You can say it either way: "hectares are" or "wasteland is". However, if you do it the second way you have to say "that is" rather than "which are". So, your choices are 1) "HECTARES of wasteland that ARE suitable for farming" 2) "hectares of WASTELAND that IS suitable for farming" A third possibility would be to make "wasteland" plural. That's your choice. I think it sounds better in the singular, but it would not be wrong to make it plural. Like so many words, "wasteland" flows freely between the categories of "countable" and "uncountable", which is just one of the reasons why I think those categories should be ignored. Have you ever noticed that dictionary definitions make no mention of those categories? That should tell you something about their importance.

Hmm, yeah 'Wasteland' is strange. It is similar to 'land,' generally. In cases where you would expect to count it, i.e. providing specific measures, it is usually uncountable. 'Behind the factory, there is 10 acres of wasteland.' Note: if you do say 'wastelands' here, it doesn't sound really weird, but not pluralizing it is better, I think. However, if we're talking about an unspecified, vague quantity of 'wasteland' we would often pluralize it. 'He looked out past the wastelands into the sunset.' You could also not pluralize it, but pluralizing it gives a sense that it's innumerable, consist of many different wastelands, etc. Similarly for 'lands:' 'He owns 25 acres of land.' Note: using 'lands' here would sound weird. Unspecific case: 'The lands of the kingdom stretched far and wide.' Or, 'many lands were under the King's dominion.' 'Borderlands' is similar also...we often pluralize it in unspecified count whereas it may be more commonly found in singular for a specified quantity.