Nitrogen Forms: Urea, Ammonium and Nitrate, Which One and When?

Two containers stand side by side. Both labels say nitrogen. One reads 24%, the other 18%. Most growers naturally pick the higher figure, because more nitrogen looks like more work done.

Yet these two containers will not do the same job in the field. They do not work at the same speed, they do not produce the same outcome under the same weather, and their loss risks differ. The total nitrogen figure on the label tells you how much nitrogen the product carries. What tells you how it will behave is the line underneath: which form the nitrogen is in.

This article covers the three basic nitrogen forms, how they behave in soil, and which one comes to the fore under which conditions.

In which form does the plant take up nitrogen?

Let us begin with a simple fact. The plant root takes up nitrogen principally in two forms: nitrate (NO₃⁻) and ammonium (NH₄⁺).

Urea is not on this list. Urea is not a form the plant can use directly in any meaningful quantity. In the soil it first converts to ammonium, then largely to nitrate. Urea is therefore a nitrogen source, not a nitrogen form.

Once you grasp this distinction, reading labels becomes easier. A label reading "Total Nitrogen 24%, Urea 12%, Nitrate 6%, Ammonium 6%" is telling you that half the nitrogen in this product is not immediately available and will pass through a conversion process in the soil.

Nitrate nitrogen: fast, mobile and exposed to loss

Nitrate is a negatively charged ion. Soil colloids are also negatively charged, so nitrate does not bind to the soil. It floats freely in the soil solution and moves with water.

Strengths:

• Taken up by the plant directly and rapidly, with no conversion wait.
• Works in cold soil. Because it needs no microbial conversion, it is dependable in early spring.
• Supports the uptake of cations such as calcium, magnesium and potassium. This is why calcium nitrate is preferred in periods when calcium uptake is critical.

Weaknesses:

• Open to leaching. Under heavy rainfall or excess irrigation it moves below the root zone.
• In waterlogged, oxygen-poor soils it can convert to gas through denitrification and escape to the atmosphere.
• Applied alone at high rates it creates a salt load.

Markka's Master Calcium Nitrate, part of the Master Comp range, carries 14.5% of its 15% total nitrogen in nitrate form alongside 26% CaO. Master Potassium Nitrat offers 13% nitrate nitrogen with 46% K₂O. These two products are for periods when you want to use nitrogen as a carrier and support calcium and potassium uptake. We covered the application detail of calcium nitrate in a separate guide.

Ammonium nitrogen: binding, slow, and staying in the root zone

Ammonium is positively charged. It binds to negatively charged clay and organic matter surfaces. This single difference changes its entire behaviour.

Strengths:

• Retained in the soil and not easily leached. On sandy soils and in wet periods this is a significant advantage.
• Stays in the root zone and provides a source over a longer period.
• Creates mild acidification around the root. On high pH and calcareous soils this can support the availability of phosphorus and some micronutrients.

Weaknesses:

• It is not permanent in the soil. It converts to nitrate through a microbial process called nitrification. In warm, aerated soil this conversion accelerates.
• At high rates and on young plants it carries a risk of root burn.
• It competes with potassium, calcium and magnesium for uptake. Excess ammonium can suppress the uptake of these cations.
• On high pH soils, surface application increases the risk of volatilisation as ammonia.

Master Comp Big Bang 21, from the Master Comp range, is the pure representative of this group with 21% ammonium nitrogen, and aims to slow the conversion with the 0.6% DCD it contains.

Urea: the most concentrated source, and the one demanding most care

Urea carries the most nitrogen per unit weight of any common source. This makes it economical, but also the source that demands the most care.

Urea applied to soil is broken down by the urease enzyme and converts to ammonium. The intermediate product formed during this conversion is ammonia, and ammonia is a gas.

One practical conclusion follows: if urea is left on the soil surface, part of the nitrogen escapes into the air. This loss, called volatilisation, increases under the following conditions:

• High soil pH and high lime content
• Warm weather
• A windy, dry surface
• Crop residue on the surface

The accepted way to reduce the loss is to incorporate urea into the soil or to move it into the soil with irrigation after application. Urea delivered through the drip line largely eliminates this risk.

Markka's Fortinato carries all of its 15% nitrogen in urea form and is positioned for those seeking a pure urea source. Markka UAN splits its 32% total nitrogen three ways (16% urea, 8% nitrate, 8% ammonium), targeting both immediate and delayed effect in a single product.

The three forms side by side

This table shows why "which is better" is the wrong question. The right question is: in this field, in this period, under these weather conditions, which behaviour do I need?

Nitrification inhibitors and DCD: keeping nitrogen in place

The conversion of ammonium to nitrate is carried out by soil bacteria. The process accelerates with temperature. A conversion that may take weeks in cool soil can complete within days in warm soil.

Here is the problem: the moment ammonium turns into nitrate it loses its ability to bind in the soil and becomes exposed to leaching. Nitrogen applied early can move into the mobile form before the plant needs it.

DCD (dicyandiamide) enters at this point. It is a nitrification inhibitor that temporarily slows the activity of the bacteria driving nitrification. The aim is to hold nitrogen in ammonium form in the root zone for longer.

Formulations containing DCD are particularly meaningful under these conditions:

• Sandy, light-textured soils with high leaching risk
• Applications made during wet periods or under heavy irrigation
• Base applications given at the start of the season and expected to last

In the Markka range, N24-DCD (24% total nitrogen; 12% urea, 6% nitrate, 6% ammonium, 0.43% DCD), Master Nitrogen DCD (16% ammonium, 10% nitrate, 0.46% DCD) and Master Comp Big Bang 21 (21% ammonium, 0.6% DCD) represent this approach.

DCD is not a nitrogen source, it is a timing tool. It affects not how much nitrogen there is, but when it will be available.

Which form comes to the fore in which situation?

The pairings below are general approaches. The final decision is made by evaluating soil analysis, irrigation water quality and the crop's growth stage together.

Early spring, soil still cold. Because microbial activity is low, the conversion of urea and ammonium is slow. Nitrate-weighted sources provide a more dependable start in this period.

Rapid vegetative growth. The plant's nitrogen demand is at its peak. Balanced formulations carrying both available and delayed nitrogen do the job here. Tribus (22% N; 11% urea, 5.5% nitrate, 5.5% ammonium) and Rover-N (25% N; 11% urea, 7% nitrate, 7% ammonium, plus 0.4% Fe and 0.2% Zn) represent this balanced structure.

Sandy soil and heavy irrigation. Leaching risk is high. Ammonium-weighted and inhibited formulations are considered alongside split applications.

High pH and calcareous soil, surface application. Do not leave urea on the surface. Incorporate it or move it in with irrigation. The drip line is the safest route.

Fruit fill and ripening. Reducing the nitrogen weighting in this period is a common approach. Excess nitrogen can push vegetative growth to the fore. Formulations using nitrate as a potassium and calcium carrier are preferred.

Periods when calcium uptake is critical. Nitrate nitrogen supports calcium uptake while ammonium competes with it. This distinction matters in calcium-related pictures such as blossom end rot, which we covered in a separate article.

Common mistakes

• Looking only at the total nitrogen figure. Two products may carry identical total nitrogen while behaving in opposite ways in the field. Read the breakdown.
• Broadcasting urea on the surface on a hot day. This means releasing part of the nitrogen into the air. Incorporate it or irrigate.
• Applying all the nitrogen at once. On sandy soil in particular, split application reduces both loss and salt pressure.
• Concentrating ammonium and forgetting calcium and magnesium. Ammonium competes with these cations. Build the programme as a whole.
• Applying nitrate-weighted products before rainfall. Nitrate moves with water. Checking the forecast is part of the fertiliser decision.
• Mistaking DCD for nitrogen. The inhibitor adds no nitrogen; it changes the timing of nitrogen.

Frequently Asked Questions

Is urea or ammonium sulphate better? There is no single answer, because the two behave differently. Urea carries more nitrogen per unit weight and is more economical, but the risk of volatilisation as ammonia is high when it is applied to the surface; it needs to be incorporated or moved in with irrigation. Ammonium sources bind in the soil and are more resistant to leaching, but they acidify the root zone and compete with cation uptake. Your soil pH, application method and irrigation regime determine the choice.

Why does nitrate nitrogen act faster? Because the plant can take up nitrate directly and does not have to wait for any conversion. Urea and ammonium only become fully available after passing through microbial processes in the soil. That process depends on soil temperature and slows in cool conditions. Nitrate carries no such dependency.

What is DCD and what does it do? DCD (dicyandiamide) is a nitrification inhibitor. It temporarily slows the activity of the soil bacteria that convert ammonium to nitrate. Nitrogen therefore remains in the root zone longer in the ammonium form, which can bind in the soil, and leaching risk falls. DCD contains no nitrogen; it affects not the quantity of nitrogen but when it becomes available.

When should nitrogen fertiliser be applied? The general approach is to split nitrogen according to plant demand rather than applying it all at once. Demand rises as vegetative growth accelerates, and the nitrogen weighting is generally reduced during fruit ripening. Avoiding nitrate-weighted applications before rainfall and taking soil temperature into account are also part of the timing. The exact programme varies by crop and region.

Which nitrogen form should be preferred on high pH soil? On high pH and calcareous soils, leaving urea on the surface increases volatilisation loss, so it must be incorporated or moved in with irrigation. The ammonium form creates mild acidification around the root and can therefore contribute to phosphorus and micronutrient availability on these soils. The final decision should be made by evaluating soil analysis and irrigation water analysis together.

Conclusion

Nitrogen management is not a question of quantity but of timing and behaviour. Nitrate works fast but does not stay put. Ammonium stays in place but converts. Urea is the most concentrated source but demands the most careful application.

Once you know these three behaviours, reading a label takes on a different meaning. The total nitrogen figure is no longer a decision on its own; the question becomes which form carries how much of it, and whether that suits the conditions of your field.

If you would like to review your soil analysis and irrigation regime together and discuss which nitrogen structure fits you, reach us via our contact page or explore our product catalogue.