introduction to soils

• Minerals include: sand, silt, clay and mineral nutrients, makes up 50%. 
• Air is held in the soil pores. Makes up 20-30 %.
• DOM includes dead leaves, roots, dead animals and faeces. 
• Biota includes: detritivores and decomposers. 
• Soil fertility is the ability of soil to sustain plant growth. 
• Things that effect soil fertility:

  • Inorganic nutrients 
  • Water
  • Organic content 
  • Aeration 
  • pH 
  • Texture 
• Layers of soil:

  • Humus 
  • Topsoil - rich in humus 
  • Subsoil - limited humus, rich in minerals 
  • Weathered rock fragments - highly compacted 
  • Bedrock 
• Inorganic nutrients, plants can only absorb these. There are two sources: weathered from the soil and from decomposition of DOM. There are two main limited nutrients: nitrogen and phosphate. Nitrogen in needed for protein synthesis and in chlorophyll. Phosphate is used for ATP synthesis. 
• Geology determines the mineral composition of the soil via weathering. 
• Nutrients are attracted to clay particles because it has a negative charge, this means positively charged metal ions are attracted. These ions and minerals are dissolved into the water and can be absorbed by plant roots. 
• Process for measuring soil nutrients:

  • Add distilled water to a sample 
  • Shake and leave to stand for nitrate to come into solution
  • Dip the test strip 
  • Wait for colour change 
  • Compare to a control strip. 
• Soil water is important for nutrient uptake, volume of soil water is determined by infiltration and water retention of the soil. Rainfall, evaporation, texture, DOM and density of surface vegetation effect the volume of water stores in pore spaces. 
• Organic content is made of dead organisms like leaves, roots or insects. Its decomposed by bacteria that release inorganic nutrients. Humus is partially decomposed DOM. 
• Aeration is important because air is needed for: decomp, nitrogen fixation, nitrification and soil detritivores. Nitrogen is needed for nitrogen fixation. Aeration is affected by texture (sand etc) and soil organisms burrowing. 
• The pH of soil dictates the macronutrients that are available. Potatoes like acid soil, cabbage likes alkaline. H+ ions are exchanged for mineral nutrient ions. In alkaline soils there are limited H+ ions so there are low nutrient levels. pH of 6.5 are the optimum pH for cation exchange. Strongly acidic soils release too many cations and are leached from the soil. 
• Soil texture is the percentage of silt and clay particles. Sand is large and round, create large pore spaces and are not charged. Clay particles are small and flat, create small pore spaces and are negatively charged. 
• Sand soils do not hold/retain much water.
• Clay soils hold a lot of water. 
• Adsorption - moves onto the surface 
• Absorption - moves into