What size auger for my post hole digger?

[CARNEYM]

I have a Kubota L3130 tractor and about to buy a post hole digger. I am building a fence and going to sink around forty 4" and six 6" post. My question is will one size auger be ok, let say a 9". Or would I do a better job getting a 6" and 9".

If I dig the 4" holes with a 9" auger will the post be too loose.
Thanks; Mickey

 

[art]

Any post that is a smaller diameter then the auger will be loose for a while until the soil becomes compressed as much as the surrounding dirt.

The most common auger sold is the nine inch! It is good to dig with that diameter, often the auger hole isn't quite straight and the larger diameter will allow you to have the room to make it vertical.

 

[CARNEYM]

Thanks Art, I have done a surch and found out massive amount of info on this subject. Should have done that first.

Mickey

 

[Mundy]

We built 1150' of privacy fence around our back yard over a three year period. All posts are 4" and 8' apart. Our soil type varied from dense clay to wet sand.

The first year we used a 6" auger and found it was very difficult to keep the fence in line. The 6" holes did not allow for much if any adjustment of the post and if the bit wandered during boring any adjustment to the hole had to be done with post hole diggers. This was a pain.

The next year we used a 9" and it was great. The hole was big enough to allow the posts to be set in a nice straight line and the backfill/compaction around the post was much easier too.

We used blast furmace slag for backfill and a 5' by 1.5" solid steel pry bar used like a ram rod to compact the slag around the post. We left the slag down about 8" and topped the hole with the clay spoils.

6" may be a little tight in the 9" hole but if most are 4" posts a little extra hand work with a few 6" posts in 9" holes would not be too bad.

Good Luck

 

[richardbro]

I'd say it has a lot to do with how well you can align your holes in order to get a nice straight fence. You get more wiggle room with a larger auger. Also, some 6" posts may be larger than 6"...
I started off with a 9 inch and ended up buying a 12 inch. The backfilled holes in either size didn't seem overly loose, but then there are tricks to backfilling...

 

[buckeyefarmer]

I use a 12" auger.

 

[logan97]

There are some educational and informative videos on YouTube. Most recently I have been putting # 57 gravel (3/4 to 1" rock) around my post and in the bottom of the hole for drainage and stability , a little more work and expense -I believe it's worth the extra effort.

YouTube - tractor post hole digging

YouTube - Post Installation

 

[thavil]

richardbro said everything I was going to say, I personally went with the 12" auger. I started off with the 9" that came with my Leinbach PHD but quickly went to TSC and purchased a 12". The 12" gave me more room for placement of my posts since I'm not a pro yet on digging straight holes. Backfilling wasn't a problem and long as you tamp as you backfill.

As you can see in the attached picture, I didn't dig a very straight hole. Looked straight from the seat but not when I got off the tractor.

 

[radman1]

For those size of posts, I would go with 9" auger. It will work with both 4" or 6" posts. A 12" auger is going to create a lot of tamping for a 4" post. You may have to use hand diggers to widen the hole on the 6" posts, if you are off the mark.

 

[buckeyefarmer]

FWIW, If the hole left around the post isn't big enough, you can't tamp it good. For a fence I just put the dirt back in, tamping as I fill the hole.