The size ranges for these separates are not purely arbitrary, but reflect major changes in how the particles behaveand in the physical properties they impart to soils.

这是not...but...结构，不是...而是...。

but后不是祈使句，而是并列句承前省略了相同的主语they (即the size ranges for these separates).

the physical properties (that) they impart to soils. that引导限制性定语从句，并在从句中作impart的宾语，that省略了。