Page 171 - Arabian Studies (II)

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Effects of Irrigated Agriculture on Soil Characteristics 163

the village of Khatt, though much of the recent expansion of
agriculture over the last five years has been on the Jiri Plain. This
agricultural development has mainly taken place on the xerosols and
some solonchak halosols, the latter in the vicinity of the Ras
al-Khaimah lagoon. As has been seen previously, this development is
of very recent nature and it would not be expected that radical
changes should have taken place in the soil. What changes have
occurred relate to organic matter contents, the salinity and the
alkalinity of the soils.
With cultivation, the initial disturbance of the soil destroys the
surface crust and the weak structure of the topsoil allowing the rapid
oxidation of any organic matter present. This is reflected in the
topsoil sample (10-16 cm. depth) from profile pit 6 at Digdaga
Agricultural Trials Station where the organic matter content was
only 0.28 per cent about a week after it had first been cultivated.
Over a period of time, the net effect of cultivation is to increase the
level of organic matter present in the soil, though the amount of this
increase will be related to the type of crop grown. A profile located
in a smallholding in the middle of the Jiri Plain in a plot that had
grown vegetables for 6 years prior to sampling had an organic matter
content in the topsoil sample (8-14 cm.) of 1.33 per cent. This
figure compares to an average value for the topsoil samples of eleven
uncultivated xerosol soils on the Jiri Plain of 0.99 per cent.
The most rapid changes in soil characteristics can occur when
irrigation water of poor quality is used and both the soluble salt I
content and the alkalinity can rapidly increase. These effects are
accentuated when internal soil drainage is poor, due to the presence
of heavy textured horizons within the profile, or if the soil occupies
a low position in the landscape. Comparative data for two xerosol
profiles (pits 6 and 7) located on Digdaga Agricultural Trials Station ! ;
show these effects quite clearly (Table 3). The high ESP value of 9.4
(relative to the upper horizons) recorded for the uncultivated profile
at a depth of 120-125 cm. probably reflects the location of the
profile pit in a topographic depression quite close to the Ras
al-Khaimah lagoon and sea water may have moved inland by
subsurface flow as a result of a local drawdown of the fresh water
table. Also, recirculation of the irrigation water may have caused an
increase in salinity in the groundwater table. At the time of sampling,
the irrigation water had a conductivity of 3.4 mmhos./cm. at 25°C. !
and its salinity had apparently been increasing for a number of years.
Two factors allegedly were responsible for this — recirculation of . !
irrigation water caused by the increasing number of smallholdings in !
the vicinity and/or local drawdown of the water table, again caused