Soil, Nutrients and Water

Chemical and Physical Properties of the Soil

Physical Properties

  1. Texture: The amount of sand, silt and clay in the soil. Good soil should have all three of these. Any soil which contains all three is called a LOAM. Clay holds minerals and water in the soil and binds the soil together. Sand allows good drainage to occur. Too much clay will allow too much swelling and shrinking of the soil to occur and make it difficult for water to enter and plant roots to penetrate as well as being difficult to cultivate (very hard). Too much sand will mean that the soil will not retain enough water.
  2. Profile: The vertical arrangement of the layers in a soil profile (horizons). Most good soils will have well developed horizons (O,A,B.C)
  3. Structure: The arrangement of the particles in a soil. The particles should be arranged into well defined PEDS and these should form distinct structures. This holds all particles together and allows water and air to enter the soil, makes it easier for plant roots to penetrate and helps to prevent erosion (if the particles are bound together it is more difficult to wash or blow them away – excessive cultivation will break down peds).
  4. Water level: The amount of water in the soil. This should be between Field Capacity and Permanent Wilting Point, so that there is sufficient water for plants to access without drowning. Field capacity is the amount of water a soil can hold after saturation and drainage for 2 days and Permanent Wilting Point is the level at which plants can no longer extract water form the soil.
  5. Air Level: The amount of air in the soil. For good plant growth and microbe activity there should be sufficient air in the soil. Very compacted or waterlogged soils will not have enough air present.
  6. Organic Matter: The amount of organic matter (dead plants, and animals, faeces and urine) in the soil. There should be a reasonable coverage ofOM on the soil. This has many beneficial effects , eg reduces soil temperature variations, retains water (reduces evaporation) returns nutrients to the soil, decomposes to humus which helps soil structure (acts like glue) helps to prevent erosion, provides spaces in the soil for plant roots. There are some disadvantages particularly of excessive amounts ofOM in the short term, it blocks machinery making it difficult to cultivate or sow paddocks, as it decomposes it reduces the amount of nitrogen in the soil, but ultimately increases the nitrogen after decomposition, reduces the pH of the soil leading to acidification.
  7. Salt Level: The amount of salt in the soil. Small amounts of salt are required in the soil, but excessive amounts will become toxic to plants and lead to salinity.
  8. Microbe Levels: The amount of microbial activity in the soil. Microbes breakdown organic matter. The greater the microbe level the better, breaks down OM faster thus freeing the nutrients for plants and not leaving excessive amount ofOM on the soil surface.