How the Soil Supplies Nutrients - Soils for Fine Wines - page 97

Nonexchangeable K

Wedge-
shaped
site

K +

K +

K +

Figure 4.8 Exfoliation of a micaeous clay on

weathering to form “wedge-shaped”
sites that hold K+ ions (White 1997).
Reproduced with permission of
Blackwell Science Ltd.

K + K +

K +

The micronutrient cations Cu 2 , Fe 3 , Mn 2 , Zn 2 , and Co 2 are also
classed as “nonexchangeable” when they form chelates with organic compounds
(section 2.3.4.2). Similarly, when the nutrient anions of P, Mo, and B are specif-
ically adsorbed on clays or oxides, they are not readily exchanged by anions such
as Cl and NO 3 . But as discussed in section 4.5.4, they are exchanged by other
specifically adsorbed anions supplied at a high concentration in solution, or when
the pH is raised. These forms of cations and anions comprise a nutrient pool of
intermediate availability to plants—intermediate between the readily exchange-
able forms and the insoluble precipitates, or unweathered rock minerals.

Exchangeable Al 3 and Soil Acidity

4.6.3

4.6.3.1 Acid Clays

A soil becomes acidic as Ca 2 , Mg 2 , K , and Na ions are leached faster than
they can be replaced by mineral weathering and atmospheric inputs. Initially, the
exchangeable cations are replaced by H ions, which are continually generated
from carbonic acid (H 2 CO 3 ) formed by CO 2 dissolving in the soil water:
CO 2 H 2 O H 2 CO 3 H HCO 3

H CO 3 2 (4.14)
Pure rainwater in equilibrium with CO 2 in the atmosphere has a pH of 5.65.
But as respiration increases the partial pressure of CO 2 in the soil air, the reac-
tions are driven to the right, producing more H ions. The intensity of acidity
that develops is measured by the soil pH, as outlined in box 4.6.
In soil, the small mobile H ions invade clay mineral lattices. At a pH 4,
mineral weathering is accelerated, releasing Al, SiO 2 , and smaller amounts of Mg,
K, Fe, and Mn. The SiO 2 combines with water to form weak silicic acid (Si(OH) 4 ),
which leaches away, and the Al, Mg, K, and some Mn are retained, initially as ex-
changeable cations. With the exception of soils containing illitic clay, K is lost
by leaching, and the clay that remains is dominated by Al 3 with some Mg 2
(fig. 4.9). Hence an acid clay becomes an Al-clay. The hydrated Al 3 ions are ex-
changeable, and the amount present is usually measured by displacement in M
KCl solution. Some H ions, produced by the hydrolysis of the Al 3 ions (see
next section), are also displaced. The sum of exchangeable Al 3 and H defines
the soil's exchangeable acidity (in cmol H /kg).