How the Soil Supplies Nutrients - Soils for Fine Wines - page 96

Box 4.5 (continued)

surfaces, the CEC at pH 7 often overestimates the effective CEC ( ECEC ), that is,
the soil's CEC at its natural pH. For such soils, methods of the second group are
preferred. Because different methods can give different values for the CEC , it is
important to specify the method of measurement.
Typical values for the CEC of the common clay minerals are given in table
B4.5.1. CEC values of viticultural soils containing these minerals will vary
according to the proportion of each mineral in the clay fraction and the total clay
content. For example, a Krasnozem with 50% clay comprising equal proportions
of kaolinite ( CEC

10 cmol ( 40 cmol (

)/kg clay) and illite ( CEC )/kg clay)

would have a CEC of 12.5 cmol ( )/kg soil. Organic matter in the A horizon

would add to the CEC value.

Table B4.5.1 CEC Values for the Common Groups
of Clay Minerals

CEC

)/kg) a Clay Mineral Group (cmol Charge (

Kaolinite 3-20

Illite 10-40

Smectite (montmorillonite) 80-120

Vermiculite 100-150

a Although the charge on the clay is negative, it is
measured as the number of moles of cation charge ( )

adsorbed.

Nonexchangeable Cations and Anions
Exchangeable cations and anions are readily available to plants. When plants ab-
sorb these ions from the soil solution, exchangeable ions are desorbed to “buffer”
the solution concentration against change. The desorbed ions are replaced on the
surface by ions that are readily available in the solution, such as H (for cations)
or HCO 3 (for anions). However, exchangeable ions of the macronutrients, such
as Ca, Mg, K, P, and S, make up only a fraction of the total inorganic content
of that nutrient in the soil. A proportion of the element may also exist in miner-
als, such as the feldspars, or in insoluble compounds such as CaCO 3 , FeS 2 , and
Ca 10 (PO 4 ) 6 (OH) 2 . For cations such as K , an important fraction is held in the
interlayer spaces of hydrous mica clay minerals, as explained in section 2.2.4.2. In
the interlayer spaces, the unhydrated K ions, as well as NH 4 ions because of
their similar size, are effectively “nonexchangeable” to cations in solution. But as
the minerals weather, they begin to exfoliate, and the interlayer K ions at the
edges become more accessible for exchange (fig. 4.8). These K ions, occupying
“wedge-shaped” sites, are still held more strongly than the hydrated, exchangeable
ions on the flat surfaces, but they are released as the concentration of exchange-
able K is depleted by plant uptake.

4.6.2