Soil warning on pesticides

By KATRINA MEGGET

Fresh research on how pesticides affect soil may have a huge impact on New Zealand's farmers.

The work, by Dr Riaz Ahmad at AgResearch in Hamilton, has found that pesticides behave differently depending on the types of soils they are in.

It was traditionally thought that pesticide sorption was directly related to the quantity of soil organic matter, such as decomposed leaves.

But Ahmad has found that additional factors also influence pesticide sorption in soil. Organic matter is made up of many different structural components.

These differences influenced how the pesticide behaved in the soil.

It was not just the quantity but the quality of the soil organic matter that had an effect, he said.


Most pesticide registration agencies used overseas data, mainly from the United States or the Britain, on how pesticides behaved, but New Zealand soil was quite different in composition.

This meant New Zealand could not rely on imported data to determine how pesticides would behave.

New Zealand's soils were acidic, so pesticides behaved differently to how they would in the alkaline soils of other countries.

This behaviour covered the strength of a pesticide's binding to soil molecules, how long it stayed in the soil, and its half life.

He said locally generated data was needed and extreme care should be taken when extrapolating data on pesticides that was based on overseas soil tests.

A thorough understanding of the factors affecting pesticide fate was needed to maximise agricultural benefits and minimise adverse environmental impacts.

He said the next step was to find ways to discover soil composition quickly and cheaply.

Pesticide use has increased 50-fold in the last 50 years. In 1945 the worldwide use was 50 million kg a year. This increased to 2.5 billion kg in 1995, along with a 10-fold increase in toxicity and effectiveness.

This effectiveness corresponded to a $3 to $5 benefit for every dollar of pesticide used, but also resulted in an environmental cost, estimated at $100 million worldwide each year.

He said the environmental impact was not new to New Zealand.

Because of leaching, pesticides have contaminated ground water supplies - although the majority are within the Ministry of Health's acceptable values.

Surface run-off can also potentially contaminate surface waters, as with the run-off of farming nutrient, which contaminates waterways.

Unfortunately, there were few studies and little published information on the degree of pesticide contamination.

Ahmad said pesticide behaviour could also affect nutrient availability, harm microbes, and impact on future crops. It could also get into the food chain and have a negative influence on human health.

In New Zealand there are more than 500 registered pesticides in use, and on arable land they are used at levels that are four times more than the OECD average. But pesticides had become an integral component of modern agriculture.

Ahmad said that short of going completely organic - an ultimately unrealistic proposition - biological controls and integrated pest management represented the most promising way forward for the primary sector.