6.4) by soluble organic compounds such as fulvic acids, derived from the break-
down of organic matter in the ‘L’ and ‘Ah’ horizons (Fig. 4.24). Dissociated func-
tional groups for example -COO-and -O-in fulvic acids, along with lone pairs of
electrons (see Box 6.4), for example on the N atom of NH 2 , coordinate with metal
ions, thereby increasing their solubility. These soluble complexed ions are then
translocated downward with percolating water. At depth however, the metal ions
reprecipitate to form an illuvial ‘Bs’ spodic horizon. This metal-rich horizon can
also be organic-matter-rich, in which case the horizon is denoted ‘Bsh’. Repre-
cipitation of the metal ions may occur because the metal chelates become unstable
due to lowered pH as they move down the soil profile. It is also possible that the
organic chelates are degraded by microorganisms as they move down the profile.
Clearly if the organic chelates are removed the metal ions will precipitate.116 Chapter Four+ Ca ++ Ca ++ Ca ++ Ca +
Clay particle deflocculation
Divalent cations, e.g. Ca2+, bind
negatively charged clay particles
together, facilitating clay particle
aggregationCaCO 3 rich from limestone bedrockAB6BtCWhen divalent cations are leached
from upper soil horizons they no
longer bind clay particles. Consequently,
the negatively charged clay particles
repel each otherCation loss causes larger clay aggregates
to fragment (deflocculate). Resultant
smaller clay particles are more readily
washed down profile (Eluvation)Deflocculated clay particles may
re-flocculate where sufficient divalent
cations are present (Illuvation)RepulsionCa2+Ca2+Water
ClayFig. 4.23Schematic diagram explaining the formation of argillic horizons in soils. The resulting horizons are
comparable to those in the alfisol (luvisol) shown in Plate 4.2(a). The master horizons are typically centimetres
to tens of centimetres thick.