Soil Biology and Organisms

Biological Characteristics of Soil

• Soil organisms add nutrients in the form of faeces
• Soil organisms decompose organic matter to humus (humification)
• Soil organisms add organic matter when they die

Soil Biomass

• Soil biomass is the total amount of organic matter living in the soil

Types of Soil Organisms

• Macro-organisms: earthworms, plants, rabbits
• Micro-organisms: bacteria, fungi
• Bacteria:
  • Convert organic matter into humus
  • Convert nitrogen into usable forms
• Fungi:
  • Responsible for humification of soil organic matter
  • Some have a symbiotic relationship with other organisms

Definitions

• Carbon Footprint: the total amount of greenhouse gases produced directly and indirectly by a farmer
• Symbiotic Relationship: a relationship where two or more organisms live in close contact, where at least one benefits, and neither are harmed
• Mutualistic Relationship: a relationship where both organisms benefit
• Parasite: an organism that lives on or in a living host, usually causing harm
• Saprophyte: an organism that feeds off dead organic matter
• Rhizosphere: the area around a plant root where the biological and chemical aspects of the soil are influenced by the root

Earthworms

• Decompose organic matter to humus and recycle nutrients
• Mix soil layers
• Improve drainage and aeration
• Improve fertility
• Improve soil structure

Conditions Required for Soil Organisms

• pH 6-8: add lime to raise pH
• Moist soil: not too dry or waterlogged
• High levels of organic matter: spread FYM or other organic manures
• Warm soil: not too cold (above 10 degrees)
• Well-aerated soil: cultivate soil

Carbon Cycle

• Steps in the carbon cycle:
  • Photosynthesis
  • Respiration
  • Consumption
  • Decay
  • Combustion
• Farmers reduce their carbon footprint by:
  • Planting more trees (afforestation)
  • Reducing diesel usage
  • Growing biofuels
  • Not burning straw
  • Minimum tillage

Organic Carbon

• To calculate Soil Organic Carbon (SOC), multiply the percentage of Soil Organic Matter (SOM) by 0.58

Nitrogen Cycle

• Nitrogen takes multiple forms: nitrogen gas (N2), nitrate (NO3-), ammonium (NH4), nitrite (NO2-)
• Processes:
  • Nitrogen fixation
  • Decomposition
  • Nitrification
  • Denitrification
• Bacteria role in the nitrogen cycle:
  • Decomposes OM to humus for energy and recycles nutrients
  • Converts nitrogen (N2) to nitrate (NO3-) (nitrogen fixation)
  • Converts ammonium (NH4) to nitrite (NO2-) then to nitrate (NO3-) (nitrification)
  • Converts nitrate (NO3-) to nitrogen gas (N2)

Flocculation

• Flocculation is the clustering of soil particles to form larger structures called floccules, which improves soil structure.
• Cations such as Ca2+ and Mg2+ promote flocculation by being attracted to the negative charges on soil particles.
• Soil particles have a negative charge, with smaller particles having more negative charges.
• Cations are adsorbed to the surface of soil particles, and polarized water holds soil particles together, trapping larger sand and silt in a floccule structure.
• High humus content is more beneficial than high clay content because humus is more fertile, holds more water, has a higher Cation Exchange Capacity, encourages more earthworm activity, and has more negative charges, resulting in an increased rate of flocculation.

Cation Exchange

• Cation exchange is the ability of the soil to adsorb cations onto its surface and exchange them for other cations.
• Cations are attracted to the negative charge of clay/humus particles and are adsorbed on the surface.
• Cations can be released into soil water (soil solution) and be replaced by others, supplying minerals to plants.
• A cation with a +2 charge (Ca2+) can replace two separate cations each with +1 charge (H+).
• Cations are removed from soil solution by plant roots.
• Colloidal humus particles have three times higher cation exchange capacity than colloidal clay particles.
• Adding organic matter to a soil with poor CEC (like sand) will increase its CEC.
• Main cations involved in cation exchange are Ca2+, Mg2+, Al3+, H+, and K+.
• Note that nitrate (NO3-) cannot be held, and sandy soils have the lowest CEC.

Soil pH

• Soil pH is the acid-base scale, measuring the concentration of H+ ions in a solution.
• Acidic soils have a pH of 0-6, neutral soils have a pH of 7, and basic soils have a pH of 8-14.
• The concentration of acidic ions adsorbed onto the surface of the soil determines the acidity of the soil.
• Acidic ions include H+ and Al3+, while alkaline/basic ions include Ca2+ and Mg2+.
• Importance of Soil pH:
  • Lime increases Ca cations available for exchange (H, K, Al).
  • As pH increases, the amount of negative charges on soil colloids increases, leading to greater CEC.
  • Liming reduces ‘acid leaching’.

This quiz covers the biological characteristics of soil, including the benefits of soil organisms, soil biomass, and types of soil organisms such as macro-organisms and micro-organisms.