I bought the small stainless
steel anchor at Walmart in the RV section.
When we got it home it
started to look too weak for the refrigerator
root cellar.
The bigger black anchor is
what people use to secure a mobile home to the ground. I found ours
today at a mobile home supply store in Coeburn for 8 dollars a piece.
It was a struggle to sink it all the way in to our clay ground, which
is a good sign that it's stuck in place and won't budge.
If you can, fix the anchor to the side walls, not to the center of the back plane. Refrigerator walls are often sandwich constructions of plastic or metal sheets with polyurethane or polystyrene foam in between. These sandwiches as a rule do not like out-of-plane loads. It's better to put the load in plane with the side walls.
I hope that the spot you've picked has enough drainage... And you might want to put a couple of plywood plates on the sides in front of the fridge to prevent the ground around the entrance from collapsing.