The populations of soil arthropods and soil bacteria under datepalm trees at Al-Hassa region (Eastern Province), Saudi Arabia, (figure 1) were studied each month from November 1977 till October 1978. The main physical properties of the studied soil were also analysed.
Annelida, Pseudoscorpionida, Araneae, Chilopoda, Symphyla and Crustaceae (wood-lice) appeared in small numbers. Oribatei was the dominant member of Acarina, followed by Mesostigmata and then Trombidiformes. Acarina also formed the highest number of soil fauna after Collembola. Members of Psocoptera, Dermaptera, Isoptera, Thysanoptera, Hemiptera, Homoptera and Lepidoptera occurred in small numbers. Diptera accounted for the highest number among Insecta after Collembola, followed by Coleoptera and then Hymenoptera. Most of the Coleoptera were members of the predaceous family Staphylinidae. Also, members of the family Formicidae were the dominant hymenopterous species. However, members of the class Insecta were found most abundantly during summer, followed by winter.
As to the soil fauna in general, it appeared in more or less equal numbers during the four seasons of the year. On the other hand, the total counts of bacteria were high during winter, slightly lower during autumn and about half as high as those in winter during spring ; these counts became very low during summer.
It was also found that when there was an increase in the total counts of bacteria, this increase was mostly followed by an increase in the number of total fauna, except during the summer months, when the total number of bacteria was very low.
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