He was born in Himeji in Harima. His grandmother was a sister of Yamauchi Kazutoyo, lord of Tosa, and his father had served the Yamauchi but became a masterless samurai after the promised lands were never granted. His mother was the daughter of an Osaka merchant. After his father's death, Kenzan returned with his mother to Tosa, starting a new path through his ties to the Yamauchi.
Around this time he is said to have been adopted by Nonaka Naotsugu, his father's cousin and a domain magistrate (bugyō). He came of age and took the name Yoshitsugu. Kenzan was a pen name, and he later also called himself Kōzan. Valued for both his lineage and his talent, he came close to the center of domain government while still young.
He was appointed a domain magistrate (bugyō) together with his adoptive father Naotsugu. He also studied Neo-Confucianism under the scholar Tani Jichū and sought to put the ideas of Nangaku, the Confucian school of Tosa, into practice in government. A junior fellow student was Yamazaki Ansai, and Kenzan is said to have later protected Ansai when he was driven from his temple.
When his adoptive father Naotsugu died of illness, he inherited the Nonaka house. The lord Yamauchi Tadayoshi ordered Kenzan to rebuild the domain's government. Kenzan increased rice output by building embankments and developing the plains, and used forests of cedar and cypress in a planned way to fund the domain. He also introduced a rotation system so that trees would not be overcut.
He began building the Yamada weir on the Monobe River. Curved and set diagonally across the current, it used huge amounts of pine timber and stone. Water drawn from the weir opened up new rice fields one after another, and the new fields developed in Kenzan's time are said to have reached 75,000 koku. The work took a full twenty-five years to complete.
He began work on Tei port, dug out of the land and regarded as Japan's first excavated harbor. Near Cape Muroto he also built a refuge port where ships could escape storms. He promoted the growing of paper mulberry for washi paper and the making of dried bonito, and invited people from other provinces to bring whaling techniques to Tosa.
Tei port was completed. To keep it from silting up with sand, the inner harbor was linked by a long narrow channel and protected by a long jetty. Around this time he brought carp, catfish and icefish from Osaka and released them into rivers to establish them in Tosa. A story tells that townspeople gathered to look at catfish, which they had never seen before.
The year before, lord Tadayoshi had retired and his son Tadatoyo had succeeded him, but Kenzan remained in high favor. This year he had the honor of meeting the shogun Tokugawa Ietsuna at Edo Castle together with Tadatoyo. Yet his policy of promoting gōshi regardless of status, and the heavy burdens he imposed, were slowly building resentment among retainers and the people.
To stabilize rice prices, he issued the first order in Tosa's history setting an official rice price. Based on Confucian ideas, he also tried to change people's lives and customs, for example by ordering that even the dead and sick without family be buried with care. On the other hand, his harsh demands on laborers in public works stirred strong resentment.
Retainers unhappy with Kenzan's government submitted a document to lord Tadatoyo accusing him. They charged that he favored gōshi while ignoring the hardships of retainers, and burdened farmers and townspeople with heavy labor and monopoly controls. After consulting a related lord, Tadatoyo removed Kenzan, who left office.
After his fall, Kenzan moved near the work site of the Yamada weir, which was dear to his heart, and soon died of illness. His family was sent to Sukumo and suffered a harsh sentence of long confinement. Even so, people secretly built a shrine to honor him, which later became Haruno Shrine. The Yamada weir is said to have been completed this year.
SenhimeTokugawa IemitsuNakae TōjuHoshina MasayukiKawamura ZuikenKumazawa BanzanAmakusa ShirōMitsui Takatoshi
People of the Edo period ・ People by period
Based on facts found in textbooks and biographies. Uncertain points are marked as such.