I had wondered the same thing myself. Not as much as I would have expected, apparently.
I did a bit of Googling around.
This site offers one of the better summaries.
From a larger perspective the area of land actually inundated by the tsunami is a relatively small percentage of Japan's agricultural total, and most of that is planted in rice. It seems that rice is fairly tolerant of occasional salt water intrusion, and the planting season had not really gotten into full swing, so some of the affected area might still be able to produce even this season if the infrastructure damage permits, and long-term effects of salinity should be negligible. Other crop species in the earthquake area more vulnerable to the effects of salt intrusion were mostly at higher elevations and not as affected by the flooding.
There appears to be more concern for the effect on the aquaculture of shrimp and fish, which was also concentrated in low-lying and tsunami vulnerable areas, but cannot be restored as easily.