MEXT and the US DOE have done some very good work, they have used aircraft to survey the cesium levels on the ground near to the accident site. They have made a series of maps which can be seen at the following site.
The yellow on the Cs-137 map is for land which has at least 1 MBq of cesium-137 per square meter, this will work out as 1 TBq per square kilometer. If we now convert this into curies it will work out as 27 curies per square kilometer. This is a high level of cesium contamination.
Page 22 of the IAEA report on the Cs-137 accident in Brazil (http://www-pub.iaea.org/MTCD/publications/PDF/Pub815_web.pdf) gives some important data for Cs-137. Using this data I have been able to estimate the gamma dose one meter above the ground due to Cs-137 when the ground is contaminated with 1 MBq of Cs-137 per square meter as being 1.6 microSv per hour, this will work out as 14 mSv per year.
You may want to look at the cesium map for the area around Chernobyl to allow you to compare the two events. Do bear in mind that the colour code on the Chernobyl map is different to the one on the Fukushima map.
Another map is present at the following site.
In the second Chernobyl map the areas which have more than 40 curies of Cs-137 are classed as closed zones, in modern units this translates into 1.48 TBq per square kilometer.
I suspect that the cesium level may have a very profound effect on farming for some years to come. One thing which the farmers of that part of Japan could do is to change to a new non-food crop.
Here is some advice which is given out by the UN regarding the people who live in the cesium contaminated areas in the former Soviet Union