sustained: maintained at length without interruption or weakening. "Verb-ed" means "done". sustainable: capable of being sustained. "-able" means "capacity", So they are used in different context and period of time. For example: He owes his success to sustained hard work. (He kept hardworking for a while in the past, and now he get success.) To make our agriculture sustainable, we have to do it.

Good question. "sustained" refers to a factual situation in the past or from the past to the present , e.g. if you talk about "sustained economic growth", then this means that there was or has been a track record or stable history of growth for some time. "sustainable" is used more when talking theoretically about the present and the future. "We can / we are able to sustain the growth" = "growth is sustainable". e.g. China has experienced sustained economic growth for many years. Even if the figures will not be as impressive for some time, some level of growth is sustainable in the long-term."