Nitrogen is one of the most important nutrients that grasses need to sustain healthy growth. It is an essential component of many of the building blocks of living organisms such as amino acids, proteins, nucleic acids (DNA and RNA) and chlorophyll. Turf grown on soils deficient in nitrogen will lack vigour, colour, wear tolerance and a natural resistance to disease.
It is not possible to obtain sufficient quantities of nitrogen from mining activities to provide the raw materials for fertiliser manufacture, so any additional resources of this vital nutrient must be taken from the atmosphere, where it accounts for about 80% of the air we breathe - in the form of dinitrogen (N2).
The triple bond between the two nitrogen atoms is very strong, which makes this molecule highly stable and practically inert. To be useful in plant nutrition, nitrogen must be ‘fixed’ in the form of ammonium (NH4+) or nitrate (NO3–) ions.Prior to the introduction of man-made nitrogenous fertilisers, the major sources of nitrogen for plants were from recycled plant and animal wastes, broken down by microorganisms, and also from nitrogen fixing bacteria living free in the soil or in the root nodules of certain leguminous plants. A small amount of nitrogen is fixed by lightning strikes where the high energy from the discharge heats the air locally to about 30,000°C, splitting molecules of N2 and O2 and forming nitrogen oxides (NO and NO2) These oxides dissolve in rain water and fall as weak nitrous and nitric acids. Lightning discharges account for about 5% of the total nitrogen fixed from the air. Nitrogen can be artificially fixed iin a process that involves the reaction of nitrogen and hydrogen to produce ammonia. Today this accounts for more than a hundred million tons of artificial fertiliser manufactured each year.
Key functions in the plant
- A fundamental component for plant growth, being important for amino acid synthesis and subsequent protein formation
- Formation of nucleic acids
- Synthesis of chlorophyll and ATP (adenosine triphosphate)
- Core role in photosynthesis and energy production
Factors affecting availability to plant
- Deficiency most common on soils prone to leaching and drought (sandy or light soils with low organic matter)
- Availability is reduced below pH 5.5 and on very alkaline soils
- High rainfall areas or heavily irrigated areas
- Drought affects nutrient uptake
- Transient deficiencies can occur during periods of rapid growth where demand exceeds the rate of nitrogen uptake
- Compacted soils and areas of poor drainage affect root development and subsequent nitrogen uptake
Nitrogen Sources
Nitrogen is available for plant nutrition in a number of different forms:
Urea – formed by reacting ammonia with carbon dioxide CO2 at high pressure. Urea contains 46% nitrogen and is soluble in water, so it can be applied as either a granule or liquid.Nitrogen (N)
Ammonium nitrate – also known as ‘Nitram’ is produced from ammonia byreaction with nitric acid. Ammonium nitrate contains 34% nitrogen and is also soluble in water.
Calcium ammonium nitrate – a mixture of ammonium nitrate and calcium or magnesium carbonate. Calcium ammonium nitrate contains 25-28% nitrogen.
Sulphate of ammonia – is useful where deficiencies of sulphur occur. It contains 21% nitrogen and 24% sulphur.
Potassium nitrate – manufactured by reacting sodium nitrate with potassium chloride. Potassium nitrate can be used in colder temperatures as it is available to the plant without the need for nitrification by soil bacteria. It contains 13.5 percent nitrogen and 45 percent water-soluble potassium as K2O, making it ideal for use to correct potassium deficiency in early spring.
Calcium nitrate – is used to remedy deficiencies in calcium and reduce soil acidity. It contains 15.5% nitrogen and 19% water soluble calcium.
Methylene Urea – provides a chemical slow release nitrogen source containing 37-40% total nitrogen; of which about a quarter is available in the short term with most of the remainder being released slowly over 2 to 3 months – depending on the ambient temperature.