20 Feb 2014

OneClickSurvey Week 2 response: Factors to consider when applying nitrogen

In our second OneClickSurvey we asked you about when you apply nitrogen and why. Usually it is to fill a feed deficit, either from an immediate need or a predicted one. Environmental factors play a key role in how pasture will respond to nitrogen and in this post we share information on the factors to consider in order to get the optimum response of dry matter to nitrogen.

When we first began our research into nitrogen fertiliser, one of the first questions we had was whether choosing one form of nitrogen fertiliser over another made a significant difference to the nitrogen response rate. Studies conducted in New Zealand show that there is little difference in effectiveness among the different forms of mineral nitrogen fertilisers commonly in use. However these studies did reveal that environmental factors such as soil temperature and moisture and the nitrogen fertiliser application rate have the biggest impact on nitrogen response rates.

Based on these studies, here is some key information to consider when deciding to apply nitrogen.

  • Soil temperature:   the warmer the soil, the greater and more immediate the response. The optimum growth rate for ryegrass pasture is around 18 to 20oC, however pasture responds to nitrogen fertiliser above soil temperatures of 10oC. Nitrogen fertiliser application should be avoided if the soil temperature is below 4oC in the spring or drops below 7oC in autumn as plant uptake is slow at these low temperatures and there is a high risk of nitrogen leaching following rain events.

  • Soil moisture: too little or too much water will lower the response. High evapotranspiration rates in summer and autumn and low reserves of soil moisture means lack of rainfall in non-irrigated pasture can limit plant growth and therefore the effectiveness of nitrogen fertiliser. Conversely, if the soil is too wet, losses through leaching or to the atmosphere can be significant and pasture growth responses will be uneconomical. New Zealand studies have found that up to 30-50% of nitrogen can be leached from winter nitrogen fertiliser applications when soils are saturated and soil temperatures are low.

  • Rate of N applied per application: nitrogen response rates decrease at higher nitrogen fertiliser application rates. New Zealand studies have found nitrogen fertiliser is most efficient when applied at rates between 20 and 40 kg N/ha per application. At application rates of 100 kg N/ha responses can be reduced by up to 40%.

While in reality soil conditions will not always be in the optimum range when you want to apply nitrogen fertiliser, understanding the factors that might limit a good N response will enable you to have more control around your nitrogen use, allowing you to delay or reschedule an application to a more suitable time. By taking these factors into account you will not only use your nitrogen more efficiently, but you will also avoid wastage and reduce your impact on the environment.

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