Could the square frame attach to a chute above perhaps? Also, the rusted legs suggests that the contents of the sack were wet - crushed apples or other fruit may be?
As mentioned above, just because the prongs were the ground doesn't automatically mean the thing was intended to be used that way. It would serve as a plant support even if it wasn't designed to do that job.
The two rusty prongs would have been sunk into the soil to a fair old depth by the look of them. A plant support surely wouldn’t need supporting to that depth so I’m tending towards something heavy like the sack of something mentioned by B3. How big is the square bit, I’m wondering?
Regarding the depth of the sturdy looking prongs, before the days of quick mix postcrete (is that what it’s called?) could the square part have been used to support some kind of upright garden structure, the supports for an arch for example?
It's either a thingamajig or a doubrifertul - you can tell which by licking both prongs according to the traditional saying: "Left lick salty prong brings the harvest home"
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.