Where does it discuss where the man was plowing the field with an ox. and he states that a man can not live on merely vegetables, but Walden knows the ox that is plowing his field eats only vegetables?

This section can be found in Chapter One of the text.

Before finishing his house, Thoreau planted two and half acres with beans, potatoes, corn, peas, and turnips. From the eleven acres he had purchased, Thoreau used deadwood from the woods, driftwood from the pond, and stumps from his vegetable patch for fuel. After paying for a team and a man to help plow the field, Thoreau ended up making $8.71 _ by selling those vegetables he didn't eat himself. The next year, he spaded only a third of an acre and realized if he grew only what he would eat, he could get by spending odd hours on it without needing oxen. Farmers, he believes, are less free than oxen. It is the oxen who have the biggest building in town, and Thoreau wishes there were as many halls for free worship or free speech.