In Death of a Salesman, why is Willy so disturbed that "nothing's planted" and "I don't have a thing in the ground?"

The things that Willie used to plant gave him something concrete that he grew from seed.  I don't think it's too big a stretch to suspect that when Willie says this, he is lamenting the fact that his own "seed" has not yielded the fruit that he had hoped for.  Neither Biff nor Happy measures up to what he had hoped for them and, no matter whose "fault" this is, it's the reality of what he perceives.

I think this could also be interpreted as Willie's statement about what he could/should have done with his life.  One of the things that Willie was good at was working with his hands.  It was remarked that there was more of him in the stoop he built than in all the sales he made.  Yet, at the end of his life, he has no place to plant seeds; the world has changed; his house is surrounded by "modern" buildings that keep out the sun, the source of his garden's growth.  And like this, the world of sales has changed.  Willie is an anachronism, no more likely to grown than the seeds he futilely plants.