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Nitrogen in agriculture: in what form?

Nitrogen Function

Nitrogen in the soil is the most important element for plant development. It is required in large amounts in the soil to avoid a deficiency in plant production and health. Nitrogen is a major part of chlorophyll and the green colour of plants. It is responsible for lush, vigorous growth and the development of a dense plant. Although nitrogen is the most abundant element (about 78%) in our atmosphere, plants can't use it until it is naturally processed in the soil by microbes.

Nitrogen is available to the soil in many forms or applications:

  • Fixation from the air by soil microbes
  • Chemical or inorganic fertiliser
  • Manures and sewage sludge
  • Compost tea and other liquid (fermented) fertilisers
  • Guano and other fish or seaweed concentrates

There is controversy involved with inorganic or synthetic nitrogen usage. Over application of chemical nitrogen leads to soil and soil water contamination through run-off and leaching. The considerable consumption of fossil fuels in the manufacturing and processing of synthetic nitrogen fertilisers is also cause for concern. Depending on your level of environmental stewardship, you may want to stick with organic sources of nitrogen. If you do use synthetic and/or inorganic, do not over apply as it can damage microbial activity and potentially up to 70% can be lost through soil leaching and volatilisation to the atmosphere as a gas.

Nitrogen Sources

The earth's atmosphere consists of 78 % nitrogen and is the ultimate source of nitrogen. In most areas of the world, the nitrogen found in soil minerals is negligible. Nitrogen may be added to or lost from soil by a number of processes. In the soil, nitrogen can undergo a number of transformations through microbial activity (eg. transformed into amino acids).

Rainfall adds about 18kg of nitrogen to the soil per hectare per year. The nitrogen oxides and ammonium that are washed to earth are formed during electrical storms, by internal combustion engines and through oxidation by sunlight. Some scientists also believe that some of the gaseous products that result from the transformation of nitrogen fertilisers may cause a depletion of the ozone (O3) layer around the earth. The extent of this possible damage has not been substantiated.

Crop residues decompose in the soil to form soil organic matter. This organic matter contains about 5 percent nitrogen. A hectare of top soil with 2 percent organic matter would contain up to 7,000kg of nitrogen. However, this is highly dependent on climate and soil health including the level of soil carbon and microbial activity in the soil. Generally, about 1 to 3 percent of this organic nitrogen is converted per year by microorganisms to a form of nitrogen that plants can use. The message here is that soil N can be only maximised in a healthy soil environment (ie. adequate soil carbon and microbial activity levels to facilitate N transformation and its availability to plants). The failure to achieve these soil conditions on farms is the key reason why much of the N fertiliser application is lost to soil leaching and volatilisation to the air.

Legumes also fix atmospheric nitrogen through their symbiotic association with Rhizobium bacteria. If plant roots are well-nodulated, the legume plant does not benefit from the addition of fertiliser nitrogen. Perennial legumes, such as alfalfa, can fix about 400 kg of nitrogen per hectare per year.

Manure contains an appreciable amount of nitrogen. Most of this nitrogen is in organic forms: protein and related compounds. Cattle manure contains about 5 to 20 kg of nitrogen per tonne. About half of this nitrogen is converted to forms available to plants during the first growing season. Lesser amounts are converted during succeeding seasons. Each tonne of applied manure is equal to about 2 to 9 kg of commercial fertiliser nitrogen.

Commercial fertiliser nitrogen comes in three basic forms: gas, liquid and dry. All forms are equally effective when properly applied. Once applied, fertiliser nitrogen is subject to the same transformations as other sources of nitrogen. There is no difference between the ammonium (NH4+) or nitrate (NO3-) that enters the plant from commercial fertiliser and that produced from natural products such as manure, crop residues or organic fertilisers

Nitrogen for the soil

Nitrogen (N) in the form of synthetic nitrogen fertiliser is the traditional form of delivery to the farm soil. Nitrogen (N) in a liquid fertiliser that has undergone fermentation will produce a result similar to the transformation of N in the soil (under microbial activity and in the presence of carbon and hydrogen) where it becomes bound into compounds such as amino acids (see image below of N in an amino acid along with O2, C and H).

Amino acids in soils are part of the humic acid fraction. This is why it helps to add humic acid as an ingredient to the liquid formulation. Also, it pays to have good carbon levels in the soil to increase humic acid production and availability of N in these compound or complex forms. This is why N (as a single element) does not act alone in a healthy soil.

The question is: do what your nitrogen (N) delivered as a chemical or as a food? Also, do you what to maximise N delivery from the atmosphere through microbial activity in the soil (this way it is free)? Microbial activity (diversity and abundance) in the soil can be sustained in the absence of chemicals and compaction, and where beneficial microbes are incorporated into the soil along with trace minerals and other nutrients (nutrient cycling).

There is about 5000+ tonnes of N in the atmosphere above 1 hectare of land and available for use by soil microbes. Most (about 70%) nitrogen supplied in a chemical form to a soil can be lost through the soil hydrology and into the air is soil conditions are poor. Therefore, it is best to deliver N in a complex form or as compounds, eg. amino acids. This is what happens or is produced in a fermentation/digestion of N and other nutrients. For example, magnesium in the fermentation will link with N to form magnesium nitrate.

Also, N in high levels can be antagonistic to (suppress) copper and potassium (see and possibly also molybdenum. Copper availability in a soil is critical to plant, animal and human health.

Nitrogen that has been transformed or complexed into amino acids, etc. in a fermentation may be the best form to deliver N to the soil as it this should make the N more useful as a soil and plant conditioner, with little or no loss of N (ie. available for immediate use by plants).

The question is: what is an appropriate/economic level of free N in a formulation (liquid or granule) to apply to a soil. It is probably about 3% of the formulation and provided in a form that is immediately available for metabolism, eg. as an amino acid/protein. This is why it may be useful to use in a fermentation the following ingredients: Nitrogen sulphate, high N plant materials (pre-fermented to extract nitrogen in a complex form), urine (because it is ready to complex), humic and fulvic acid, fish and seaweed concentrates and sea minerals to obtain the minerals that N needs to complex with to form amino acids and other compounds for the soil and microbes. I also add nitrogen fixing bacteria (eg. EM3)

Over 90% of the nitrogen N in the surface layer of most soils occurs in organic forms (eg. humic acids), with most of the remainder being present as NH4- which is held within the lattice structures of clay minerals (often not available due to poor microbial activity). The surface layer of most cultivated soils contains between 0.06 and 0.3% N. Peat soils have high N contents to 3.5% due to the presence of high carbon levels.

Plant remains and other debris contribute nitrogen N in the form of amino acids.

Nitrogen as compounds

Nitrogen is an essential building block of amino and nucleic acids, essential to life on Earth. As part of the symbiotic relationship, the plant converts the 'fixed' ammonium ion to nitrogen oxides and amino acids to form proteins and other molecules, (eg. alkaloids). In return for the 'fixed' nitrogen, the plant secretes sugars to the symbiotic bacteria

Elemental nitrogen in the atmosphere cannot be used directly by either plants or animals, and must be converted to a reduced (or 'fixed') state in order to be useful for higher plants and animals. Precipitation often contains substantial quantities of ammonium and nitrate, thought to result from nitrogen fixation by lightning and other atmospheric electric phenomena. This was first proposed by Liebig in 1827 and later confirmed. However, because ammonium is preferentially retained by the forest canopy relative to atmospheric nitrate, most fixed nitrogen reaches the soil surface under trees as nitrate. Soil nitrate is preferentially assimilated by tree roots relative to soil ammonium.

Nitrogen occurs in all living organisms, and the nitrogen cycle describes the movement of the element from air into the biosphere and organic compounds, then back into the atmosphere. Synthetically-produced nitrates (NO3) are key ingredients of industrial fertilisers, and also key pollutants in causing the eutrophication of water systems. Nitrogen is a constituent element of amino acids and thus of proteins and nucleic acids (DNA and RNA). It resides in the chemical structure of almost all neurotransmitters, and is a defining component of alkaloids, biological molecules produced by many organisms. The human body contains about 3% by weight of nitrogen, a larger fraction than all elements save oxygen, carbon, and hydrogen.

Nitrogen transformations

Nitrogen exists in a number of chemical forms and undergoes chemical and biological reactions.

Organic nitrogen to ammonium nitrogen (mineralization). Organic nitrogen comprises over 95 percent of the nitrogen found in soil. This form of nitrogen cannot be used by plants but is gradually transformed by soil microorganisms to ammonium (NH4+). Ammonium is not leached to a great extent. Since NH4+ is a positively charged ion (cation), it is attracted to and held by the negatively charged soil clay. Ammonium is available to plants.

Ammonium nitrogen to nitrate nitrogen (nitrification). In warm, well-drained soil, ammonium transforms rapidly to nitrate (NO3-). Nitrate is the principle form of nitrogen used by plants. However, it leaches easily, since it is a negatively charged ion (anion) and is not attracted to soil clay. The nitrate form of nitrogen is a major concern in pollution to water bodies.

Nitrate or ammonium nitrogen to organic nitrogen (immobilization). Soil microorganisms use nitrate and ammonium nitrogen when decomposing plant residues. These forms are temporarily "tied-up" (incorporated into microbial tissue) in this process. This can be a major concern if crop residues are high in carbon relative to nitrogen. Examples are wheat straw, corn stalks and sawdust. The addition of 9 to 30 kg of nitrogen per tonne of these organic residues is needed to prevent this transformation. After the residues are decomposed, the microbial population begins to die back.

Nitrate nitrogen to gaseous nitrogen (denitrification). When soil does not have sufficient air, microorganisms use the oxygen from NO3- in place of that in the air and rapidly convert NO3- to nitrogen oxide and nitrogen gases (N2). These gases escape to the atmosphere and are not available to plants. This transformation can occur within two or three days in poorly aerated soil and can result in large loses of nitrate-type fertilisers.

Ammonium nitrogen to ammonia gas (ammonia volatilization). Soils that have a high pH (pH greater than 7.5) can lose large amounts of NH4+ by conversion to NH3 gas. To minimize these losses, incorporate solid ammonium-type fertilisers, urea and anhydrous ammonia below the surface of a moist soil.

Proteins, nucleic acids, and other organic chemicals contain nitrogen, so nitrogen is a very important atom in biological organisms. While nitrogen makes up 78% of Earth's atmosphere most organisms cannot use nitrogen gas (N2). N2 enters the trophic system through a process called nitrogen fixation. Bacteria found on the roots of some plants can fix N2 to organic molecules, making proteins. Again, animals get their nitrogen by eating plants. But after this point, the nitrogen cycle gets far more complicated than the carbon cycle.

Animals releases nitrogen in their urine. Fish releases NH3, but NH3 when concentrated, is poisonous to living organisms. So organisms must dilute NH3 with a lot of water. Living in water, fish have no problem with this requirement, but terrestrial animals have problems. They convert NH3 into urine, or another chemical that is not as poisonous as NH3. The process of releases NH3 is called ammonification.

Because NH3 is poisonous, most of the NH3 which is released is untouchable. But soil bacteria have the ability to assimilate NH3 into proteins. These bacteria effectively eat the NH3, and make proteins from it. This process is called assimilation.

Some soil bacteria does not convert NH3 into proteins, but they make nitrate NO3- instead. This process is called nitrification. Some plants can use NO3-, consuming nitrate and making proteins. Some soil bacteria, however, takes NO3-, and converts it into N2, returning nitrogen gas back into the atmosphere. This last process is called denitrification, because it breaks nitrate apart.

Amino acids

Amino acids exist in soil in several different forms, like:

As free amino acids
  • in the soil solution
  • in soil micro pores
As amino acids, peptides or proteins bound to clay minerals
  • on external surfaces
  • on internal surfaces
As amino acids, peptides or proteins bound to humic colloids
  • H-bonding and van der Waals' forces
  • in covalent linkage as quinoid-amino acid complexes
As muco-proteins
As a muramic acid

Amino acids are readily decomposed by microorganisms, and have only nephemeral existence in soil. Therefore, the amounts present in the soil solution at any one time represent a balance between synthesis and destruction by microorganisms.

The free amino acids content of the soil is strongly influenced by weather conditions, microbial activity, moisture status of the soil, type of plant and stage of growth, level of soil carbon and incorporation of organic residues (eg. stubble).

Amino sugars

Amino sugars occur as structural components of a broad group of substances, the muco-polysaccharides and they have been found in combination with muco-peptides and muco-proteins. Some of the amino sugar material in soil may exist in the form of an alkali-insoluble polysaccharide referred to as chitin.

Generally the amino sugars in soil are of microbial origin. From 5 to 10% of the N in the surface layer of most soils can be accounted for in N-containing carbohydrates or amino sugars.

An alpha-amino acid has the generic formula H2NCHRCOOH. Amino acids are critical to life, and have many functions in metabolism. One particularly important function is to serve as the building blocks of proteins, which are linear chains of amino acids. Amino acids can be linked together in varying sequences to form a vast variety of proteins.

  • Nitrogen in the air is the ultimate source of all soil nitrogen. This can be achieved with adequate soil carbon levels and a diverse and abundant microbial activity.
  • Nitrogen may enter the soil through rainfall, plant residues, nitrogen fixation by microbes (soil organisms), animal manures and commercial fertilisers.
  • There is ultimately no difference between the nitrogen that enters the plant from commercial fertilisers and that from organic products.
  • Nitrogen may be lost from the soil by plant removal, volatilisation, leaching or erosion.
  • Leaching of nitrate (NO3-) is a pollution hazard; control nitrogen losses with proper soil management practices. This will occur in sandy soils (low clay levels), low soil carbon (eg. <2%), low microbial activity, low soil moisture, compaction and in the presence of chemical residues.
  • Nitrogen is only maximised in the soil by plants when the soil carbon levels are adequate (eg. above 2%) and microbial activity is diverse and abundant.
  • Nitrogen delivered to the soil as a compound or in a complex form (eg. amino acids) in a liquid formulation (ie. fermented) may be the best form to deliver nitrogen to the soil, rather than as N and in an organic form which cannot be used by plants until it is gradually transformed by soil microorganisms to ammonium (NH4+)
  • Soil microorganisms use nitrate (NO3-) and ammonium (NH4+) nitrogen when decomposing plant residues. These forms are temporarily "tied-up" (incorporated into microbial tissue) in this process. However, the soil carbon to nitrogen ratio needs to be balanced.