Fjölrit RALA - 15.06.2004, Blaðsíða 97

Abrasion pH and abrasion solution composition in reference European
volcanic soils
E. García-Rodeja, J.C. Nóvoa-Munoz, A. Martínez-Cortizas and T. Taboada
Dpto. Edafología y Química Agrícola. Facultad de Biología. Universidad de Santiago de Compostela
Stevens and Carron (1948) established the use of abrasion pH as an aid in mineral
dentification. Later, Grant (1969) used abrasion pH as an indicator of rock weathering and
Ferrari and Magaldi (1983) proposed it as an index of potential fertility of soils. The abrasion
pH is obtained by grinding the minerals into distilled water; its value is affected by the
quantity of residual cations released from primary minerals and the amount and type of clay
minerals. In consequence, the higher values are expected in soils that are rich in fresh and
weatherable minerals and, as weathering proceeds and the clay content of the soil increases,
the abrasion pH tends to decrease.
In this study abrasion pH and abrasion solution composition were determined in 15 COST
action 622 soils (72 horizons) developed from volcanic materials in different European
volcanic regions: Italy (Napoli: Nl, N2; Rome: N3, N4), Azores (N5, N6), Iceland (N7 to
N9), Tenerife (NIO to N12), Santorini (N14), France (N16) and Hungary (N19), with the aim
to evaluate its use as an index of weathering degree and/or of potential soil fertility in these
particular soils using a set of samples that covers a wide range of volcanic materials, climatic
conditions and degree of soil development.
Abrasion pH was measured in peroxidized samples (to minimize the homogenizing effect of
organic matter in the pH values) following the method of Grant (1969) that consists of
measuring the pH of a soil (20g):distilled water (40mL) suspension after a grinding period of
2lA min (+ 2 min for settling) in an agatha mortar. After centrifugation of an aliquot of the
suspension, base cations (Ca, Mg, Na, K) and Fe, Mn Si and A1 were measured.
The results showed a wide range of abrasion pH values (4.5-7.7), with the lower in the soils
from Azores (4.5-5.3) and the higher in those from Santorini and Hungary (>7) a fact that can
be related to the different climatic conditions (udic vs xeric) determining their weathering and
pedogenesis. In some cases the variations along the profile are small (Nl, N2, N5, N6, N14,
N19) although the lower values for each soil tend to correspond to the A horizons. In other
soils the variation of abrasion pH along the profile is more complex and, frequently, can be
associated to discontinuities in the parent material or to different cycles of soil formation. For
example, the soil N3 has more acid abrasion pH in the subsurface horizons than in the upper
part of the profile with the limit located at a discontinuity marked by a stone line; in the soils
NIO, N12 and N8 the buried horizons, with higher degree of weathering and pedological
evolution, also have lower pH.
Other approach to evaluate the abrasion pH as a weathering index in volcanic soils was to
make a comparison to the weathering index of Parker (1970) (WIP), considered the most
appropiate for soils on heterogeneous parent materials materials because it only includes the
highly mobile alkali and alkaline earth elements in its formulation (Price and Velbel, 2003).
(WIP = (100) [(2Na20/0.35)+(MgO/0.9)+(2K20/0.25)+CaO/0.7)]). From the comparison of
both parameters (see figure) two groupsof soils can be differentiated. In one side are those
from Italy and the N5 from Azores, with higher WIP, developed from more alkaline materials
(trachytic and phonolitic), than the other ones, mainly formed on basaltic or andesitic parent
materials, which tend to have lower WIP for the same abrasion pH. When both parameters are
compared for each soil profile, the expected parallelism between them is only found in two
soils (N9, N10), while in other, like N4 or N16, they follow opposite trends.
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