Choosing the Right Nitrogen Source

There are many different types of nitrogen fertilizer, but the form of the nitrogen in those fertilizers varies. It’s important that farmers work with their agronomists and fertilizer retailers to choose the right source of nitrogen for their operations. This article will dig into these different forms and help you choose the best source for your farm.

There are three forms of nitrogen that can be found in a fertilizer: urea (NH₂), ammonium (NH₄⁺) and nitrate (NO₃^-). These forms vary in the way they interact with soil, soil bacteria and plant availability.

Soil Nitrogen Transformations

Nitrogen from Urea

Urea is both a form of nitrogen and a fertilizer product. Granular 46% urea is a very common nitrogen fertilizer in Ontario and around the world. As its name would suggest urea as a fertilizer is made up of 100% urea nitrogen. Other fertilizers, though, contain different amounts of nitrogen in the form of urea.

Urea when applied to the soil is not immediately available. A very small amount of the nitrogen present as urea in the soil can be absorbed directly by plants. The vast majority, however, must undergo a transformation into ammonium before it can be taken up by plant roots. This transformation is known as hydrolysis. Enzymes in soil bacteria perform this transformation, which may take between one day and a week to complete. Cool and dry conditions will cause the process to take longer, while warm and moist soil conditions will speed it up. Regardless of it not being immediately available to plants urea nitrogen can still be a valuable nitrogen source on most farms. Typically, as a pre-plant or top-dress nitrogen source.

Nitrogen from Ammonium        

Ammonium is the second step in a three-step pathway. Urea is transformed into ammonium by soil bacteria. Once in the ammonium form the nitrogen becomes much more available to plants; though it is still not the preferred nitrogen source for plants. It is, however, the preferred nitrogen source for some soil bacteria. Ammonium nitrogen must undergo a second transformation to become the preferred nitrogen source for plants. This conversion step is also moisture and temperature dependent and may itself take several weeks.

Compared to urea ammonium nitrogen is initially more plant available, will take less time to be transformed to nitrate and become fully plant available and is also not subject to volatilization like urea. Ammonium nitrogen is not soil mobile and needs to be intercepted by plant roots. It is also possible to lose some of the nitrogen intended for a crop to bacteria in the soil.

Nitrogen from Nitrate

Nitrate is the preferred nitrogen source for plants. It is easily absorbed by plants roots and is immediately available for plant use. As it is a negatively charged anion it also facilitates the uptake of other positively charged soil nutrients like magnesium and potassium. Being highly soil mobile and highly plant available it is a great source of nitrogen in a banded starter. Bulk applying nitrate is not recommended as it will leech from the soil if applied at too high of a rate. For bulk applications a fertilizer with a higher urea or ammonium content is ideal. Those products will have their release into the soil delayed due to the need for transformation.

Composition of Common Nitrogen Fertilizers

Key Take-aways

Nitrogen fertilizers are composed of different nitrogen forms in varying ratios. In the soil, nitrogen follows a pathway ultimately ending at nitrate, the preferent nitrogen source for plants. How many steps are needed to get to nitrate and how long this process takes depends on the initial source of nitrogen applied. Depending on when the nitrogen fertilizer is being applied, to what crop, and how it is being applied, different fertilizers may be more effective than others. Below is a summary of all three-nitrogen form discussed.

Urea Nitrogen (NH₂)

• Two transformations required before becoming the preferred source of Nitrogen for plant roots, creates opportunities for loss.
• These transformations may be beneficial at times as nitrogen release is spread out over time. 
• No significant absorption by plant roots.

Ammonium Nitrogen (NH₄⁺)

• No risk of volatilization.
• Transformation from NH₄⁺ to NO₃^- can delay release into the soil.
• The need for transformation may result in some loss to soil bacteria.
• Low soil mobility.

Nitrate Nitrogen (NO₃^-)

• Preferred nitrogen source for plants.
• Immediate available and highly mobile in soil.
• Instant nitrogen source but may leech.
• Great fit for a starter or band.

Learn more about YaraLiva® Calcium Nitrate

Learn more about YaraVera^® AMIDAS™ Nitrogen and Sulphur Fertilizer