Large animal farms often pollute the area around them in a very ironic way -- they overfill the land with plant-nutrients. The trees turn neon green and the excess leeches into the waterways, causing algal blooms and bad smells.
On the other hand, some crops, particularly Cotton and Tobacco, have been particularly famous through farming history for needing quite a lot of these resources. I think we can use one to solve the other.
In an abandoned animal farm, grow quite a lot of Nitrogen-needy plants. They will absorb the pollution. Do not make cigarettes/textiles out of them, as they're probably infected with all kinds of heavy metals and bacteria to be trusted near humans. The crops can be chemically separated into any useful components.
Nearby trees should indicate the nitrogen-balance of the soil. When their leaves change from an unhealthy-looking neon-green to a lush summer-green, this indicates that the soil is now ready for conventional agriculture. Before that, the soil should test free of heavy-metal contaminants (which we will test for chemically), and then we can turn the farm over to a farmer of some kind.