The nitrogen cycle - Plant nutrition

Nitrogen fertilisers are not always directly assimilated by plants. Before the plant can take up the fertiliser, it must be in the right chemical form, and the nitrogen cycle is responsible for this.

Plants can generally only absorb nitrogen in two chemical forms, ammonium and nitrate. Plants have a predilection for nitrate and it is always more easily absorbed in this form.

• Ammonium NH⁺4 -> Bioavailable, but not the preferred source of absorption.

Ammonium is already a bioavailable element so its effect is immediate, although not optimal. Ammonium once in the soil can have 4 pathways:

1. Plant assimilation
2. Ammonia formation and volatilisation
3. Soil fixation in cases with high cation exchange capacity
4. Input to the nitrification cycle, during the nitrification process nitrous oxide and nitrogen monoxide can be generated, which is volatile if soils are acidic.

• Nitrate NO3- -> Bioavailable and the preferred source of nitrogen.

Therefore, any fertiliser applied cannot be absorbed until the cycle has reached nitrate or ammonium.

Each nitrogen fertiliser has a different decomposition cycle.

• Urea CO(NH2)2

Urea is a very potent source of nitrogen. Its concentration is very high and its bioavailability very fast. Its nitrogen concentration is 46% and its bioavailability is between 6 hours and 48 hours.

When urea is applied to soil with a pH above 6.3, hydrolysis occurs, generating ammonium and bicarbonate. If the pH is below 6.3, hydrolysis generates ammonium, carbon dioxide and water. Their leaching is low, their action is long lasting and they tend to slightly acidify the soil. There is a sulphureous and phosphorous version, which adds sulphur and phosphorus respectively.

Partial incompatibilities with ammonium sulphate, calcium nitrate, potassium nitrate, monoammonium phosphate, phosphoric acid and ammonium nitrate can be mixed, but it is not recommended to leave them in the tank for a long time!

Ammonium fertilisers have very similar reactions.

Ammonium fertilisers are generally poorly leached and have a strong acid reaction in the soil.

• Ammonium sulphate (NH4)2SO4

Ammonium sulphate has a 21% Nitrogen and a 60% Sulphur. In clay soils or soils with poor infiltration it can cause problems of excess sulphur, but in soils with good infiltration there are usually no problems.

Ammonium sulphate is totally incompatible with calcium nitrate, monoammonium phosphate and phosphoric acid. It is partially incompatible with urea.

• Monoammonium phosphate NH₄H₂PO₄

Its concentration is 11% Nitrogen and 52% of phosphorus.

The combination of these two nutrients works as a synergy between them as it favours the absorption of phosphorus and reduces the tendency of phosphorus fixation in soils with high cation exchange capacity. It is recommended for use in alkaline soils or soils with high pH water. Monoammonium phosphate should not be mixed with ammonium sulphate and calcium nitrate. And it should be used quickly mixed in the tank if mixed with urea. It must not be mixed with calcium and magnesium fertilisers.

Some products such as nitric acid also provide nitrogen, which must be taken into account for professional work.

• Diammonium Phosphate (NH4)2HPO4

It differs from its smaller sibling in that it increases nutrient concentrations. Its concentration is 18% Nitrogen and 54.6% of phosphorus. In all other respects it is the same as monoammonium phosphate.

Diammonium phosphate should not be mixed with ammonium sulphate and calcium nitrate. And it should be used quickly mixed in the tank if it is mixed with urea.

Nitrate fertilisers have very similar reactions.

Nitrate fertilisers are highly leachable and have a low soil life. And they do not have an acid reaction.

• Potassium nitrate (KNO₃)

It has an NPK ratio of 13-0-44. It is frequently used in fertigation and its bioavailability is high and immediate, although it leaches quickly. It has no incompatibilities, but if mixed with Urea it should be used quickly and not left in the tank for too long. It is very popular in commercial granular fertilisers.

• Calcium nitrate (Ca(NO₃)₂)

Calcium nitrate normally has a ratio of 15% of nitrogen to 26TP3T of calcium, although it comes in many forms and can vary. It is very effective because its calcium concentration has the ability to regulate the osmotic potential of the soil, allowing the plant to absorb water more easily.

It has one major drawback: it is incompatible with many other fertilisers such as ammonium sulphate, monoammonium phosphate, phosphoric acid, potassium sulphate or ammonium nitrate. Nor should it be mixed for a long time with Urea or potassium nitrate.

• Ammonium nitrate (NH4NO3)

The prince of nitrogen with a concentration of 34% behind urea, although it is more stable and does not volatilise nitrogen into the atmosphere. It is very soluble and therefore performs well in fertigation. It behaves better against the environment than urea and has an acid reaction and its nitric part stays longer in the soil.

It is incompatible only with calcium nitrate, and should not be mixed for a long time with Urea and Phosphoric Acid.

Nitrates are an indispensable part of soil nutrition and a perfect example of soil fertility.

The fertility of the soil can be known with the Ionometers Laqua. These sensors facilitate the task of monitoring fertility by taking immediate, in situ readings. Any field can have an Ionometer in its facilities and monitor nitrates to know the real nitrogen needs of the plants at all times.

Do you want to know more about nitrogen or ionometers? Contact Tiloom at info@tiloom.com and we will inform you.