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Year 2 week 9 presentation

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    Year 2 week 9 presentation Year 2 week 9 presentation Presentation Transcript

    • RHS Level 2 Certificate Year 2 Week 9 – Review of soil water and of nutrients and pH.
    • Learning objectives
      • To identify the effect that soil texture has on soil pores and soil water
      • To state and define the different types of soil water
      • To correctly define Field Capacity, Permanent Wilting Point and Available Water Content
      • To state the mechanism of CEC
      • To define pH correctly
      • To identify the effect of pH on nutrient availability for specific nutrients
      • To state that excess of certain nutrients may make others unavailable
      • To apply this knowledge to the diagnosis of plant problems
    • Soil water and soil pores
      • Soil pores are found mainly between soil aggregates.
    • What water is available to plants?
    • Definitions of soil water states
      • Saturation Point: all the pores are filled with water, there is no air.
      • Field Capacity: the water remaining in the intermediate and micro-pores once gravitational water has drained.
      • Permanent Wilting Point: All capillary water is used; only unavailable hygroscopic water remains.
    • Available Water Content
      • The amount of water that a soil holds between the point at which Field Capacity is reached and Permanent Wilting Point.
      • The total of the capillary water remaining in the soil at any time.
      • The water that plants can use.
    • Soil Water Worksheet answers
      • Soil texture can be defined as the relative proportions of sand , silt and clay particles in a soil. Soil mineral particles form aggregates when they are bound together by clay and humus in the soil. Soil pores are the spaces between soil aggregates .
      • Sand – largest; silt – next largest; clay- smallest
    • Soil Water Worksheet answers
      • 3. 1m 3 of clay – smaller particles means less volume per unit of surface area.
      • 4. (a) Saturation Point; (b) Field Capacity; (c) Permanent Wilting Point; (d) Available Water Capacity.
      • 5. (a) the water that drains from the soil; (b) the water held in soil pores that plants can use; (c) water held tightly to the surface of soil particles that plants cannot use.
      • 6. The water lost from the surface of plant leaves.
    • Cation Exchange Capacity
      • Clay and humus particles have a negative surface charge.
      • Certain nutrient (and other) ions found in soil water have a positive charge (cations).
      • Negative charges attract positive charges, so holding these ions in the soil.
      • Which cations are held depends on their relative concentrations in the soil water.
    • What is cation exchange?
      • The ability of one cation to displace another from the negative charge site on the clay or humus particle.
      • This means that the nutrient cations can become available to the plant as they are displaced.
    • Cation exchange
    • Soil pH
      • A measure of how acid or alkaline a soil is. Technically a measure of concentration of hydrogen ions.
      • Scale runs 1 – 14 where 1 is very acid, 14 very alkaline
      • Plants grow best (with some exceptions) in the range 6.5 -7.5
    • Soil pH scale
      • Logarithmic scale – each point is 10 times greater than the one before. So pH 7 is 0 (balanced), pH 6 is ten times more acidic, pH 5 is 100 times more acidic (10 x 10), pH 4 is 1000 times more acidic (10 x 100) etc. Alkaline soils are less acidic so pH 8 is 10 times less acidic than pH7 etc.
    • Soil pH – link to cation exchange mechanism
      • Cations fall into two types – acidic and basic (or alkaline). H +1 (hydrogen) and Al +3 (aluminium) are acidic; Ca +2 (Calcium), Mg +2 (Magnesium) and K +1 (Potassium) are alkaline or ‘basic’ (meaning that they have acid neutralising capacity).
      • So the more acidic the soil the more H +1 ions will be either free in the soil water or attached to the negative charge points on clay or humus soil particles. This displaces the nutrient cations that can then leach from the soil.
      • Al +3 increases in concentration in significantly acidic soils as it is soluble in acidic conditions. Aluminium is toxic to plants.
    • Plant nutrients and pH
      • So high soil acidity can lead to soils being unable to hold alkaline (basic) nutrient cations. This means that these are subject to leaching and may become deficient.
      • Soil pH can also affect the way that plant nutrients form chemical compounds with other elements. If these compounds are less soluble in soil water then the nutrients may become unavailable to the plants.
      • This is called Immobilisation
    • Plant nutrient immobilisation
      • For example: Phosphorous forms an insoluble compound with Aluminium at low pH and with Calcium at high pH.
      • Excessive application of calcium will lead to an insoluble compound being formed with potassium.
      • Another possible cause of deficiency in the plant is ion antagonism – that is high concentration of one ion (like calcium) may block the root surface membrane and prevent it from taking in another (here, magnesium).
    • Plant nutrients and pH
    • pH Worksheet answers
      • The concentration of hydrogen ions in soil solution
      • More alkaline (the higher the number above 7 the more alkaline it is)
      • Positively charged (cations)
      • 1000 (7=0, 6=10 times, 5= 100 times, 4 = 1000 times)
      • Hydrogen ions will be exchanged for calcium ions on the surface of clay or humus particles. The calcium is displaced into the soil water .
    • pH worksheet answers
      • 6. Iron
      • 7. As the level of calcium (lime) in the soil itself is normal it is unlikely to be low pH. Either potassium (forms insoluble compound) or magnesium (ion antagonism) is in excess . Nutrient tests should diagnose
      • 8. Nitrogen deficiency. Either the soil lacks nitrogen because no N fertilizer or organic matter has been applied for some time or the soil pH is below about 5 or above 8.5. To tell which it is carry out a soil nutrient test for N and a pH test.
    • Learning Outcomes
      • To identify the effect that soil texture has on soil pores and soil water
      • To state and define the different types of soil water
      • To correctly define Field Capacity, Permanent Wilting Point and Available Water Content
      • To state the mechanism of CEC
      • To define pH correctly
      • To identify the effect of pH on nutrient availability for specific nutrients
      • To state that excess of certain nutrients may make others unavailable
      • To apply this knowledge to the diagnosis of plant problems