Supercompost

Super compost is a better version of compost. Compost and super compost cut chances of your crops becoming diseased as well as increase how many items you will receive upon harvest. To make super compost, find a compost bin, which is found at any allotment patch. Put 15 items into it. To make super compost, these items must be ones such as pineapples (which are easy to get on Karamja), watermelons, higher level berries, as well as high level herbs. You gain experience for filling a bucket of super compost as well as using a bucket of it on a patch. As you put 15 items in, you get 15 buckets of compost.

Please note that the composting process takes a bit of time, as do all things Farming related.