MUTSUO BANBA’s rice farm in Ishikawa prefecture, on the north-west coast of Japan’s main island, is a mosaic of plots, many separated by land belonging to others. In the season, Mr Banba, a full-time farmer, takes care to water his rice every day. Others, he says disapprovingly, do not. These part-time farmers, he complains, “stay inside with their air-conditioning, while their rice dries out and cracks”. This angers him, because their harvest is mixed with his when the local co-op picks up the crop for sale.

Part-time farmers mounting their tractors in their spare hours are a much-loved part of the Japanese landscape. Often elderly, they have other employment too, or their families help them financially: either way, farming is not their sole source of income. The sheer number of such farmers drags down the sector’s productivity. Of Japan’s 1.5m farmers, only 420,000 are engaged in farming full-time. Part-timers tend not to invest, and often farm badly.

Yet by force of numbers, they wield political influence, through the national network of local farm co-operatives called Japan Agriculture (JA). With its tight links to the Liberal Democratic Party (LDP) and the agriculture ministry, and employing an astonishing 240,000 staff in Tokyo and around the country, the JA is probably Japan’s most powerful lobby. It campaigns to keep high import tariffs on farm goods: the tariff on rice is 777.7%, that on butter is 360%, while sugar attracts a 328% levy. So the announcement in March by the prime minister, Shinzo Abe, that Japan intends to enter talks to join the Trans-Pacific Partnership (TPP), a free-trade grouping, came as a shock. The LDP is expecting a backlash. Farmers may prove the biggest barrier to entering the TPP. The farm ministry will also object, claiming that nine-tenths of Japanese rice production would go with the TPP, and 3.4m jobs overall.

Letting in cheap foreign rice is the most controversial part of entering the talks. Rice, says Kozo Watanabe, who until recently represented JA interests in the Diet, is a “spiritual cornerstone”. The communal efforts that were required to grow it shaped Japanese culture and identity. Moreover, says Mr Watanabe, Japanese small-scale farming will collapse in the face of American agribusinesses sowing seed from planes. Yet, in the case of rice, an aim of protection is to keep the domestic price high by restricting production. That is fine for farmers paid to grow less, as well as for the JA, but a lousy deal for consumers.

Rice cultivation has the highest concentration of part-timers. Elsewhere, farming has diversified towards other crops, market gardening and livestock. There, professionals dominate. But, apart from in livestock, their farms are small, with an average size of 1.5 hectares.

The farm lobby argues that tiny farms are a natural result of Japan’s history and mountainous landscape. After 1945 a crucial land reform redistributed land from large landlords to tenants, with an average plot size of around 3 hectares. Under the devoted care of the small landowners, agricultural yields shot up, laying a crucial foundation for growth.

The hope was that small farmers would before long take other jobs as new industries were born, says Masatoshi Wakabayashi, who was agriculture minister in 2007 during Mr Abe’s first term as prime minister. They were expected to sell their land to other farmers seeking scale and efficiency. But as the value of land soared during the bubble era, he says, farmers preferred to hang on in the hope of selling their plots for development. Today, with rural areas increasingly depopulated, perhaps a tenth of all plots are abandoned to weeds. Farmers are often old: in 2010, the average age was 70. Few offspring want to follow them into farming. Yet tight land laws make it hard for just anyone to come in and buy. Japanese agriculture, says an official at the agriculture ministry, has a choice to make between improvement and decline.

Joining the TPP and lowering tariffs is just the medicine, says Mr Banba, who is 56 and once won a government award for his farm management. “Strong farmers”, he adds, “are not afraid of the TPP”. Entry would allow professionals like him to amass more land. He has a point. A 74-year-old neighbour with three small fields says that on the day Japan signs up to the TPP to let in cheap American rice, he will quit and lease his land to Mr Banba, who has plans to join up with a fish company and start selling frozen sushi to California.

If Japan joins the TPP, it is likely to insist on being given several years to dismantle protection for its farmers, especially for key products such as rice. Yet radical steps towards agricultural reform, says Masayoshi Honma, an economics professor at the University of Tokyo, need to be taken before then. Entering trade negotiations, he says, might help hasten such steps. Japan needs policies to concentrate farmland into the hands of capable farmers. Newcomers, including companies, should be allowed to buy, not just rent.

Mr Abe’s team is currently studying options for agricultural reform, to be announced later in the year. Trying to negotiate TPP entry and introduce wider farming reforms at the same time will be politically hard. Yet lowering tariffs without taking steps to boost competitiveness might prove disastrous.

With both the TPP and wider reforms, the main hurdle will be JA. Its influence and profits depend on the size of its membership. So it has an interest in maintaining something like the current distribution of farmland. Yet new co-ops are also springing up that do a better job of serving the needs of full-time farmers like Mr Banba, as well as of urban consumers who want more choice and competition. And all the while the JA’s members are ageing. Japan’s most powerful lobby is losing its potency along with them.