Clay can be used to create a variety of recipes for Fertiliser to improve your crops or a variety of structures within your farm. The key use of Clay is that it is needed to create a Silo in order to store Hay to provide for your animals, as this is costly when purchased individually it is wise to create a Silo as soon as you have a Coop on your farm as then you can create Hay simply by collecting the grass from your farm.