The shovel has one of the longest reaches of any melee weapon in the game, making it effective for fending off large groups of enemies. The fertilizer shovel is great for literally whacking your enemies heads off in free-aiming. The shovel's poison damage stacks with each hit, so massive amounts of damage can be piled on quickly.
It can be repaired with other shovels.
You can get it from the tribalCroatoa (keeper of the punga fruits at Ark & Dove Cathedral), with the Child at Heartperk, or by passing a speech check after asking about his 'secrets'. You may also kill him and retrieve it from his body, but this method is not recommended unless you have already completed both the Walking with Spirits and Hearing Voices quests. It does -4 HP for 10 seconds on strike, likely due to the fertilizer it was used with. That, and Croatoa has warned the player not to cut themselves on it for, "it will make you sick."