Page 169 - Arabian Studies (II)

Effects of Irrigated Agriculture on Soil Characteristics 161

of moderate medium angular blocks which break to small angular
blocks and granules. Even under irrigation, there is a noticeable
stability of structure in these cultivated soils.
(b) Decomposition of organic matter releases acids and these
appear to have caused a certain amount of decalcification to have
taken place. The outwash, on which the soils have developed, is ! •
predominantly derived from the limestone uplands and, in the area
around Shimal, uncultivated yermosol profiles have carbonate
contents of about 70 per cent (Tabic 2). However, cultivated profiles
have carbonate contents of less than 60 per cent in, at least, their top
50 cm., though there is some increase in their carbonate content at
depth. Brammer (1968) commented on the decalcification of
Gangetic alluvium in East Pakistan and he ascribes this process to the
reducing conditions that prevail during seasonal flooding. CaC03 is
transformed into Ca(I/C03)2 in the presence of organic matter and
remains in solution due to the flood conditions. Different rates of
decalcification are due to the variable levels of organic matter, the
length of flooding and to biotic mixing of the soil. A similar process
could contribute to decalcification of the soils of the date gardens
for, despite their characteristic of free drainage, the irrigation regimes
practised by the owners mean that the soils are flooded at regular
intervals and during these periods reducing conditions will prevail.
While a slight reduction in pH values is also noticeable in these
cultivated soils, values still remain above 8.5.
Sir W. Halcrow and Partners (1969), in their water resources
survey of the Trucial States, classified water from wells and pumps
between Hail and Shimal as being either C4*S,4 or C4.S3 categories, i.e.
having a very high salinity hazard and a high or very high alkalinity
(sodium) hazard. Such irrigation water is not suitable for irrigation
under normal conditions and even on these very freely draining
outwash deposits there has been some increase in soluble salt
content, as measured by conductivity and ESP (Table 2). Neither
has, however, increased to such an extent that the growth and
productivity of the date palms have been affected, even though
cultivation is of undoubted antiquity, since the ground-water table is
at sufficient depth not to interfere with the root growth of the
palms.

(Hi) Commercial Smallholdings and their Effects on the Character­
I
istics of Xerosol and Halosol Soils
;i i
The main area where the commercial smallholdings are situated lies
between the southern extremity of the Ras al-Khaimah lagoon and
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