U of IL printed this a couple of days ago:

http://www.ipm.uiuc.edu/bulletin/article.php?id=990

A clip from the article says:

The third step is to know the nutrient content of the straw.
A ton of wheat straw typically has 9 to 12 pounds of nitrogen (N),
3 to 4 pounds of phosphorous (P) in the form of P2O5, and 25 to
45 pounds of potassium (K) in the form of K2O. The large variability
in K content arises from the fact that K is not incorporated into
organic compounds and thus can be easily leached out of the residue.
The time elapsed and the amount and frequency of precipitation since
the crop reached maturity together with the time the straw is removed
from the field impact how much K there is. Typically, K leaching
occurs rapidly. It is not unusual to see a decrease in K content
of as much as 80% after the straw has been rained on a few times.
On the other hand, N and P content in the straw are more or less
stable because they are present in organic forms and require
microbial decomposition before they can be leached out.