Posts for (large) woodshed

[jonatha]

Hi, I was wondering if there are any good pieces of advice to be found here.
I'm going to build a large woodshed, just over 11 meters long and just over 3 meters deep, with three simple walls (back wall) and a roof of old (thin and lighter than usual) concrete tiles I have lying around.
I want to be able to back in with a homemade pallet trailer with firewood on pallets and I'm not very good at reversing with a trailer, so I want as few obstacles as possible, therefore only one post in the middle (at the front, the back of the woodshed can have more).
Now I'm considering how to attach the posts (front corners and middle post) in the ground with a reasonable amount of work. The ground is a lawn, quite firm soil, and not too wet. I'll probably make my own posts from small spruces to be able to use force without going broke (I think something like 20*20 cm will be solid enough?), and the height of the roof is just under three meters.
But how do I best attach it to the ground? I don't want it to rot, but I want it to stand firmly sideways (probably quite common requests when it comes to posts, I would imagine). If it can also be adjusted slightly in height later, that would be best.
The back of the shed will have very sturdy corners; old pollarded trees with a diameter of over a meter but without a large and high crown, so the rear corners will be able to take large loads in both lateral and vertical directions.
Suggestions?
(and, despite the size, it's just a woodshed, I like it cheap, simple but robust)

 

[Nefrums]

Cast your own footings and insert two plinth irons, setting them at the right distance to fit the posts.

 

[jonatha]

Yes, maybe that's best? Would it suffice to dig holes about 30 - 40 cm in diameter and maybe 60 deep, pour in concrete and insert the bars, or do you need the paper tubes?
I haven't quite understood why you need tubes, it should be more stable if the concrete lump fits the hole perfectly, right?

 

[Nefrums]

Sometimes the cardboard tubes only have the top decimeter just to make the plinth look nice. But it's not necessary.

 

[Dan_Johansson]

Yes, maybe that's best? Would it suffice to dig holes about 30-40 cm in diameter and maybe 60 cm deep, pour in concrete and insert the iron, or do you need the paper tubes?
I haven't quite understood why you need tubes, it should be more stable if the concrete block fits the hole perfectly, right?

It also uses less concrete... Hard to dig as "small" as a tube...

 

[jonatha]

Less concrete is needed too... Hard to dig as "small" as a pipe...

Yes, of course... But then the concrete post just stands there and wobbles, will it be just as good?

 

[Dan_Johansson]

Yes, of course... But if the concrete pillar then stands there wobbling, will it be just as good?

Wobbling?

You dig wider and deeper (down to frost-free depth), then fill up with coarser gravel, insert the pipe and then concrete... Fill up with soil next to it...

Edit: don't forget reinforcement and what you're going to attach the posts to.