Soil and Soil Fertility Notes

Intro to Soils and Soil Fertility

Formation and Properties of Soil

  • Introduction to Geology of the Caribbean
  • Soil Formation and Classification
  • Chemical Properties of Soil
  • Physical Properties of Soil

Soil Physical Properties

  • Physical properties are characteristics of soil that can be measured by physical means and expressed in physical terms.
  • Examples: color, density, porosity, hydraulic conductivity, structure, texture, and depth.

Soil Physical Properties

  1. Structure and aggregation
  2. Bulk density
  3. Total and air porosity
  4. Hydraulic conductivity
  5. Infiltration rate
  6. Water holding capacity
  7. Thermal conductivity
  8. Diffusion coefficient
  9. Air permeability
  10. Erodibility

Soil Organic Matter

  1. Pool
  2. Composition

Gaseous Emissions

  1. CO₂ emission
  2. CH₄ emission
  3. Nitrification, denitrification, and N₂O/NOx emissions.

Soil Physical Processes

  1. Crusting
  2. Compaction
  3. Runoff
  4. Erosion
  5. Aeration
  6. Nutrient diffusion
  7. Aggregation
  8. Elemental transformation
  9. Evaporation
  10. Transpiration

Soil Properties

Physical Properties

a. Horizonation
* Soil physical properties
* Soil texture

b. Soil color
c. Soil texture
d. Soil structure
e. Soil consistence
f. Bulk density

  • Soil structure
  • Surface areas
  • Soil density
  • Soil porosity
  • Soil colour
  • Soil consistence.

What is Soil?

  • Provides air, water, and nutrients to plants
  • Provides mechanical support to plants
  • Consists of weathered materials, decaying organic matter, air, and water

How is Soil Formed?

  • Decomposing animals and plants:
    • Fungi and bacteria feed on the material to break it down until it is released into the soil.
  • Rocks and minerals break down through weathering (freezing, thawing) and mechanical forces to create soil texture.

Soil Profile

  • TOPSOIL: roots, bacteria, organic matter, fungi, insects, and earthworms
  • SUBSOIL: roots, bacteria, fungi, insects, and earthworms
  • PARENT MATERIAL: limestone, bedrock, or other mineral substance

Average Soil Composition

  • 45% Mineral
  • 5% Organic Matter
  • 25% Air
  • 25% Water

Soil Components

  • Microcolonies of bacteria
  • Quartz
  • Air
  • Organic matter
  • Clay particle
  • H₂O
  • The complexity of soil

Horizonation

  • Soil “horizons” are discrete layers that make up a soil profile.
  • Typically parallel with the ground surface
  • Show evidence of the actions of the soil-forming processes

Layers of Soil

  • O HORIZON: Organic, Loose and partly decayed organic matter
  • A HORIZON: Topsoil, Zone of leaching
  • B HORIZON: Subsoil, Accumulation of clay transported from above
  • C HORIZON: Parent material, Partially altered parent material
  • Bedrock: Parent material

Soil Color

  • Aerated soils, oxidized or ferric (Fe^{+3}) iron compounds: brown, yellow, and red colors
  • Iron reduced to the ferrous (Fe^{+2}) form: gray color
  • Reduced iron color persists in shades of green or blue.
  • Upon aeration, reduced iron can be reoxidized and redeposited, sometimes creating a variegated or mottled color pattern.
  • “Redoximorphic features” indicate an anaerobic state, ranging from brown with a few mottles to complete gray or “gleization” of the soil.
  • Brown or yellow mottles indicate hydric conditions.

Soil Texture

  • Sand is the largest particle size, allowing for more air and water movement.
  • Clay soils are heavy and hold a lot of water.
  • Loamy soils are intermediate between sand and clay, providing both water-holding capacity and fertility.

Soil Texture

  • Different sized mineral particles give soil its texture

    • Sand
    • Silt
    • Clay

  • Worms help water flow through the soil!

Soil Texture Size Ranges

  • Sand = <2 to 0.05 mm
  • Silt = 0.05 to 0.002 mm
  • Clay = <0.002 mm
  • Sand and silt are the “inactive” portion of soil.
    • Do not contribute to a soil’s ability to retain soil water or nutrients.
    • Comprised of quartz or some other inactive mineral.
  • Clay has a large amount of surface area per unit mass due to its small size and sheet-like structure.
    • Surface charge attracts ions and water.
    • Clay is the “active” portion of the soil matrix.
  • In mineral soils, the proportion of sand, silt, and clay always adds up to 100 percent.
  • Percentages are grouped into soil texture “classes”, which have been organized into a “textural triangle”.

Soil Texture & Associated Permeability

  • Sand: Fast Permeability
  • Sandy Loam: Moderate Permeability
  • Clay: Very Slow Permeability

Soil Structure

  • Soil separates can become aggregated together into discrete structural units called “peds”.
  • Peds are organized into a repeating pattern that is referred to as soil structure.
  • Between the peds are cracks called “pores” where soil air and water are conducted.
  • Soil structure is described in terms of the shape of the individual peds that occur within a soil horizon.

Description of Structure Shape

  • Granular: Roughly spherical, like grape nuts. Usually 1-10 mm in diameter. Most common in A horizons, where plant roots, microorganisms, and sticky products of organic matter decomposition bind soil grains into granular aggregates
  • Platy: Flat peds that lie horizontally in the soil. Platy structure can be found in A, B, and C horizons. It commonly occurs in an A horizon as the result of compaction.
  • Blocky: Roughly cube-shaped, with more or less flat surfaces. If edges and corners remain sharp, it is called angular blocky. If they are rounded, it is called subangular blocky. Sizes commonly range from 5-50 mm across. Blocky structures are typical of B horizons, especially those with a high clay content. They form by repeated expansion and contraction of clay minerals.
  • Prismatic: Larger, vertically elongated blocks, often with five sides. Sizes are commonly 10-100mm across. Prismatic structures commonly occur in fragipans.
  • Columnar: The units are similar to prisms and are bounded by flat or slightly rounded vertical faces. The tops of columns, in contrast to those of prisms, are very distinct and normally rounded.
  • Single grain/structureless

Soil Consistence

  • Ease with which an individual ped can be crushed by the fingers.
  • Depends on soil moisture content.

Moist Soil:

  • Loose: Non-coherent when dry or moist; does not hold together in a mass
  • Friable: When moist, crushed easily under gentle pressure between thumb and forefinger and can be pressed together into a lump
  • Firm: When moist, crushed under moderate pressure between thumb and forefinger, but resistance is distinctly noticeable

Wet Soil:

  • Plastic: When wet, readily deformed by moderate pressure but can be pressed into a lump; will form a “wire” when rolled between thumb and forefinger
  • Sticky: When wet, adheres to other material and tends to stretch somewhat and pull apart rather than to pull free from other material

Dry Soil:

  • Soft: When dry, breaks into powder or individual grains under very slight pressure
  • Hard: When dry, moderately resistant to pressure; can be broken with difficulty between thumb and forefinger

Consistency of Soil

  • Volume (v)
  • Water Content (w)
  • Solid, Semi-Solid, Plastic, Liquid

Bulk Density

  • Bulk density is the proportion of the weight of a soil relative to its volume.
  • It is expressed as a unit of weight per volume and is commonly measured in units of grams per cubic centimeters (g/cc).
  • Indicator of the amount of pore space available within individual soil horizons
  • Inversely proportional to pore space: Pore space = 1 – \frac{bulk density}{particle density}
    • For example, at a bulk density of 1.60 g/cc, pore space equals 0.40 or 40%. At a bulk density of 1.06 g/cc, pore space equals 0.60 or 60%.

Bulk Density Determination

  • Soil is made of solids and pore spaces.
  • To calculate Bulk Density:
    • Bulk Density = \frac{Weight of Soil}{Volume of Soil}
    • For example, let's assume we have 1 cubic centimeter of soil that weighs 1.33 grams.
      • Bulk Density = \frac{1.33 grams}{1 cm³} = 1.33 grams/cm³

Bulk Density

  • The ratio of oven-dried soil (mass) to its bulk volume (g/cm³).
  • Range: 1.0 to 1.7 g/cm³.
  • Used to convert soil water content in percent by weight to percent by volume.
  • Used to calculate porosity.
  • Calculation: BD = \frac{Oven-Dry Soil Weight}{Core Sample Volume}
  • Indicator of: Compaction, aeration, root growth, microbial activity, infiltration, and drainage.

Growth Factors: What do plants need to grow?

  • Light
  • Water
  • Nutrients
  • Oxygen
  • Carbon Dioxide
  • Temperature

Soil Structure and Infiltration

  • Regular shapes, Large particles leave large pores, Infiltration
  • Irregular shapes, Small particles leave small pores, Ponding or runoff
  • Well-structured, aggregated soil
  • Dispersed clay platelets

Soil Management

  • What are your soil uses?
  • SOIL TEST - DON’T GUESS!
  • What are the needs of your plants?
    • pH
    • Fertility
  • Compaction
  • Soil Depth
  • Slope

Guidelines for Soil Sampling

Overview

  • Soil test values are no better than the soil samples you collect.
  • Proper soil sampling procedures must be followed to obtain meaningful test results for fertilizer decisions.

Guidelines

  • The best guideline for determining fertilizer needs is a reliable analysis of a soil sample that is representative of the field.
  • Proper procedures must be followed to collect representative soil samples.

Why Soil Test?

  • Determine the average nutrient status in a field
  • Obtain a measurement of nutrient variability in the field

Sampling the Management Units

  • Proper Sampling Depth
    • Surface (tillage layer) samples are used for determining soil pH, lime need, organic matter, phosphorus, potassium, sulfur, and zinc.
    • Soil test correlations and calibrations for these tests are based on surface samples.
    • Usually, the tillage layer is considered to be the 0-6 inch or the 0-8 inch depth. It is best to use the same sampling depth from year to year so soil test values can be more accurately compared.

General Guidelines

  • Proper random sampling can provide an accurate picture of the average nutrient level in the field.
  • Grid sampling can provide an opportunity to obtain even more information. If individual samples from a grid sampling pattern are analyzed separately, they can be used to produce nutrient level maps of the field.