Friday, July 23, 2021

Aqua-ammonia vs. liquid urea – Round 2

• By Bruce Linquist •

ucce 2017 urea trials
Courtesy Dr. Bruce Linquist

Following my blog post about a week ago (see below) about using urea or aqua-ammonia (aqua), a number of people have been asking to see the data. The study was conducted in 2017 at the Rice Experiment Station.

Aqua and liquid urea were applied at three rates (50, 100 and 150 lb N/ac). Both N fertilizers were injected into the soil at 3 to 4 inches depth. Treatments were replicated three times.

Across the N treatments, yields were similar between the aqua and liquid urea treatments. Yields were low, but overall state-wide yields in 2017 were low as well.

In other studies (data not shown), we used dry urea banded into dry soil before flooding to the same depth as aqua we saw almost identical yields across seven on-farm studies. Based on these data, liquid or dry urea that is injected or buried into a dry soil before flooding, performs the same as aqua.

fertilizer rig
Photo courtesy University of California

Aqua in short supply — what are my options?

I have heard there may be a shortage of aqua-ammonia (aqua) fertilizer for the start of this year’s rice season. This begs the question as to what are the options.

In previous studies, we have compared aqua to urea fertilizer. In one study, we compared aqua to liquid urea. In another study, we compared aqua to granular urea.

The liquid urea was applied exactly as one would apply aqua (injected 3 to 4 inches below the soil surface). The granular urea was applied to the soil surface but then lightly incorporated. How much you incorporate depends on the state of the seedbed.

If you have a cloddy seedbed, a lot of the urea will fall into the cracks between the clods and relatively little is needed (maybe a light harrow in front of a roller). If you have a fine seedbed, it may require a light disking.

Importantly (and good news) is that urea dissolves relatively quickly and the flood water also helps move the dissolved urea farther down the soil profile (this is not the case with ammonium sulfate).

In both studies, we found that urea performed similarly to aqua (in terms of yield and N uptake efficiency). However, to get these results, you must make sure that the urea is applied to a dry soil before flooding and it be managed so it gets incorporated below the soil surface before planting (or banded as you do with aqua).

I have found that when urea is used, the crop never seems as uniform as when aqua is used as the main N source; however, this has not affected rice yields. It may affect your ability to assess the crop mid-season for an N deficiency.

If you flood the field before applying your nitrogen fertilizer, then you have to apply all of your fertilizer N into a flooded rice field. In this case, your N use efficiency will be less and the cost of application will be higher.

If you are in this position, apply N in four split applications. Apply 20%, 30%, 30% and 20% of the N rate at around two, four, six and eight weeks after planting, respectively. The first N application should be ammonium sulfate or a starter blend of fertilizer (one that contains N, P and K).

We do not recommend urea for the first N application. The other three applications could be urea. Efficiency will be lower in this system, so you may need to increase your overall N rate.

Dr. Bruce Linquist is University of California rice Extension agronomist. He may be reached at

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