Is it too dry for N

Published 18 July 14

Is it too dry to apply inorganic fertiliser nitrogen?

The simple answer is – if it hasn’t rained in two weeks and the forecast is for it not to rain for the next two weeks, then halve or eliminate the inorganic fertiliser N application you are about to make, says Dr George Fisher, British Grassland Society.

Don’t bother playing catch-up either, by applying more later on; the opportunity has gone – just make sure you make applications when the rain comes back.

Grass does not need much water to take up an application of nitrogen, but it does need some. Dew in the morning and again in the evening is not enough on a soil that has been dry for a couple of weeks. 

The dew might be enough to make the fertiliser prills or granules disappear from a dry soil, but the way the grass roots take up nitrogen in the form of nitrate dissolved in water means that transpiration needs to happen. Transpiration is the movement of water from the soil, through the root, into the above ground growing point, stem (which is there at this time of year) and leaves – and that takes moisture in the soil, not just dew.

The problem is that if the water is not there and the fertiliser is applied, there is a risk of loss of nitrogen from that fertiliser to the atmosphere, through the process of volatilisation. This is the loss of Ammonium-N from the soil as Ammonia gas, and the conditions that favour volatilisation most are warm and drying soils, particularly with a breeze. 

Volatilisation losses from urea can be much higher than from ammonium nitrate, because urea has to go through an ammonium stage before it becomes nitrate, and available for grass root uptake.  So, if you are applying urea at this time of year, don’t – the risk is too high.

The term drought has different definitions. These are spelt out in the great book ‘Improved Grassland Management’ by the late John Frame  and updated by Scott Laidlaw. John states: ‘A ‘dry spell’ is a period of at least 15 days in which no single day receives more than 1mm or more of rainfall, while a ‘partial drought’ is a period of at least 29 consecutive days in which the mean daily rainfall does not exceed 0.2mm per day. The most severe form of drought is an ‘absolute drought’ in which at least 15 consecutive days receive no rainfall or less than 0.1mm per day.’

So it’s up to you as the grassland manager to decide if you are in a region prone to summer dry periods. If so, you really should be measuring rainfall on the farm, like you measure spring soil temperature.  When you are in a dry spell, partial drought or absolute drought, and it’s not forecast to break in the next week or two, halve or eliminate your ammonium nitrogen fertiliser application, and restart when the rain comes.