I tried to find some information about the troops that Grant might have had in reserve in case of a prolonged siege. Unfortunately I've misplaced by copy of "Army of the Heartland" by Connelly and the other source I have didn't provide an answer.

So from memory, I don't think Grant was prepared for a long seige at all. After Henry fell with only token resistance I had the feeling Grant expected a quick victory. The number of troops available to each side is also confusing. I've seen estimates as high as 18,000 for Confedeate strength. Buckner estimated the number who surrendered at 9 to 10,000. Even after subtracting those who escaped it difficult to get to 18,000, 15,000 maybe. Grant extimated his strength at 27,000 at the end of the battle.

Earlier I stated that Grant didn't need to storm the fort but could just wait the Confederates out. However that did not appear to be his intention. In his famous note to Buckner demanding unconditional surrender, he stated his intention to "move immediately upon your works". Given his actions later in the war I have no doubt he would have attack the fort had the Confederates not surrendered.

For years I though such an attack was likely to fail. Rereading after action reports from both sides, now I'm not sure. All three Confederate generales were convinced that could not hold the fort without sizable re-inforcements.

If this battle had occured later when rifled guns and minie balls greatly extended the range and accuracy of the defenders weapons, I have not doubt a frontal attack would have failed as they did at Malvern Hill, Fredericksburg, and many other places. However the defenders had smooth bore guns and in some case shotguns. This would have greatly reduced their ability to repulse the attack. Still Grant had, at best, a two to one advantage in forces. That's low.

I'll keeping looking for the "Army of the Heartland" book. It should have additional information regarding A S Johnson's plans and feeling at the time.