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  • Writer's picturePhilip

The Four “R’s”

Updated: Mar 8, 2021

The Right timing.

When should I apply the first dressing of nitrogen to my crop?

Always a great question and since we are dealing with the UK climate and what it does after you have made the application its a difficult one to be accurate about.

If we consider a few of the basic principles first then maybe we can come up with a more informed answer.


Ammonium (NH4+) is positively charged so it is attracted by electrostatic forces to the negatively charged soil. Ammonium is not leached or lost by denitrification (conversion to nitrogen gas). Therefore, it will stay in soil even if the soil becomes excessively wet.


Nitrate, which is produced by soil microbes from ammonium in a process called nitrification, is a negatively charged ion, is repelled by the negatively charged soil, and is leachable and subject to denitrification.

The soil nitrogen cycle.

Since nitrification is a microbial-mediated process, the rate is influenced by several factors that affect biological activity of which the greatest influencer is soil temperature. The way to slow conversion of ammonium to nitrate is to have cold soil temperatures

The optimum temperature for nitrification is around 32° C. Below 10° C the rate slows rapidly, but continues until 0° C. is reached

Optimising Ammonium Conversion – Temperature effect.

The date to apply ammonium N is not the first day that temperatures reach 10° C, (at 50 mm) but rather when there is a trend for this temperature to be sustained.

So, “are we there yet!”


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