Sponsored by the Crop Protection Association and the England Catchment Sensitive Farming Delivery Initiative

Good management of soil and handling areas is key to keeping chemicals out of water. In the first of two Academies, Voluntary Initiative manager Patrick Goldsworthy highlights how to minimise losses from the field.

About 60% of pesticides detected in surface water are lost from the field due to surface run-off and drainage. Poor filling and handling practices account for the rest.

The amount of pesticide lost in these ways is normally very small, rarely affecting product performance. But water companies incur substantial costs cleaning water to meet the 0.1 parts per billion EU pesticide drinking water standard.

How do pesticides move from soil to water?

Most pesticide losses from soils occur during the autumn and winter, although heavy rain can cause problems at any time. Pre- and early post-emergence cereal and oilseed rape herbicides, as well as metaldehyde for slug control, occur most often in water.

Soils that are dry and cracked let pesticides move quickly through to field drains. But if soils are too wet, surface run-off and drainage will carry more mobile pesticides into water.

Pesticides can dissolve in rain, or soil water, and flow through drainage to watercourses. Alternatively, active substances can attach to soil particles which leave the field in surface run-off or in field drains. Pesticide losses will depend on the chemistry of the pesticide, soil type, structure and moisture, rainfall patterns and the length of time between application and rainfall.

How does soil structure influence movement of pesticides?

Good drainage is important for successful crop establishment and development. But drainage can also carry pesticides swiftly to water. So soil structure should be managed to balance the needs of crop and the environment.

Begin by assessing current soil structure, checking for compaction and pans using a spade. If there is no pan, sub-soiling is not necessary. If you find a pan, ensure the sub-soiler is set to operate just below it.

Cultivation techniques have changed markedly in recent years and many modern implements do several cultivations in one pass and may include deep-set tines within the combination. Unless there is a pan to bust, it is better to restrict these tines to previous tramlines or the plough layer.

Newly installed mole drains can increase deep cracking and pesticide losses, so discuss the best pesticide to use in these situations with your agronomist.

How do you reduce soil losses? 

The ideal time to spray is when the soil surface is moist, the subsoil is dry and heavy rain is not likely for at least 48 hours. But, as this rarely occurs in practice, a balance must be struck. The key is patience.

• Wait until dry, cracked soils have rehydrated before application
• Delay pesticide application if heavy rain is forecast
• Wait until drains have stopped flowing on saturated fields before applying pesticides
• Consider making tramlines in the spring.

In the longer term, aim to increase organic matter content in the soil. This will improve soil structure and stability, and can also increase water holding capacity. Your Soil Protection Review should look to amend cropping and cultivation practice on erosion-prone soils and slopes (over 5% or 1 in 20).

Solutions can include establishing 6-20m grass strips across the slope on the entire field, strategically establishing beetle banks, or grassing down valley bottoms and steep slopes leading to any watercourse.

How does pesticide choice affect losses?

The basic chemistry of a crop protection product determines its characteristics but popularity, rate and the timing of application all affect whether or not a product is likely to end up in water.

Extra advice for pesticides commonly reaching water, including pre- and early post-emergence cereal and oilseed rape herbicides, is now available in Water Protection Advice Sheets (WPAS) which can be found in the library section at www.voluntaryinitiative.org.uk.

The WPAS explains how the pesticide is reaching water, gives advice on the steps that will reduce the chances of it reaching water, and provides a checklist of key measures that need to be followed when deciding if you should spray. For local advice on pesticides and soil management consult your BASIS-registered adviser.

Do

• Manage soils to reduce risk of erosion and pesticide losses
• Manage soils to retain organic matter and protect structure
• Apply pesticides to dry soil profiles with moist surfaces
• Set tramlines to run across or around slopes, where possible
• Leave trash on soil surface, to minimise impact of rain and subsequent erosion
• Check local weather forecasts before spraying and ensure there is at least a 48-hour window before heavy rainfall
• Consult an agronomist for advice on the best crop protection options for crop, soil and season.

Don't 

• Fill and clean in compacted gateways or next to ditches
• Spray dry and cracked soils
• Spray saturated or frozen soils
• Overwork soils, which can cause capping or slaking
• Spray just before heavy rain is forecast.

Now take the test on water stewardship or view the Academy on pesticide handling