Erosion takes toll on growers

By Libby Middlebrook

The top vegetable growers in Auckland are being forced to rethink their farming practices to stop their land being washed away.

Franklin district - the country's biggest vegetable growing area with annual production worth up to $120 million - is suffering from severe erosion thanks to a string of torrential storms which started with a 100-year flood in 1996.

Huge chunks of soil have already been swept down the Pukekohe hill into drains and streams and some growers fear erosion could cost them thousands of dollars in damaged crops and remedial work if it is not reduced.

"The land has been absolutely thrashed," said the president of the Pukekohe Vegetable Growers Association, Keith Vallabh.

"When growers have their land running away from them it's going to affect their incomes and we've got to concentrate on preservation."

In July 1998, the 400-member association set up a research programme called the Franklin Sustainability Project, focusing on better soil and water management for district growers, whose crops include 90 per cent of New Zealand's $100 million onion exports.

The three-year programme, which receives $150,000 a year from the Franklin District Council, Environment Waikato and the Auckland Regional Council, aims to find better cropping practices for hillside growers, whose fields are particularly prone to soil movement during winter's high rainfall.

The Agriculture New Zealand consultant who is managing the research project, Fiona Bell, says Pukekohe is one of the only areas in New Zealand where growers crop on a hill.

"In this area more and more we're getting very high intensity rainfall events and that doesn't have any real affect on yield but over a period of time you're actually losing a valuable resource - soil."

More than two years after the project started researchers have come up with practical hints which could drastically reduce soil movement.

These include constructing drains to collect runoff water, and developing silt traps to prevent sediment moving off a property.

Researchers have also found ways to minimise groundwater contamination and lift organic matter in the soil, which had been reduced to zero by years of intense cropping.

For example, growers can plant cover crops such as oats and rye grass to soak up excess nitrogen from chemical overuse and prevent leaching into groundwater supplies.

Cover crops can increase the level of soil organic matter after being churned back into the earth.

"A lot of growers use these techniques, but they haven't necessarily used them widely," said Fiona Bell.

Trials were also under way to examine whether chemical fertilisers could be applied to crops in smaller amounts more frequently to reduce nitrogen leaching into the ground.

"Growers aren't going to reduce fertiliser if it's going to have an adverse impact on yield, but we are trying to find different ways to apply it which could minimise leaching."