition: The capacity of soil to hold positively charged ions (cations) due to the negative charges present on clay particles and organic matter.

  • Soils with a high CEC are generally more fertile and can hold a greater quantity of nutrients.

Soil pH

  • Optimal soil pH for most field crops is between 6-8.

  • Soil pH influences nutrient availability and affects the growth of beneficial soil organisms (e.g., Rhizobia).

Salinity

  • Salinity arises from the accumulation of soluble salts (e.g., sodium, calcium, magnesium, potassium) that dissolve in water.

  • Typically found in dry regions where salts evaporate and accumulate, leading to:

    • Plant dehydration

    • Breakdown of soil structure

  • Most crops show intolerance to high salinity levels.

Soil Biological Component

  • Soils host numerous organisms that play essential roles in:

    • Interactions with plants

    • Decomposing plant and animal residues

    • Affecting organic matter accumulation and nutrient cycling.

Soil Microorganisms

  • Classified into:

    • Bacteria

    • Actinomycetes

    • Fungi

    • Algae

    • Protozoa.

  • Some microorganisms are pathogenic while others, like mycorrhizal fungi and Rhizobia, form beneficial symbiotic relationships with plants.

Importance of Soil Microorganisms

  • They contribute to sustainable crop production and the organic matter content within soil.

  • A teaspoon of soil can contain more microorganisms than there are people on earth; up to 10 billion bacterial cells may inhabit each gram of soil in the rhizosphere.

Arthropods in Soil

  • Include insects, millipedes, and centipedes:

    • Responsible for breaking down organic matter and residues.

    • Can be crop pests (e.g. cutworms, root maggots) but also beneficial (predators of pests, weed seed eaters).

Earthworms

  • Function to:

    • Break down crop residues and promote decomposition.

    • Mix mineral soil with organic matter.

    • Excrete soil organic matter “casts” that contribute to aggregation.

    • Create tunnels that aerate soil and facilitate water and root movement.

Soil Fertility

  • Definition: The capability of a soil to supply essential nutrients or elements vital for plant health.

  • Naturally fertile soils are derived from minerals rich in essential elements, whereas others maintain high nutrient levels due to organic matter (e.g., tallgrass prairie soils).

  • Over time, most soils will require fertilization for maximum crop yields.

  • Well-managed soils, particularly with higher clay and organic matter content, correlate with greater inherent soil fertility, moisture retention, and nutrient supply.

Relationship Between Soil Nutrients and Crop Yields

  • A strong relationship exists between levels of soil nutrients and crop yields for different crops and varieties, demonstrated in a yield curve indicating diminishing returns as nutrients increase.

Diminishing Returns in Soil Fertility

  • Law of Diminishing Returns: States that increasing one factor of production will yield a point where adding more will reduce the returns on output.

References

document: 84107-NitrogenFertilizationinCropProduction_ - _02-17 (2).pdf

  • Please refer to the Province of Manitoba | Agriculture for revised nitrogen fertilizer guidelines relevant to Wheat, Barley, and Canola.