Why was it difficult for early humans to cultivate land prior to the end of the last ice age?

...For one, because the landmass was covered in ice...but also because they didn't have the tools. So even if the the conditions for plant life was to be met, they'd still have to fertilize the land they were trying to cultivate. A good way for that to happen is for decomposition of animals or some other way...problem was, was that instead of animals decomposing and fertilizing the land, they were being frozen...