Soil Properties and Environmental Factors

1
Soil Properties and Environmental Factors
2
Soil Pore Space

• Affects rate of movement of air and water through
  soil
• Determined by soil texture and aggregate size
• Based on particle size alone sandgtsiltgtclay
• But clay particles can form larger aggregates, so
  depends on aggregate size

3
Pore Space

• Macropores space between soil aggregates ---
  drainage
• Micropores space within soil aggregates ---
  water holding

4
Pore Space
5
Soil Moisture, Micropores, and Water Holding
Capacity

• Water holding capacity depends on soil type
  (especially amount of micropores)
• Sand light
• Silt medium
• Clay heavy
• Some clays hold more water than others
• Organic matter has high water holding capacity

6
Soil Moisture, Macropores, and Water Movement

• Water movement depends on size of macropores
• Rapid movement leaching of nutrients
  drying of soil and wilting of plant
• Plant growth between wilting point and field
  capacity

7
Soil Water and Plant Growth

• Wilting point soil water not available to
  plants
• Field capacity micropores full of water

Best Growth
Plant Growth
OK Growth
0 WP FC
Soil Moisture
8
Measurement of Soil Water

• Gravimetric moisture content (water content in
  soil)
• Moisture tension or suction tensiometer
  measures tension with which water is held in soil

9
Chemistry Review

• Many chemical elements often exist in ionic form
• H H K K Ca Ca2
• Ionic forms soluble in water
• Plants must take up these elements (nutrients) in
  water to roots

10
Soil pH

• 0 acidic 7.0 (neutral) alkaline
  14
• Measures relative amounts of H and OH- (acids
  and bases)

11
Soil becomes acidic from

• Leaching of bases by precipitation
• Organic and inorganic acids formed from organic
  decay or microbial action on fertilizers
• NH4 2O2 2H NO3- H2O

12
pH affects plant growth

• Not much affect except at extreme pH values
• Many plants have optima from 5.8-7.0, but varies
  with plant species
• Liming may be necessary on low pH soils (add Ca
  or Mg materials)

13
pH affects nutrient availability

• Soil chemistry changes in acidic vs alkaline
  soils
• Critical for some elements like Fe, P
• Remember, nutrient must be in proper ionic form
  for uptake by plant
• This pH effect on nutrient availability is much
  more important to plant than direct effects of pH
  on plant !!

14
Cation Exchange Capacity

• Ability of colloids in clay or humus to bind or
  tie up metal ions (micronutrients)
• Affected by pH !

15
Cation Exchange Capacity
Or Clay
Brady, 1974
16
Plant Nutrients and CEC
Ions in soil water K Ca2
Plant Uptake
Clay Colloids
Leaching
17
CEC is a good thing !

• Ions stored and easily exchanged and available to
  plant
• Exchanges depend on relative concentrations of
  ions present
• AlgtCagtMggtKgtNa

18
Salinity

• Accumulation of levels of some salts may affect
  plant growth
• Na Cl- often greatest problems
• Salts from fertilizers
• Measured by electrical conductivity (EC)

19
Aeration

• Depends on pore space
• CO2 higher in soil (1 up to 5) than in
  atmosphere (0.04)
• O2 level can be similar to air, but may be very
  low due to flooding or other situations with poor
  aeration

20
Topography

• Affects ease of cultivation
• Affects erosion
• Surface runoff affected by degree and length of
  slope

21
Local Erosion Note topsoil and OM deposited in
low area
22
Cultivation

• Requires energy
• Improves root zone for plants (see Tivy, 1992 for
  details)
• Can lead to compaction
• Can lead to erosion
• How much is necessary ?

23
Biological Factors

• More on this later.

24
Uneven Distribution of Soil Properties Affects
Plant Growth
25
Uneven Distribution of Soil Properties
Topography and Organic Matter
26
Precision Agriculture

• Method for managing uneven distribution of soil
  properties
• Identify problem areas and map by Global
  Positioning System (GPS)
• Adjust fertilizer rates, etc., as required for
  each area

27
References

• Text, pp. 85-89.
• Brady, 1974. Ch. 7, 14.
• Tivy, 1992. Ch. 4.