Best fertilizer practice

Each kg of nitrogen kept in the soil increases nitrogen efficiency and plant uptake.

What are the practical measures to put in place?

Nitrogen forms make a difference

Ammonia from urea?

More than 72 % of ammonia emissions from fertilizer application are caused by urea and UAN. Ammonium nitrate generates 90% less ammonia emissions per unit of nitrogen than urea. Replacing all urea and UAN by ammonium nitrate could save 63% of overall ammonia losses from fertilizer application in Europe. With a potential emission reduction of about 470 kt NH3, this is the single most efficient measure to reduce ammonia volatilization. When volatilization risk is high, only CAN or AN shall be used.

Urease inhibitors

Urease inhibitors slow hydrolysis of urea. More time therefore is available for diffusion into the soil, reducing ammonia concentrations and increasing the soil volume available for buffering pH. Urease inhibitors can mitigate ammonia losses from urea by about 70 % and from UAN by about 40%. For this reason the new German fertilizer ordinance requires either use of urease inhibitors or incorporation of urea when urea is applied from 2020 on. However, ammonia emissions still remain more than 3 times higher then those from CAN/AN. Urease inhibitors can improve environmental and agronomic outcome but do not overcome other weak points of urea such as lower spreading accuracy and lower reliability. Furthermore, degradation of the inhibitor on urea bears the risk of much lower ammonia emission control than claimed.

Nitrogen compounds at a glance:

NH3

Ammonia: a pungent smelling gas and air pollutant causing soil acidification, eutrophication, ground level ozone and a precursor of secondary particulate matter.

NH4+

Ammonium: a cation found in low concentration in soil solution and fixed by clay mineral.

NO3-

Nitrate: an anion found in the soil solution. Preferred nitrogen form taken up by plants.

N2O

Laughing gas: a 300 times more powerful greenhouse gas than CO2.

NOx

Nitrous oxides: an abbreviation designating both, NO and NO2. Important air pollutant causing ground level ozone and a precursor of secondary particulate matter.

N2

Dinitrogen: an unreactive gas that is abundant in the atmosphere

Applying fertilizer optimally

Incorporation upon spreading

Incorporation of urea into the soil immediately upon spreading, either by closed-slot injection or by cultivation, reduces potential volatilization losses by up to 70 %. However, also in this case, ammonia emissions still remain more than 3 times higher than those from CAN/AN. Depth of injection and soil texture influence reduction efficiency.

Weather conditions

Spreading urea under hot and windy conditions with no rainfall expected upon spreading shall be avoided. With dry soils, diffusion of ammonium and urea in the soil is slow and volatilization losses are high. Humid soils improve diffusion. Rainfall upon fertilizer application significantly reduces ammonia emissions by better distribution of fertilizer in the soil and mitigation of pH peaks.

Soil conditions

Alkaline soils (high pH) result in higher volatilization losses. Urea and UAN therefore shall not be spread on such soils.

Split application

Split application reduces ammonia concentrations and volatilizations risks.

Conclusion

Use of nitrate-based fertilizers and split application are the most efficient means to mitigate ammonia losses to the atmosphere. Urease inhibitors reduce ammonia volatilization from urea but remain less performant than ammonium nitrate.