Nitrogen is the building block of amino acids, proteins and chlorophyll. Plants can absorb nitrogen either as Nitrate (NO3-) or Ammonium (NH4+), and therefore, the total uptake of nitrogen usually consists of a combination of these two forms.
The ratio between Ammonium and Nitrate is of a great significance, and affects both plants and soil/medium.
For optimal uptake and growth, each plant species requires a different ammonium/nitrate ratio. The correct ratio to be applied also varies with temperature, growth stage, pH in the root zone and soil properties.
First we need to understand the different ways these two nutrient forms are metabolized:
Ammonium metabolism consumes much more oxygen than metabolism of Nitrate.
Ammonium is metabolized in the roots, where it reacts with sugars.
These sugars have to be delivered from their production site in the leaves, down to the roots.
On the other hand Nitrate is transported up to the leaves, where it is reduced to Ammonium and then reacts with sugars.
At higher temperatures the plant's respiration is increased, consuming sugars faster, making them less available for Ammonium metabolism in the roots. At the same time, at high temperatures, Oxygen solubility in water is decreased, making it less available as well.
Therefore, the practical conclusion is that at higher temperatures applying a lower Ammonium/Nitrate ratio is advisable.
At lower temperatures Ammonium nutrition is a more appropriate choice, because oxygen and sugars are more available at root level. In addition, since transport of Nitrate to the leaves is restricted at low temperatures, basing the fertilization on Nitrate will delay the plant's growth.
As we already established, sugars need to be transported down from the leaves to the roots to meet the Ammonium.
In growing fruits and plants in which the majority of the growth is in the leaves (e.g. Chinese cabbage, lettuce, spinach), sugars are consumed quickly near their production site and are much less available for transport to the roots.
Thus, Ammonium will not be efficiently metabolized and use of a lower Ammonium/Nitrate ratio is preferred.
Electrical balance in the root cells must be maintained, so for each positively charged ion that is taken up, a positively charged ion is released and the same is true for negatively charged ions.
Thus, when the plant takes up Ammonium (NH4+), it releases a proton (H+) to the soil solution. Increase of protons concentration around the roots, decreases the pH around the roots.
Accordingly, when the plant takes up Nitrate (NO3-) it releases bicarbonate (HCO3-), which increases the pH around the roots.
We can conclude that uptake of Nitrate increases pH around the roots while uptake of Ammonium decreases it.
This phenomena is especially important in soil-less media, where the roots may easily affect the medium pH because their volume is relatively large compared with the medium's volume. To prevent medium pH from rapidly changing, we should keep an appropriate Ammonium/Nitrate ratio, according to the cultivar, temperature and the growing stage.
It is noteworthy that under certain conditions, the pH may not respond as expected due to nitrification (conversion of Ammonium into Nitrate by bacteria in the soil). Nitrification is a very rapid process, and the added ammonium may be quickly converted and absorbed as Nitrate, thus increasing pH in the root zone, instead of decreasing it.
Ammonium is a cation (positively charged ion), so it competes with other cations (Potassium, Calcium, Magnesium) for uptake by the roots. An unbalanced fertilization, with too high Ammonium content, might result in Calcium and Magnesium deficiencies. Potassium uptake is less affected by the competition.
As already mentioned, Ammonium/Nitrate ratio may change the pH near the roots. These pH changes may affect solubility and availability of other nutrients.