Nitrogen fertiliser use on grassland during dry periods

Prolonged dry weather makes it harder to manage grass growth and fertiliser use. Find out how nitrogen behaves in dry conditions and what to do when the rain returns.

Nitrogen is a key driver of yield and forage quality. However, applying it during a drought or extended dry spell is often ineffective without careful planning.

Nitrogen uptake by plants depends on soil moisture, root activity, microbial processes and the movement of nitrate in the soil.

In dry conditions:

• Grass growth slows, so nitrogen demand falls
• Soil microbes become less active, reducing natural N cycling
• Applied nitrogen remains in the soil, often in ammonium form, and is less available for uptake
• Volatilisation losses can increase, especially with urea-based fertilisers on dry, warm soils

As a result, nitrogen applied during very dry periods is less likely to give a return on investment. It might also increase the risk of nutrient losses when weather breaks.

If grass growth has slowed and soil is dry, it’s usually best to delay nitrogen applications until rain is forecast.

Focus on maintaining plant health and protecting existing grass cover.

If you must apply nitrogen:

• Keep application rates low
• Use products with a lower risk of loss, such as protected urea or ammonium nitrate

Use available information to match applications to crop need:

• Monitor sward height and growth rates
• Use weather forecasts and soil moisture sensors to guide timing
• Use Forage for Knowledge or local agronomy services

Don’t apply missed nitrogen in a single large dose. A split application approach is more effective and reduces the risk of overloading the crop.

Rainfall after a dry period often triggers a rapid increase in soil nitrogen supply.

Microbial activity restarts, leading to mineralisation of organic matter and the release of ammonium and nitrate.

This can result in a short-term flush of nitrogen, which may temporarily meet crop needs without further fertiliser.

Test soil nitrogen supply (SNS) post-drought before applying more fertiliser and monitor grass regrowth to determine whether additional nitrogen is needed.

Also be cautious of luxury uptake, where plants take in more nitrogen than needed, which can affect silage quality.

Dry weather followed by heavy rainfall increases the risk of nutrient loss. Recently applied nitrogen can be more prone to leaching, especially on sandy or shallow soils.

Urea volatilisation risk is high during dry, warm weather; consider protected urea or ammonium nitrate instead.

Be aware of compliance with NVZ rules. Some may restrict application on parched soils or specify timings based on crop need.

Reassess total fertiliser requirement based on actual grass growth and silage yields.

Consider the role of clover or other legumes in the sward, which fix nitrogen naturally and are especially useful during low-input periods.

Use partial budgeting to compare costs and benefits of mid- or late-season applications versus conserved feed alternatives.