Check with the "Call before you Dig company" then drive four sticks of 1-2"diameter X 30-36" pipe in the ground, leaving enough pipe out of the ground to fit just under the bottom step on the ladder & just inside of leg of the ladder.
You can tie the leg to the pipe with wire or use a "U" clamp to clamp the pipe to the ladder.
You can get the "U" at hardware or big box store.
You will need to take the pipe with you to fit the clamp.
You will have to drill holes in the ladder leg to bolt it to the pipe.

great ideas to secure the ladder. I'll have to use them. I have an old ladder in the garage. Suppose I could find a second one at a garage sale. Thanks!