Why does the speaker admire his father?

The speaker admires his father for his ability to dig and work the land of a potato farmer, just as his grandfather had done. The description given about how well his father handles a spade and the amount of earth moved in a day show the author's respect for his father's abilities. He further describes the ability of his father and grandfather to dig down to the most fertile soil, which is necessary for successful potato farming. It is not just the physical ability to dig that the author admires. He also acknowledges his father's ability to know where the most fertile soil is and how to work the soil to grow potatoes.
The author himself has no such abilities and does not have a spade to use. If he did, he likely wouldn't know how to use it efficiently or where to find the most fertile soil. Instead, the author cultivates with pen and paper, as a writer. He compares using the pen to write with his father using the spade to dig up potatoes and spread the sprouts into the fertile soil.