What does that rich earth a richer dust concealed mean?

If the poet dies in war and is buried overseas, then the farmland of a foreign country (that rich earth) will be enriched by his decomposing body (richer dust concealed). He imagines that his foreign burial will not only add substance to the soil, but something metaphysical too, his soul perhaps. Note the echo from the English burial (and Ash Wednesday liturgy): "Dust thou art and to dust thou shalt return."