Nitrogen is the building block of amino acids, proteins and chlorophyll. Plants can absorb nitrogen either in the nitrate form (NO3-) or the ammonium form (NH4+). The total uptake of nitrogen usually consists of a combination of these two forms.

The ratio of ammonium-to-nitrate affects both plants and the soil or grow medium. For optimal uptake and growth, each plant species requires a different ammonium-to-nitrate ratio. The correct ratio also varies with temperature, growth stage, pH in the root zone and soil/grow medium properties.

How the Temperature of Soil Affects Nitrogen Metabolism

To understand how temperature affects plants’ abilities to metabolize the different forms of nitrogen, first we need to understand the different ways these two nutrient forms are metabolized by plants. When plants metabolize ammonium, more oxygen is consumed than with 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, plants’ respiration is increased, consuming sugars faster and making them less available for ammonium metabolism in the roots. At the same time, at high temperatures, the oxygen solubility in water is decreased, making it less available as well. Therefore, at higher temperatures, applying less ammonium is advisable.

At lower temperatures, ammonium nutrition is a more appropriate choice, because oxygen and sugars are more available at the root level. In addition, since transport of nitrate to the leaves is restricted at low temperatures, basing a fertilization program on this form of nitrogen will delay plant growth.

Plant Species and Growth Stages

As already discussed, sugars need to be transported down from the leaves to the roots so plants can metabolize the ammonium. When growing plants where the majority of the growth is in the leaves (Chinese cabbage, lettuce, spinach, etc.), 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 using a lower ammonium-to-nitrate ratio is preferred.

Ammonium, Nitrate and pH

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. When the plant takes up ammonium, it releases a proton. An increase in protons around the roots decreases the pH. Alternatively, when the plant takes up nitrate it releases bicarbonate (HCO3-), which increases the pH around the roots. Therefore, we can safely conclude that nitrate uptake increases pH around the roots while uptake of ammonium decreases it.

This phenomenon is especially important in soilless media, where the roots may easily affect the grow medium’s pH because their volume is relatively large compared with the medium’s volume. To prevent the grow medium’s pH from rapidly changing, we should keep an appropriate ammonium-to-nitrate ratio, according to the cultivar, temperature and the growing stage.

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 rapid process, and the added ammonium may be quickly converted and absorbed as nitrate, increasing pH in the root zone instead of decreasing it.

Other Nutrients

Ammonium is a cation (positively charged ion), so it competes with other cations such as potassium, calcium and magnesium for uptake by the roots. An unbalanced fertilization program, with ammonium content that is too high, might result in calcium and magnesium deficiencies, while potassium uptake is less affected by the competition.

Nutrient Requirements of Different Crops

Almonds

Nutrient requirements for a yield goal of 7 ton/ha:

N - 83 kg/ha

P - 40 kg/ha

K - 100 kg/ha

Ca - 70 kg/ha

Mg - 16 kg/ha

Celery

Nutrient requirements for a yield goal of 80 ton/ha:

N - 185 kg/ha

P - 60 kg/ha

K - 450 kg/ha

Ca - 145 kg/ha

Mg - 12 kg/ha

Corn

Nutrient requirements for a yield goal of 9.5 ton/ha:

N - 191 kg/ha

P2O5 - 89 kg/ha

K2O - 235 kg/ha

CaO - 57 kg/ha

MgO - 73 kg/ha

Eggplants

Nutrient requirements for a yield goal of 40 ton/ha:

N - 207 kg/ha

P - 20 kg/ha

K - 330 kg/ha

Ca - 40 kg/ha

Mg - 24 kg/ha

Garlic

Nutrient requirements for a yield goal of 9.8 ton/ha:

N - 142 kg/ha

P - 24 kg/ha

K - 105 kg/ha

Ca - 57 kg/ha

Mg - 9 kg/ha

Onions

Nutrient requirements for a yield goal of 40 ton/ha:

N - 155 kg/ha

P - 60 kg/ha

K - 132 kg/ha

Ca - 52 kg/ha

Mg - 20 kg/ha

Oranges

Nutrient requirements for a yield goal of 40 ton/ha:

N - 135 kg/ha

P - 16 kg/ha

K - 110 kg/ha

Ca - 90 kg/ha

Mg - 12 kg/ha

Peas

Nutrient requirements for a yield goal of 10 ton/ha:

N- 125 kg/ha

P - 20 kg/ha

K - 105 kg/ha

Ca - 45 kg/ha

Mg - 37 kg/ha

Potatoes

Nutrient requirements for a yield goal of 40 ton/ha:

N - 170 kg/ha

P - 22 kg/ha

K - 220 kg/ha

Ca - 30 kg/ha

Mg - 28 kg/ha

Rice

Nutrient requirements for a yield goal of 9.8 ton/ha:

N - 217 kg/ha

P - 68 kg/ha

K - 256 kg/ha

Ca - 27 kg/ha

Mg - 23 kg/ha

Greenhouse Tomatoes

Nutrient requirements for a yield goal of 154 ton/ha:

N - 370 kg/ha

P - 63 kg/ha

K - 560 kg/ha

Ca - 205 kg/ha

Mg - 67 kg/ha

Table Tomatoes - Open Field

Nutrient requirements for a yield goal of 60 ton/ha:

N - 136 kg/ha

P - 24 kg/ha

K - 192 kg/ha

Ca - 240 kg/ha

Mg - 22 kg/ha

Hydroponic Tomatoes

N - 245 ppm

P - 39 ppm

K - 400 ppm

Ca - 240 ppm

Mg - 55 ppm

Source: smart-fertilizer.com