A "third sector" company in Chiba was recycling the soil from the plastic greenhouses and selling it to plant nurseries, until 57,000 becquerels/kg of radioactive cesium was found recently.
From Jiji Tsushin (2/13/2012):
Chiba Engei Plastic Kako (Chiba Horticultural Plastic Processing) in Togane City in Chiba Prefecture, a third-sector company with investment from the Chiba prefectural government, announced on February 13 that the maximum 58,000 becquerels/kg of radioactive cesium was found from 140 tonnes of dirt that had been washed off from the used plastic greenhouses the company had collected from farmers in Chiba. The amount of radioactive cesium is more than 7 times the amount allowed by the national standard for regular disposal which is 8,000 becquerels/kg.
The company washes down the dirt off the used plastic greenhouses and recycles the dirt collected. The dirt has already been brought in at 7 plant nurseries. The company plans to recall all the dirt, and will store it in the company's facilities according to the guidance from the prefectural government. The water used for washing was treated, and no radioactive materials were detected.
After I tweeted the Jiji news yesterday, people immediately dug up information of this company. It turns out the company is one of three that was partially owned and operated by the agricultural producer co-op, JA, in Chiba Prefecture. But on the JA Chiba's website, the company has gone missing. One person searched the cache, and found that the JA's site had this company on the page until January 29.
My guess is that's when they found out about radioactive cesium in the recycled dirt, but had kept quiet for 2 weeks.
The JA Chiba has two other "subsidiaries", no doubt third-sector companies: LPG (liquefied petroleum gas) company and a funeral home.
By the way, Chiba's governor is an ex-TV actor turned politician, a mind-numbingly unthinking person who uses his stage name instead of his real name (really, very pedestrian).