N-form and water use

But which nitrogen form is best suited if water supply is the limiting growing factor?

Many studies have shown the superior performance of nitrate-based fertilizers over urea and UAN under dry conditions.

N availability in dry soils

In a pot trial performed by Yara [3], columns of 5 cm height were filled with arable topsoil (silty loam or sandy soil, respectively). Nitrogen was applied on the surface of the soil column as CAN or urea. Moisture content was kept constantly low at 7.5% and 5.0%, respectively, for three weeks at 25°C. At the end of the trial, soil columns were cut into slices of 1 cm. The content of nitrate, ammonium and urea was measured for each slice.

More than 80% of nitrogen from CAN has been recovered, compared to only 40% with urea, indicating significant better N availability even in deeper soil layers as compared to urea. Though residual soil moisture was sufficient to convert urea to ammonium, more than 60% of the nitrogen applied as urea was not recovered mainly because of volatile ammonia losses. Furthermore, with urea hardly any nitrate nitrogen became available in deeper soil layers, holding potentially still more available water.

Urea or ammonium nitrate?

With limited water availability, choosing the right N-form is important. Dry soil conditions boost nitrogen volatilization losses as ammonia from urea. Volatilization losses are unpredictable and thus hamper precise nutrition plans for high yield and protein content.

Ammonium nitrate dissolves fast without any losses and nitrate is more mobile and penetrates deeper into the soil under dry conditions. Readily available nitrate nitrogen can be rapidly taken up and metabolized and therefore offers the best features to ensure targeted nitrogen supply when water becomes a limiting factor.

Higher Yield with Nitrate Fertilizer Under Drought

Independent field studies conducted in France compared yield for different nitrogen sources under adequate water supply and drought stress. 130 kg/ha of nitrogen were applied at stem elongation (GS 30) as AN or UAN. Part of the surface was covered by movable glass roofs and received no rain for 23 consecutive days after spreading, while the other part received rain on day 1 and 7 after spreading. Yield was lower for UAN than for AN under both, normal and dry conditions, but the difference was more marked under dry conditions.

Overall this shows that crops can produce more grain based on a limited amount of water when ammonium nitrate is applied instead of UAN. This is an accumulated effect of less losses,higher mobility in the soil, better root growth and a more efficient use of water with nitrate nutrition.