Page 167 - Arabian Studies (II)

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Effects of Irrigated Agriculture on Soil Characteristics 159

The date gardens were originally located to take advantage of the
finer deposits on the edges of the outwash fans. However, the
subsequent irrigation over long periods has led to a redistribution of
this fine material within the profile for there has always been a
tendency to over-irrigate. This redistribution of fine material is
marked in most cultivated profiles (e.g. Profile 3) where, although
silt and clay are present throughout the profile, the large cobbles in
the gravelly silty loam horizon, at a depth in excess of 75 cm., have
accumulations of silt on their upper surfaces. In addition to the
redistribution of fine material, the irrigation water also adds a small
quantity of insoluble residue to the topsoil and, though there is
eventual downward translocation, there is a tendency for there to be
an increase in the fine fractions of topsoil samples. This is shown in
Table 2.
A major effect of cultivation has been to increase the organic
matter content of the soil (Table 2). At any one time the amount of
organic matter present in the soil represents a balance between the
addition of undecomposed organic material and the processes of
decomposition, and consequently there will be seasonal variations.
As decomposition proceeds, losses first fall on the carbonaceous
parts of the organic material and, when the organic material is well
humified, as in the uncultivated profiles, the C : N ratios appear to
be of the order of 10-13 in topsoil samples. Under cultivation, the
C : N ratios of the surface horizons are wider (18—20) because there is
the periodic addition of organic matter from the date palms, ground
crops, weeds and livestock. Biological activity is also very different in
the cultivated soils due to the profound change in soil climate. For
instance, in profiles examined in the date gardens, a species of small
worm was present — such activity is totally absent in the hot, dry
uncultivated profiles. By such activity, through the decomposition of
old root systems in situ in the soil, and also as a result of the
downward percolation of irrigation water, organic matter is not
restricted to the top few centimetres of the soil profile but can occur
at considerable depths. In profile pit 4 (Fahlain), channels of grey
mineral matter having an organic matter content of 1.2 per cent (in
contrast to values of 0.21 per cent for the surrounding soil) were
observed down to depths of at least 1 metre.
The increase in organic matter also influences two other soil
characteristics:
(a) The structure of these long cultivated soils is noticeably
different from that of the surrounding uncultivated soils. The
structure of the latter consists of weak angular blocks which break
down to unaggregated material. In contrast, as a result of the organic
matter, the structure of the topsoil of cultivated soils tends to consist

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