About five activists from Greenpeace and Climate Justice Taranaki have erected signs and chained themselves to material blocking the plant's entrance.
Greenpeace agriculture campaigner Gen Toop said they were preventing the distribution of synthetic nitrogen fertiliser, citing the chemical's impact on climate change.
"We're doing this because this climate-wrecking chemical is being used to intensify dairying and drive up cow numbers and that's causing the climate crisis to get worse."
Toop said about half a million tonnes of synthetic nitrogen fertiliser was used in New Zealand every year, predominantly in the dairy sector.
"Since 1990, synthetic fertiliser use has climbed 672 per cent and in that time the number of dairy cows has nearly doubled, causing greenhouse gas emissions from intensive dairying to soar," Toop said.
"Today we're taking matters into our own hands, stopping Ballance from selling the synthetic fertiliser that's propping up industrial dairying and driving the climate crisis."
When applied to farmland, the chemical emitted more greenhouse gases than the entire domestic aviation industry, Toop said.
She said the government's recently announced a cap on synthetic fertiliser use - of 190kg per hectare, which would come into force mid next year - was a good start, but did not go far enough.
"That's why we a calling on all political parties before the coming election to commit to phasing out all synthetic fertiliser and to invest in backing New Zealand farmers to make the shift away from intensive dairying and into regenerative farming."
Ballance has partnered with hydrogen energy company Hiringa Energy in a $50 million project to producing some "green urea" at Kapuni using hydrogen generated via a wind turbine rather than gas.
Climate Justice Taranaki spokeswoman Emily Bailey was not convinced that was the answer.
"Urea fertiliser, whether made from natural gas or hydrogen, drives industrial farming which degrades our soil and waterways, disrupts our climate and locks our farmers in debt," Bailey said.
"We need an urgent shift to regenerative agriculture to supply local markets with real, healthy food that nourish the communities and the farmers, without harm to our soils and waterways. Fossil fuels and industrial farming are not our future.
"We have more than enough energy to live comfortably if we use it wisely and don't allow corporations to squander it for profit making."
In a statement, Ballance Agri Nutrients said the protest had not caused any distruption to its operation.
"We have made sure the protesters outside our site are safe, and have politely asked that they allow our hardworking local team to enter or exit as needed. The protesters' presence has not caused any disruption to our daily operations."
The company said it was available to have a conversation with the activists to share science-backed information about its products.
"We are a farming nation and agriculture is our largest industry. Nutrients are needed as part of New Zealand's sustainable farming future. Nutrients, such as nitrogen replenish and nourish the soil and provide the right nutrients to ensure good quality crops.
"We determine nutritional requirements and soil deficiencies based on science - soil and herbage testing, plant and animal research and New Zealand based soil and pasture research.
"Modern agriculture is dependent on inputs from fertiliser and other sources to maintain productivity. If you removed all fertiliser inputs this would reduce the worlds food production by half."
Police said they had been alerted to the protest action at 9.50am.
A spokesperson said they were there to ensure the safety of all and uphold the law, while recognising the lawful right to protest.