Soil, Earth
A dirt nurse is someone who nurses dirt. When nursed, dirt becomes soil.
Soil generally has more nutrients in it. It makes sense for dirt that's been nursed to become soil.
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Soil is a thing that Joe (じょう), the huge farmhand, works on improving. If he makes the soil better, the farm will yield more crops.
That kind of makes Joe a dirt or soil nurse, if you think about it. Not sure he’d like that title, though…