Arenas need to be harrowed frequently since this can prevent horses from sustaining serious injuries once all has been said and is now out of the way. However, you do need to use the proper way to use such a harrow in the first place. The reason behind this is that sometimes people use harrows in such a way that they are unable to truly take advantage of the benefits that they can provide, and this most often has to do with things like the depth with which the harrowing process is implemented.
You should look into making the harrows as deep as possible so that the ground can be softened all the way through. Failing to do so would mean that your harrow for riding rings would leave the top soil relatively loose and soft, but this would be quickly beaten away by the pounding hooves which means that the harder soil underneath will be revealed, thereby causing all of the injuries that you were trying to prevent in the first place.
Shallow harrowing can make the chances of your horses being safe lower than might have been the case otherwise, so you should try not to cut any corners with this sort of thing. Optimizing your harrowing is all about making sure that the whole patch of earth in the arena is made soft yet firm, so make sure that you don’t make it too loose since this would also end up causing quite a few problems. It’s all about finding the right kind of balance that would enable you to do the best harrowing you can which is quite necessary if you care about the safety of your horses.