Urea as an Efficient & Cost Effective Nitrogen Source

With longer days and improving soil temperatures, the demand for applied N is increasing as grass and crop growth takes off, Mark Plunkett, Soil & Plant Nutrition Specialist, tells us more.

Currently grass growth rates of up to 25 kg DM/ha/day are predicted for the coming week.  Urea will be one of the main N sources available compared to other years as straight N (46% N) and in N-P-K fertiliser blends.  This is due to the global unrest and high energy costs.  Urea is up to 30% cheaper per kilo of N compared to a calcium ammonium nitrate (CAN).  This offers major savings at a time of high fertiliser prices.

Urea 46% N

Urea contains 46% N and has the highest N content compared to any other fertiliser N type on the market.  This brings advantages, as there is more N in each 1 tonne of urea fertiliser plus when it comes to spreading there is more N in the fertiliser spreader and it will cover more acres per spreader fill.

Urea N is a very safe form of N especially in the early part of the season, while N in CAN is in the nitrate (50%) and ammonium N (50%) form.  Nitrate can be easily lost through leaching / denitrification – both processes releasing harmful forms of N to the environment (nitrate to water / nitrous oxide to air).   When urea is applied to the soil it converts to ammonium which is plant available for uptake and conversion which can take a number of days depending on soil conditions.

Protected Urea / Urease Inhibitors

To reduce ammonia emissions, urea based fertilisers are treated with a urease inhibitor, for example NBPT, NPPT or 2-NPT.  Urea treated with these urease inhibitors are called protected urea products and offer a cost-effective solution to reducing emissions. These inhibitors slow the transfer process of the urea to ammonium, thus reducing ammonia losses by up to 79% and increases availability of N for plant uptake. Therefore, protected urea can be applied throughout the growing season as a very efficient and effective source of N for grass and crop production. Research over the last 8 years at Johnstown Castle shows that protected urea grows ~ 13% more grass compared to unprotected urea.  Therefore, N rates can be reduced where protected urea is replacing unprotected urea.

Urea Spreading and fertiliser calibration

When it comes to spreading urea, it is important firstly to understand granule density. Granule density is the mass to volume ratio of granules. This is a measure of the physical weight of 1 litre of fertiliser by a weighting scale.

Urea is a low-density fertiliser 25 % less density than CAN.  Urea is 0.75kg / litre vs CAN at 1 kg / litre, respectively. 

This has a large impact and difference on spreading distances of both products as one is denser than the other.  Denser sized particles will spread wider at high spinning disc speeds.

Other fertiliser quality characteristics to consider such as granule size, shape and hardness are important for example for even application aim to have 80% of granules between 2 to 4mm in size. 

Figure 1: Aim to have 80% of fertiliser granules between 2 to 4 mm in size for an even application.

For an even, an accurate spread it is important to check fertiliser spreader manufacturers spreading calibrations set up for specific products and subsequently carry out a tray test.

For further information on calibrating the fertiliser spreader & fertiliser quality log onto https://www.teagasc.ie/crops/soil--soil-fertility/fertiliser-spreaders/