Nitrogen – Urea Vs CAN

Slow grass growth makes fertilizer application decisions more difficult

Poor grass growth and dwindling supplies of winter feed are causing major difficulties for farmers around the country. There is a lot of uncertainty surrounding how best to manage fertilizer applications during the current spell of cold and dry weather.

In a number of cases, nitrogen has been applied in early spring in anticipation of milder weather and normal grass growth rates. Since there has been very little growth, the question of whether this nitrogen will still be available when growth conditions improve is being asked by farmers. This nitrogen could only have been removed from the soil by either plant uptake, volatilisation, leaching, run-off, or denitrification. Since soil and air temperatures, and rainfall, have been very low, the loss of nitrogen via any of these pathways was low. Therefore, a large proportion of the early nitrogen application is still available to grass. Nitrogen application rates in the coming days should be adjusted to reflect this.

In the current cold and dry weather conditions, farmers are also questioning whether to use CAN or urea fertilizers. Losses of nitrogen as volatilised ammonia from urea can occur in dry and warm weather conditions. Best results from urea fertilizer will be obtained when application is followed by approximately 10mm of rainfall within 2 days. Obviously, farmers need to be vigilant to avoid heavy rainfall events when targeting rainfall based on weather forecasts, as removals through run-off and leaching must be avoided.

Even though current conditions are cold, the sunshine and high mid-day temperatures may still cause some nitrogen loss from urea as ammonia volatilisation, especially on fields where there is little or no grass cover. If you are spreading urea, watch the weather forecast and try to spread before light or moderate rain. If spreading in dry weather, CAN will be more reliable.

With grassland looking brown and hungry after the frost, farmers should also see a good response to phosphorus and potassium fertilizers this spring. Application of compound fertilizers rather than straight nitrogen should be considered, especially on silage fields and where soil test levels are low.

Issued on behalf of The Fertilizer Association of Ireland
March 15, 2010