buy up something

I would read that to mean the farmland itself.

The phrasal verb "buy up" means to "buy all of". So as DrewDad says, it refers to the land and the "up" just adds the meaning of buying it all to the verb to buy.

What Robert Gentel said is correct of course, that to "buy up" means to "buy all of", but this doesn't make much sense in the current example.
That is, the purchasers clearly did not buy up all the farmland in Africa.
Rather we could assume that, in a particular area, they bought all the land they could, maybe all that came onto the market in a particular timeframe.