Leaving Certificate Agricultural Science

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Leaving Certificate Agricultural Science
Soil 4 Soil Properties
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Humus

• Humus is the product of the breakdown of organic
  matter.
• It is the remains of dead animals and plants.
• Fully decayed material forms CO2, water and
  mineral salts.
• But the incomplete product is a dark sticky
  material called humus.
• Humus in the soil absorbs the water and can
  greatly improve the quality of otherwise poor
  (sandy) soils.

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Humus
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Humus

• Humus is useful to the soil because
• It contains minerals (the type and amount depend
  on the source of the humus).
• It holds minerals in the soil due to its high
  Cation exchange capacity.
• It improves and strengthens the crumb structure
  of heavy soils.
• It forms clay humus complexes, which improves
  soil stability.
• Its dark colour improves the warming capabilities
  of the soil (i.e. it is able to absorb more heat
  from the sun)
• Can make the soil more acidic (an advantage and
  disadvantage depending on the use of the soil).

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Humus
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Soil Micro organisms

• The micro-organisms in the soil include bacteria,
  fungi and algae but bacteria are the most
  important group.
• The most important function of bacteria is the
  breakdown of organic matter into humus and play a
  major role in the Nitrogen carbon cycle.

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The Earthworm and its role in the soil

• The earthworm plays a major role in the structure
  of the soil.
• The number of earthworms is an indication of the
  fertility of the soil.
• In good (fertile) soil there may be as many as
  150-200 worms per metre squared.
• That is the same as 1.5 2 million worms per
  hectare.

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The Earthworm and its role in the soil

• Earthworms improve the soil in the following
• ways
• They eat their way through the soil and mix the
  ingested material with mucus in their guts. This
  helps to improve soil crumb structure.
• Depositing soil in different places and mixing
  horizons.
• Improve drainage of heavy clay soils
• Introduces more air into the soil.
• When they die the further increase the amount of
  organic matter.

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The Earthworm
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Soil Water

•  Water is important in the soil for some obvious
• reasons
• Required for the growth of plants and animals
  (like earthworms and even bacteria)
• All chemical reactions that take place in the
  soil require water for them to proceed.
• But too little or too much water can be harmful
  to the soil and its inhabitants.
• It is the worlds best known solvent.
• It forms salts readily with chemicals found in
  parent rock these are then taken up by the plant
  roots.

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Soil Water
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Soil Water

• There are four extreme conditions to consider
• when discussing soil water
• 1. Flooding When all available space in the
  soil is taken up by water and there is excess on
  the surface of the soil.

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Soil Water

• 2. Water logging When all available spaces are
  taken up by water in the soil but there is no
  excess on the surface of the soil.
• The presence of the water is not the problem here
  but the resultant absence of air.
• The area where all the air spaces is taken up by
  water is called the Water Table
•  
• 3. Wilting Point Is the condition of a
  particular plant when it has extracted all the
  available water from the soil.

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Soil Water

• Field Capacity - When a soil has just enough
  water and is not flooded or water logged.
• Measured in mm of water
• Clay soils can hold more water and are said to
  have a greater field capacity.

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Soil Air
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Soil Air

• Air is required in the soil for the growth of
  roots, bacteria and earthworms. It is essentially
  the same in composition as atmospheric air.
• 78 Nitrogen
• 21 Oxygen
• 0.03 Carbon Dioxide
• 0.97 Other gases

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Soil Air

• The most important constituent of soil air is
  oxygen.
• This gas is vital for the process of respiration
  by both plant roots and macro and
  microorganisms.
• Respiration involves burning up food in the
  presence of oxygen and releasing energy.

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Respiration

• Aerobic In the presence of Oxygen (O2)
• Anaerobic In the absence of Oxygen (O2)
• Food Oxygen ? Carbon Dioxide Water
  Energy
• C6H12O6 6O ? 6CO2
  6H2O Energy

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Need for Oxygen

• If it happens that the Oxygen content is lt 20 of
  its desired capacity then root growth is impeded
  (stunted).
• This can happen by compaction by heavy machinery,
  frequent flooding or water logging and in severe
  cases over use of pesticides killing off helpful
  bacteria and earthworms.
• Plants use O2 for the uptake of minerals from the
  soil. Most the useful bacteria in the soil are
  aerobic (i.e. they need Oxygen to live) Anaerobic
  bacteria do not need oxygen. Certain types of
  bacteria can use the Nitrogen in the soil and
  change it into nitrates and other compounds which
  are useful to the plant.

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Mineral Nutrients

• These are elements or compounds that are either
  bound chemically to clay particles or are
  dissolved in water in the soil.
• Plants require them as nutrients for growth and
  repair of their cells and also for certain
  chemical reactions to take place.
• They are classified by their amount of use.
• The minerals required in large amounts are called
  Macro Nutrients. Examples are N, P, K, Ca and
  Mg.
• Minerals required in small amounts are called
  Micro Nutrients. Examples are Fe, Mn, Cu, and
  B. They are also known as minor elements or trace
  elements.

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Soil Temperature

• Water is very slow to attract heat.
• This means that soil with a lot of water in it
  harder to heat up than dry soil.
• In fact 1kg of water needs more than ten times
  the heat to raise its temperature by 1 degree, as
  does 1 kg of dry soil.
• Clay soils, as mentioned earlier can attract more
  heat than sandy soils because of their colour but
  are generally wetter soils therefore need more
  heat to raise its temperature.

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Soil Temperature

• This is why clay soils are referred to as cold
  soils and sandy soils are called warm soils.
• On average, clay soils need 50 more heat than
  sandy soils to raise their temperatures by 1
  degree.
• Aspect, colour and altitude also affect the
  heating capabilities of a soil.

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Soil Colour
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Soil Aspect

• This is the direction that land is facing.
• North facing slopes never get the sun
• South facing slopes always get the sun
• Land with a S, SW aspect will be warmer in spring
  and therefore more productive.

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Soil Altitude

• This is the height above sea level. For every
  150ft the soil temperature drops by 10C.

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Soil pH
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Soil pH

• Soil pH is a measure of the acidity of a soil.
• pH stands for the power of the Hydrogen ion
• Plants require certain pHs to grow well if pH
  changes the plant may not be able to produce as
  much, or may in fact die as a result. Earthworms
  and soil bacteria thrive in neutral or near
  neutral conditions.
• In soils, pH ranges mainly from 4 to 9.

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Soils pH values

• lt 4.5 Very Acidic
• 5.2 Strongly acidic
• 5.3 5.9 Moderately acidic
• 6.0 6.5 Slightly acidic
• 6.6 6.9 Near Neutral
• 7.0 Neutral
• 7.1 7.5 Slightly Alkaline
• 7.6 8.3 Moderately alkaline.
• In Ireland, soils tend to become acidic with
  time, and most soils need to be limes (increase
  the pH) every four years or so.

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Organic Matter

• This refers to dead and partially decayed plant
  leaves, roots, twigs, and micro and macro
  organism.
• The final degradation or break down of organic
  matter results in a compound called humus.

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Organic Matter

• Advantages of organic matter
• It is dark brown/black in colour. This means it
  will absorb heat and warm soils (especially in
  spring) will have an early germination capacity
  (Tillage Barley and Wheat)
• It has a crumb structure and is therefore friable
• It has 4 times the C.E.C. of clay particles.
  Therefore soils rich in organic matter will be
  very fertile.
• By promoting flocculation it hugely improves
  aeration. This increases the population of earth
  worms and bacteria of decay