These days, you can buy 2 kinds of treated post...... one kind is made for ground contact and the other is not. The treated landscaping timbers sold at the discount home building stores are not made for ground contact, they will rot in a few years, if placed on the ground or in cement. I learned this by building flower beds for the wife and a dog pen. 4-5 years and the bottom post was rotted in the flower beds and I broke all the dog pen post off at the top of the cement, with 1 hand, they were rotted.
You used the word "parish", so you might be in La. Because of the abundance of rain, I'd go with a post that is made for ground contact and to stay on the cheap, I'd just backfill and tamp good with soil. Back when we had cattle and had to build fences, we'd use creosote (I dont think you can get them anymore) post and just backfill and tamp good. Good post would last 15-20 yrs. 2' deep on a post the will only be 5' out of the ground is plenty to hold up the signs you mentioned.
