SOIL-PLANT RELATIONSHIPS Flashcards
- It is the reversible process by which cations are exchanged between solid and liquid phases and between solid phases if in close contact with each other.
CATION EXCHANGE
– Cation exchange reaction
are equilibrium reactions and are
reversible.
Reversible
– The velocity of exchange reactions involving counter ions is extremely rapid.
Instantaneous reaction
– The exchange of cations between the colloid surface and the solution takes place in equivalent amounts.
Electrical neutrality
– A greater proportional replacement occurs as the concentration increases.
Concentration of replacing cation
– refers to the net negative charges present in the soil measure in terms of equivalents (or milliequivalents) of the different cations rather than weight, expressed per100 g soil.
Cation Exchange Capacity (CEC)
Cation Exchange Capacities of Soil Colloids
(
me/100 g soil)
Organic matter . . . .200
Kaolinite . . . . . . . . . . . 8
Vermiculite . . . . . . .150
Hydrous oxide . . . . . . .4
Montmorillionite . . . .100
Illite . . . . . . . . . . . . . . 30
Allophanes . . . . . . . 100
CEC of Phil soil. . 20 to 40
•the capacity for holding anions, increases with the acidity.
•at neutral soil pH, certain anions, like Cl- and NO3- are adsorbed very slightly or not at all by many soils, unlike H2PO4- and HPO4=, which are bound at higher as well as the lower pH.
•Since at pH values usually encountered in cultivated soils, the capacity of soil for adsorbing Cl-, NO3- and SO4= is slight, hence these ions are easily loss by leaching.
ANION EXCHANGE
• Root CEC is much higher for dicots than for monocots, thus dicots adsorbed divalent cations preferentially over monovalent cations.
ROOT CATION EXCHANGE CAPACITY
– growth of plants is limited by the plant nutrient present in limiting amounts.
Law of minimum
– fraction of nutrients
in the soil which is accessible to plant roots
- nutrients that are positionally available to
plants
Nutrient Availability
Ions are adsorbed by roots from the soil soluti
Soil solution theory –
Ions are adsorbed from surfaces of soil colloids.
Contact exchange theory
– nutrients come in contact with roots as the roots push their way through the soil.
•roots grow through the soil
•contact soil particle surfaces
•root surfaces contact/adsorbed ions
Root interception
- nutrients must be dissolved in water and as the water moves through the roots, the nutrients are also carried and hence become positionally available.
Mass flow
– Cation exchange reaction
are equilibrium reactions and are
reversible.
Reversible
– The velocity of exchange reactions involving counter ions is extremely rapid
Instantaneous reaction
– The exchange of cations between the colloid surface and the solution takes place in equivalent amounts.
Electrical neutrality
