Fjölrit RALA - 15.06.2004, Side 153

Use of volcanic muich for saline-sodic rehabilitation: short and long term
experiences in the Canary Islands
M. Tejedor, C. Jiménez and F. Díaz
University La Laguna, Tenerife, Canary Islands
Soil salinisation and sodification, both of which are frequent processes in arid regions, lead to
degradation of the land and are a major obstacle to land use. When the degradation is highly
advanced, desertification can result. The use of mulch to rehabilitate saline-sodic soil in dry
farming conditions has merited relatively little attention in the literature. In a previous work,
(Tejedor et al., 2003a) we compared the salinity and sodicity of the arable layer of soils which
had been covered with 10-15 cm of basaltic tephra more than 20 years ago and adjacent
uncovered soils. The results demonstrated the important reduction in the salinity and sodicity
of the mulched soils.
What rate did/do these processes occur at? In this paper we present preliminary results of
monitoring of the evolution of the saline profile in a clay-loam soil since it was covered with
12 cm of coarse grain basaltic tephra (with dominant grain size 2-8 mm, 77 %). The results
are compared with those obtained in an adjacent uncovered soil. The study zone was located
in Fuerteventura, an island with annual rainfall of less than 100 mm. A 100 m2 plot was
covered with tephra in May 2001. Sampling has been carried out since then in the covered and
uncovered soil after each dry and wet period. In each case, samples were taken every 10 cm to
a depth of 1 m in 10 profiles (5 in the mulched soil and 5 in the uncovered soil) and the
electrical conductivity (EC) was measured in 1:1 extract. The salt profiles obtained in this
short experience (after only two years of tephra cover) were compared with those of adjacent
soils which have been under mulch for over 20 years. The non-parametric Mann-Whitney U
test was used for statistical analysis, with differences considered statistically significant at
p<0.05.
Figure 1 shows the evolution of the saline profile in the recently-covered soil and the
uncovered adjacent soil, during the first year of the study. The initial sample (May 2001,
figure la) -prior to covering the soil- was taken at the beginning of the dry period, when the
soil had not yet reached its highest salinity levels. Following thefirst rainy season (83.5 mm,
April 2002, figure lb) considerable leaching of the salts was already evident in the top 30 cm
of the tephra-covered soil, the EC falling to below 7 dS m"1. Subsequent accumulation of salts
was seen at around 50 cm, with a maximum value of 28 dS m"1. In the soil without tephra the
saline profile after the wet period was similar to the initial one. The differences between the
covered and uncovered soils were statistically significant at 0, 10, 20 and 30 cm. After the dry
period, (0 mm rain since the previous sample, October 2002, figure lc), as expected, the flow
was reversed and the salts rose again in the mulched soil, even though EC values were still
low in the first 20-30 cm. At the same time, less salts were present in the zone in which it had
accumulated during the wet period. No significant differences were found between the
covered and uncovered soils. After the secondperiod ofrainfall (115.3 mm since the previous
sampling) the leaching was considerably deeper although the saline profiles were similar to
those of the first period,. Figure 2 shows the profiles after the second dry period (0 mm of rain
between the two sampling periods, August 2003). The salts rise in the covered soil was lower
than in the first dry period and the maximum accumulation threshold had reached 60-70 cm.
Differences between the covered and uncovered soil were significant at 50, 60, 70 and 80 cm.
The results obtained in this short and long term experience show that soils under tephra
undergo a rapid leaching process, with subsequent accumulation. A comparison between this
tendency and the saline profile of soils covered over 20 years ago (Figure 2) enables us to
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