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Nitrogen and Nitrogen Fertilizers

Nitrogen (N) is a key nutrient in manipulating plant growth. Most nursery/floral producers use large quantities of N fertilizers in a "blanket" attempt to meet the needs of their crops. However a thorough understanding of N nutrition Can be useful in optimizing both the concentration and form of N best suited for the plant species, stage of growth, time of year and production objectives.

Plants require N in relatively large quantities and in forms that are readily available.

Nitrogen metabolism is a well studied and a vital aspect of plant growth. Nitrogen is one of the important building blocks in amino acids:




Amino acids are typically made up of an amino group (NH2), carbon (C), a carboxyl group COOH), and a variety of molecular structures (R) which define individual amino acids (glycine, serine, licine, alanine, etc.). When these amino acids link together in long chains they form proteins. Proteins are also vital components in a variety of metabolic pathways and processes. Proteins makeup the molecular structure of DNA, RNA and a host of other critical metabolic processes required for plant growth.

When N is deficient in plants restricted growth of tops and roots and especially lateral shoots may occur. Plants also become spindly with a general chlorosis of entire plant to a light green and then a yellowing of older leaves. This condition may proceed toward younger leaves. Older leaves defoliate early.

Plants can take up N in 4 forms:

NH4 Ammonium
NO3 Nitrate
Organic Nitrogen
Molecular Nitrogen

Regardless of the N source (inorganic fertilizer, organic fertilizer, manure, etc.) plants can only take up N in these 4 forms. That means that some conversions must occur in the growing media/root zone (rhizosphere) before some sources of N can be taken up by the plant. All 4 forms of available N have unique characteristics that influence plant growthin different ways. Understanding these characteristics is very important in matching the best N fertilizer with plant species, stage of growth, time of year and production objectives. The following is a brief description of these 4 N forms and some additional information on the most common fertilizer sources for each.

Nitrate NO3 and Ammonium NH4 Nitrogen:
The roots of most plants absorb N from the growing medium in the form of NO3. Nitrogen in this form, however, is not directly used by the plant but must be reduced to ammonia (NH3) before it can be assimilated by the plant. The process of nitrate reduction to ammonia is a 3 step process:

NO3 a NO2 a NH3
Nitrate Nitrite Ammonia

This conversion is dependent of the presence of several enzymes (i.e. nitrate reductase) for the conversion to complete it's cycle. These enzymes, and the microorganisms that indirectly produce them, are effected by several factors including: temperature, moisture, etc. If the conversion process stops at the nitrite stage serious damage may occur. Nitrite is toxic to plants at low to moderate levels and can cause significant reductions in growth at low levels.

Both nitrate and ammonium fertilizers are commonly used to provide supplemental nutrition for nursery/floral crops. Ammonium (NH4) fertilizers must first be converted to nitrate NO3 before it can be used by the plant. This is a 2 step process in which ammonium is first converted to nitrate and then the nitrate is subsequently converted to ammonia. This process, known as nitrification, is dependent on several soil microorganisms (Nitrosomnas, Nitrobacter). These microorganisms are effected by several factors including: temperature, moisture, etc.

2NH4 + 3O2 a 2NO2 + 2H2O + 4H
Ammonium Oxygen Nitrate Water

and then

NO2 + O2 a 2NO3
Nitrite Oxygen Nitrate

Ammonium is the most common, and perhaps the lowest cost supplemental source of N for plant growth. Research has shown that the balance between nitrate (NO3), nitrogen (N) and ammonium (NH4) can effect plant growth. In Texas it is recommended that no more than 50% of the N supplied should be in the NH4 form. Increased amounts of NH4 in the growing media may result in severe ammonium toxicity (nitrites??).

Organic Nitrogen:
Many plants are capable of using organic, as well as inorganic N. As they breakdown in the growing medium, many of the amino acids, amides and proteins provide available N for plant growth. However, urea is perhaps the most commonly used source of organic N for nursery and floral crops.



Urea must first be converted to ammonia before it can be used by the plant. This conversion is dependent on the enzyme urease. Urease is another compound that is effected by factors such as temperature, moisture, etc.


NH2 C NH2 a 2NH3 + CO2
Urea Ammonia

Under cool temperatures urease is often rendered inactive and little, if any, N is available for plant growth.

Molecular Nitrogen:
Many plants are capable of fixing N directly from the atmosphere (legumes). This process usually requires the indirect mediation of soil microorganisms. Perhaps the best example of N fixation is in soy beans. Beans are inoculated with specific N fixing microorganisms prior to planting. Nodules are then formed on the root system which indirectly provide atmosphereic N to the plant.

Although several nursery/floral crops have the capability to fix N from the atmosphere, most growers provide supplemental fertility to compensate for the potential lack of these specific microorganisms in soilless growing substrates.