[TowerTalk] One more ground radial question

[Jerry Connelly]

Hello
I do not want to flog the proverbial dead horse but I am
trying to understand something here.  My inverted L for 160m
seems to work fairly well but the tuning is far from o.k.  Right now
I have 5 on-ground radials.  Three are 1/4 wave long and two sre 30' 
shorties. I intend to end up with 12 to 24 but weather hasn't been usable yet. 
I got the length pruned to lowest swr (about 1.6) but everytime it rains or snows
it goes up to 3 or 4.  I realize a tuner will handle it but I like tuning for freq. of
interest and only using a tuner when away from that freq. range.  My yard is
average to poor soil.  It has lots of shale and a small amount of granite.
Is the moisture improving the soil's conductivity and lowering the impedence??
Does this mean I need more (possibly many more) ground radials??  I wanted to bury
them several inches down....so would that help even the swr curves some??
Forgive my ignorance but ground radials are new to me and I don't want to copperplate
the backyard only to find out something else is really the culprit.
Thanks
JerryC
KC8TES