The Ammonium/Nitrate Ratio Importance in soilless growing medium
|Ammonium metabolism requires much more oxygen than Nitrate exchange in the roots|
|Adi Nave, Gat Fertilizers Agronomist, firstname.lastname@example.org|
|September 8, 2020|
Soilless growing medium cultivation is considered the most advanced system in modern agriculture. This is primarily because when growing in soil, the ability to influence in the plant's underground section is limited, whereas control can be maximized with soilless growing medium crops (worldwide defined as 'hydroponics' method), both over the above-ground section of the crop as well as over the root system.
Appropriate pH control may be done by controlling acidification by injection acid to irrigation water. However, it's important to remember that nitrogen ions composition is significant in this aspect.
Ammonium metabolism requires much more oxygen than Nitrate exchange in the roots. Ammonium metabolism occurs in the roots, where it reacts with sugars, so that the plant invests energy in order to transfer sugars from their production site in leaves to the roots. On the other hand, Nitrate is transferred from the roots to the leaves, where it interacts with the sugars.
The plant maintains electrolyte balance in the root cells, so that each positively charged ion taken from the ground solution releases a positively charged ion from the root cells into the solution. This process is also correct for negatively charged ions.
Therefore, when the plant's roots absorbs Ammonium (NH4+) it releases a proton (H+) to the substrate solution, the increase in protons concentration around the roots reduces the pH, By contrast, when the plant absorbs Nitrate (NO3- ), it releases hydroxide (HO-), which increases the pH around the roots (Rhizosphere)
These facts are particularly important when growing on soilless. since the roots are likely to easily influence the solution's pH', defending on the nitrogen configuration absorbed. This is occurred because their volume is relatively great in the substrate's volume. In order to prevent an extreme decline in acidity to a level injurious to the plant, an appropriate Ammonium / Nitrate ratio must be maintained, according to the type of crop, the temperature and the growth stage.
Diagram 1: Nitrification Schema
For most plants' root systems, when absorbing nitrogen, there is a clear preference for Ammonium as opposed to Nitrate. But by contrast, the plant's ability to self-ferment the area of the root zone is so efficient that a plant growing in a solution containing Ammonium as an exclusive source of nitrogen will reduce the solution's pH down to less than 3.5, and inflict on itself irreversible damage.
Graph 1: Growing tomatoes in soilless growing medium
Ammonium is a Cation (positively charged ion) and therefore competes with other cations (Potassium, Calcium, Magnesium) in being absorbed by the roots. Unbalanced fertilization, with too much Ammonium, is liable to cause Calcium and Magnesium deficiencies (absorption of Potassium is less affected by this competition). Fertilization with too much Ammonium is likely to cause a Calcium deficiency in the fruit and appearance of Blossom-End Rot, especially in a warm, dry climate.
To conclude, the Ammonium / Nitrate ratio is likely to change the pH near the root zone. This change is likely to influence the solubility and availability of other basic nutrients. Hence, using fertilizer containing Ammonium for growing in soilless growing medium is a step requiring extreme caution, especially with crops that are sensitive to Calcium and Magnesium deficiencies (tomato and pepper). The latter is totally true during the winter when the rate of Nitrification in the substrate decreases precipitously as the temperature drops. Most of the Ammonium nitrogen provided to the plant remains as Ammonium, is absorbed as Ammonium and will cause serious damage to the plant.