Fjölrit RALA - 15.06.2004, Page 143

Response of physical properties of Andisols and andic soils from Tenerife to
saline and sodic treatments
S. Armas-Espinel1, C.M. Regalado2 and J.M. Hernández-Moreno1
1 Dept. Edafología-Geología, Universidad de La Laguna (ULL)
2 Dept. Suelos y Riegos, Instituto Canario Investigaciones Agrarias (ICIA).
Although the stabilising effect of variable-charge minerals on soil structure is well known,
calibration studies using accepted methodologies are scarce, especially in the case of andic
materials coexisting with layer silicates. In previous field work on soils from Tenerife, the
authors found that under certain combinations of exchangeable cations distribution, salinity,
and clay mineralogy, the aggregating effect of the non-crystalline materials could not
counterbalance soil structure deterioration for certain threshold values of the andic parameters
(Armas-Espinel et al., 2003). Therefore, further investigation is needed to calibrate the
combined effect of electrolyte concentration and exchangeable Na (Mg) on soil structural
stabihty in soils with a range of andic properties. To this end, an experiment was set up to
study the influence of sodicity and electrolyte level (C) on soil saturated hydraulic
conductivity (Ks), moisture retention and shrinkage behaviour.
Soil samples (2 mm-sieved) were packed at field bulk density in 100 cm3 columns (5 cm i.d.)
and percolated through with the highest concentrated solutions with different SAR values until
equilibrium was reached (three replicates each). Next, Ks was determined at decreasing
electrolyte concentrations for each SAR value in a constant head laboratory permeameter, until
reductions in permeability and dispersed clay in the percolate were observed. Finally, of the
three replicates, first one was used for water retention measurement, second for soil shrinkage
determination (remoulded samples), and third for soil analysis.
This work presents the first results obtained for selected high SAR values with natural and
cultivated soils from Tenerife with a range of andic properties. For the selected soils, the
parameter (Al0 + l/2Fe0) decreased in the order: 6 % (soil AV, Andisol), 3 % (soil X), 2.1 %
(soil F) and 2 % (soil B). To describe the results, we shall use the following definitions:
Threshold concentration Cth (meq/L) = 0.56- SAR+0.6 (Eq. 1). Concentration causing a
15% decrease of permeability (Quirk, 1971).
Turbidity concentration Ctu (meq/L) = 0.16-SAR+0.2 (Eq. 2). Concentration when the
percolating solution is reduced to about one-quarter of the Cth and dispersed clay appeared in
the percolate (Quirk, 1984).
The relative change in hydraulic conductivity is defined as the ratio of Ks for a given solution
to the respective Ks calculated for the initial solution.
At the SAR value studied no influence of C was observed in the structural stability of the
Andisol. In the case of the andic soils, the Ks values were unaffected until concentrations of
about 0.02-0.0013 mol/L (soil F), 0.01-0.015 mol/L (soil B) and 0.01 mol/L (soil X), in which
a 15% Ks reduction occurred (Fig. 1). Therefore, these concentrations represent the Cth values.
In the case of soil F this value agreed with that predicted by Eq. 1, while soils B and F were
more stable than predicted. Clay dispersion (Ctu) appeared from 0.005M for the less andic
soils (B and F) and no turbidity was observed for the more andic soil (X) up to the more
diluted solution studied (0.001M). These values were in agreement with Eq. 2 for soils B and
F.
In conclusion, at SAR 25, the main mechanism of Ks reduction up to 0.005 mol/L seems to
be swelling by layer silicates, since structural stability in allophanic material is little affected
by the nature of exchangeable cation (Warkentin and Maeda, 1980). In spite of the sharp
reduction of the relative Ks, it still presented high (soils F and X) to moderate values (soil B).
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