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If your dirt is clay, or compacted, it is a real time and energy saver to let a special bit made for this purpose help you do the task.
This set has two different sizes so you can choose the best one for you specific needs.
I've used these regularly to help me go down deeper when I need to plant something that has long roots, needs to be in a larger hole or to loosen the soil at the bottom so I can add some compost before planting.