In fact Harth is a metonymy for the vulva of women. The similarity between 'tilth' and 'vulva' is that man put his seed in the vulva of women. This seed will grow up till it becomes a child. Using this metonymy in Arabic language is very common even before the Qur'an was revealed. Allah used this beautiful example to tell Muslims that however the seed was put in the 'tilth', the result will be the same.
Women are ungrateful to their husbands.
Thus, if women did not do those two sins, then they won't be the majority of the Hell-Fire. So, the Prophet was saying to them that since they make those two sins, then they should make more good deeds to erase those sins. I have not seen anyone more deficient in intelligence and religion than you. A cautious sensible man could be led astray by some of you. They replied in the affirmative.