Type of cable to use for long > 100m serial connection
From TI's RS485 design guide, RS485 can support:


10-Mbps maximum data rate (at 40 feet)
4000-foot maximum cable length (at 100 kbps)


The following, from the same document, appears to conflict with the above:



Reading the fine print of (1), it looks like they dropped the somewhat important "M" from "Mbps" on the X-axis of the chart.  Perhaps most useful for your application is the equation:

Line length(m) X data rate(bps) &lt; 10^7 

At 100m and 9800bps, it looks like you've got an order of magnitude margin on the guideline.

As for environmental protection, I'd recommend burial-grade PVC conduit rather than direct burial-rated cable.  Laying the conduit might be a little more work, but you'll 1) be able to use much less expensive cable 2) be protected against errant shovels and 3) have the ability to pull additional conductors in the future if your application requires it.  

At the length and baud rate you're contemplating, nearly any available twisted pair cable should be adequate.