Soil

• Eutrophication

  The tap in phosphates and nitrates
• Soil

  • Provides anchorage for roots
  • Provides a medium that contains water and nutrients
  • All nutrients in biogeochemical cycles pass through soil
• Components of soil

  • Mineral skeleton (soil texture)
  • Air
  • Soil water
  • Living organisms
  • Dead organic matter
• Soil particles

  • Produced by the weathering of rocks that produce regolith
  • Clay particles are the smallest, silt is medium sized and sand particles are the largest
• Soil texture

  The proportion of sand, silt and clay particles. Affects nutrient levels, soil drainage, aeration and water content
• Air in the soil

  • Found in the spaces between the solid particles which are not occupied by water
  • Provides the gases for aerobic organisms to use in decomposition and nitrogen fixation
  • A lack of oxygen gives anaerobic conditions which leads to incomplete decomposition, leaving organic matter such as peat releasing methane gas and producing acidic conditions in which nutrient uptake is difficult
• Soil water

  • Found in the spaces between solid particles
  • Essential for plants as they can only take nutrients up that have been dissolved in water and need water for many physiological functions
• Living organisms in soil

  • Plants-roots hold soil together, dead vegetation provides nutrients and humus
  • Detritivores-e.g. worms, millipedes, woodlice, dung beetles. They break up dead organic matter, releasing nutrients. The passages they create in the soil aid drainage
  • Decomposers-e.g. bacteria and fungi. They digest dead organic matter left by detritivores
  • Nitrogen cycle bacteria-nitrogen fixation, nitrification, denitrification
  • Mycorrhizal fungi-fungal network in soil provides plant roots with nutrients in return for receiving sugars
• Dead organic matter

  • Important as a source of food for soil organisms
  • As it breaks down it release nutrients and produces humus (a complex mixture of organic materials, including organic acids and the breakdown products of lignin from wood)
  • Humus helps to hold soil together, retains water, aids drainage and acts as a thermal insulator
• Aeration

  • Well-aerated soils mean aerobic processes such as decomposition and microbial nutrient cycling occur rapidly
  • High air content reduces thermal capacity
• Water drainage, infiltration and retention

  • Some water is essential to enable nutrient absorption
  • If crops are short of water then stomata will close to prevent dehydration, preventing carbon dioxide being absorbed so photosynthesis and therefore growth, would stop
• Thermal capacity

  • The composition of the soil affects its ability to retain heat and therefore the rate at which it heats up and cool down
  • This affects how soon growth can occur in spring and the rates of chemical and biological reactions in the soil
• Soil structure (crumb, blocky, platy peds)

  • The aggregation of soil into peds affects the ease of movement of air, water and roots through the soil
  • Platy peds are large and flat-they slow drainage and reduce aeration
  • Crumb peds are small and rounded - they aid drainage, aeration and root penetration so fertility is improved
• pH

  • Extreme acidic or alkaline conditions will denature root cell proteins and kill plants
  • Acidic conditions make soil nutrients more soluble, but low pH inhibits nutrient uptake by roots, so the ideal soil acidity is 5.5-6.5 which is slightly acidic
• Soil texture

  Affects drainage, nutrient retention and root penetration, so it also affects fertility and productivity
• Soils are important because they provide food, habitat, carbon storage, a medium for crop production, a producer and absorber of gases, a medium for plant growth, a great integrator, a snapshot of geologic, climatic, biological, and human history, a home to organisms, a source material for construction, medicine, and art, and an essential natural resource that filters water and wastes
• Soil fertility

  The ability of soil to sustain plant growth i.e. to provide plant habitat and result in sustained and consistent yields of high quality
• Features that determine soil fertility

  • Water content
  • Soluble materials
  • Air content
  • Dead organic matter
  • pH
  • Soil biota
  • Soil structure
• Water content

  • Hygroscopic water, capillary water, gravitational water
  • Available water for plant growth is between the wilting point and field capacity
• Soluble materials

  • Contain macronutrients like nitrogen and potassium in ionic form as nitrate and potassium ions
  • Contain micronutrients like copper and iron
  • Toxic ions like aluminium and heavy metals are absorbed onto mineral particles so they cannot dissolve in water and harm organisms
• Air content

  Living organisms and processes that increase soil fertility are aerobic, so aerated soil is more likely to be fertile
• Dead organic matter

  Increases water retention, provides food for soil biota, and releases nutrients as it decomposes
• pH

  • Fertile soil pH is between 5.5 to 7, the range of tolerance for most plants and soil biota
  • Acidic soils increase leaching of nutrients and damage root cell membranes
  • In alkaline conditions phosphates become insoluble
• Soil organisms and their functions

  • Beetles - break up dead organic matter to increase surface area, create tunnels
  • Decomposers (bacteria and fungi) - break down dead organic matter by secreting digestive enzymes
  • Detritivores (e.g. worms) - break up dead organic matter, release nutrients, create passages for drainage
  • Nitrogen-fixing bacteria - convert gaseous nitrogen into ammonium ions
  • Nitrifying bacteria - oxidise ammonium ions to nitrite ions then to nitrate ions
  • Mycorrhizal fungi - form symbiotic relationships with plant roots, aid phosphate uptake
• Soil structure

  • Soil particles form aggregates (peds) bound by polysaccharide gums, fungal hyphae, roots, soil biota, and clay particles
  • Platy peds are large and flat, reducing drainage, aeration and root penetration
  • Crumb peds are small and rounded, aiding drainage, aeration and root penetration, improving fertility
• Soil texture

  • Controlled by the proportions of sand, silt and clay
  • Loam soils have an ideal mix of 40:40:20 sand, silt and clay for cultivating most crops, providing good drainage, water retention and high nutrient content
• Effect of particle size on soil properties

  • Sand - larger pore spaces allow rapid drainage, reducing water content but increasing aeration, poor nutrient absorption
  • Clay - tiny pore spaces allow capillary rise of water, poor aeration, high nutrient absorption
• How to assess soil texture

  Soil sieve method - dried, crushed soil placed in stacked sieves of decreasing mesh size, shaken, and the % composition of the 3 portions calculated
• Method 2 Sedimentation

  1. Larger objects removed from dried soil using 2mm sieve

  2. Soil is crushed to ensure particles are separated

  3. A measuring cylinder is about half filled with soil then topped up with water

  4. Top is sealed & cylinder is shaken repeatedly

  5. Suspension is allowed to settle
• Soil components after settling

  • Sandy soil
  • Clay
  • Silt
  • Sand
• Soils prone to wind erosion

  • Dry, especially with low clay content, likely to be loose with little cohesion between particles to hold soil together - crumb peds most susceptible
  • If windy & soil is unprotected - blow away
  • Can cause problems where soil is deposited - cover crops or end up in urban areas
• Soils prone to water erosion - surface runoff

  • Caused by surface runoff when infiltration capacity of soil is exceeded
  • Occurs from heavy or prolonged rainfall or if soil is impermeable so more water flows over ground surface
  • Platy peds are more likely to become waterlogged & cause surface runoff
• Soils prone to water erosion - slumping & landslides

  • Soil on slopes becomes very wet, the increased mass & lubrication makes downward movement of large soil more likely
  • Often occurs when deep soil on steep slopes becomes less stable from deforestation
  • Roots that help soil together decompose - lose hold on soil & landslides from heavy rain likely
  • Platy peds are prone to waterlogging which leads to slumping & landslides
• Soils prone to water erosion - rain splash erosion

  • Soil particles are dislodged by the splash of raindrops
  • Soil particles are dispersed in all directions but those going downhill will travel further
  • Overtime it can cause the downhill movement of large amounts of soil
  • Crumb peds (small soil particles) will be more susceptible to rain splash erosion
• Measuring soil components after sedimentation

  1. The total depth of the settled soil components is measured after 2 min, 2 hrs & 2 days

  2. The proportion of the total volume of each textural category can then be calculated
• Erosion

  A natural process where soil particles are removed by wind or water
• Types of soil erosion

  • Wind
  • Water
• How vegetation reduces soil erosion

  • Vegetation acts as a natural wind break > reducing wind velocity > reducing kinetic energy to carry away soil particles
  • Vegetation cover & leaf litter reduce impact of raindrops > soil particles less likely to be dislodged
  • Soil organic matter, including colloidal material humus > help bind soil particles together
  • Plant roots hold soil together
  • Plants help increase infiltration of water > reduces runoff > reduces water erosion
• Human activities that increase soil erosion

  • Vegetation removal
  • Ploughing
  • Vulnerable soils
  • Overgrazing
  • Cultivating steep slopes
  • Reduced soil biota
• Effects of soil compaction

  • Detritivores & decomposers break down DOM, releasing plant nutrients which may increase vegetation occur
  • Decomposition produces humus which increases adhesion between soil particles
  • Worms aerate the soil, increasing drainage rates & the infiltration capacity of the soil
  • Use of heavy machinery, high livestock density & reduction in soil detritivores make it more likely that soil will become compacted
  • Compacted soil has smaller interstitial spaces which reduces the infiltration rate so it's more likely that rainfall will produce surface runoff & cause erosion