Know your nitrogen

The role of nitrogen in greenhouse crop production is very important. Nitrogen affects the growth and morphology (i.e. size) and quality (i.e. appearance) of our crops. Too much nitrogen can result in excessive growth. Too little nitrogen results in pale or chlorotic foliage and plants that may be too small.

In addition to the direct effect of nitrogen on plants, nitrogen can also affect the root zone pH (Figs. 1 & 2). Unlike other nutrients, there are different forms of nitrogen in fertilizers, including nitrate (NO₃^-), ammonium (NH₄⁺), and urea (CH₄N₂O). This article aims to highlight how the different forms of nitrogen can affect root zone pH and how you can use these different forms of nitrogen to improve your crop culture.

Nitrate

Nitrate (NO₃^-) is the form of nitrogen that can cause the root zone pH to increase over time. When plants take up nutrients, they must maintain an electrochemical balance in their cells. Since nitrate is a negatively charged ion, plants exude a hydroxide ion (OH^-) for each nitrate ion taken up by roots. By releasing a negative ion for every negative ion taken up, the plant remains electrochemically balanced (Fig. 2).

While nitrate can increase the substrate pH, it is dependent on crop growth. If plants are growing slowly (or not at all!), little to no nitrate is taken up. As a result, no hydroxide is released and the pH will not drift upwards. In order for the nitrate to influence root zone pH, plants must be actively growing and taking up nitrogen.

Ammonium and urea

Ammonium (NH₄⁺) and urea (CH₄N₂O) are the forms of nitrogen that can decrease root zone pH over time. While ammonium is a positively charged ion, urea doesn’t have a charge. However, urea is converted to ammonium and therefore, is considered to be a form of ammoniacal nitrogen, with respect to its function in the root zone.

Ammoniacal nitrogen is taken up by roots and, like nitrate, plants maintain an electrochemical balance during this process. However, instead of a hydroxide ion, a positively charged hydrogen ion (H⁺) is exchanged for the positively charged ammonium ion. As the concentrations of hydrogen increase, the root zone pH decreases (Fig. 1).

Ammoniacal nitrogen can also decrease root zone pH as it is converted to nitrate. Positively charged hydrogen ions are released when ammonium is converted to nitrogen by bacteria in the substrate, reducing the pH. Unlike nitrate, ammonium can still affect substrate pH even if plants are growing slowly. While no hydrogen ions will be released due to diminished uptake, the ammonium may still be converted to nitrate and acidify the substrate. One caveat is that the bacteria that convert ammonium to nitrate are less active when temperatures are cooler and, if ammonium is not being converted to nitrogen, there is a risk of ammonium toxicity.

Selecting the right fertilizer

So which fertilizer are you going to choose? “It depends” would be the correct answer! There are a wide range of fertilizers available, so to choose the one that is best for you, you’ll need to consider the plant material you’re fertilizing, the growing environment, and the chemical properties of your growing substrate and irrigation water.

There are some fertilizers that are comprised of 100 percent ammonium (ammonium sulfate, diammonium phosphate) or nitrate (calcium nitrate, potassium nitrate). However, research has shown that plants tend to grow better when both forms of nitrogen are present in a fertilizer, and the majority of complete, balanced water-soluble fertilizer and controlled release fertilizers contain a blend of the different nitrogen forms (Fig. 3).

The next question, then, is what proportion of each form? Again, it depends! You will need to consider several factors, including crop requirements, irrigation water quality (alkalinity and pH), substrate pH, and environmental conditions. If you are growing crops that are efficient at taking up micronutrients (the “geranium group”), have very low alkalinity, and/or are growing at cool temperatures, select a fertilizer that has more nitrate and will increase pH. Alternatively, if you are growing crops that are inefficient at taking up micronutrients (the “petunia group”) or if your water has higher alkalinity and pH, select a fertilizer with more ammoniacal nitrogen that will decrease pH.

Bottom line

The different forms of nitrogen in your fertilizer can affect crop growth and development differently by affecting the root zone pH. By knowing what type of fertilizer would be best for your crops, growing conditions, and cultural practices, you can avoid problems during production. Familiarizing yourself with the multitude of different fertilizers available will give you greater flexibility during production and control over your crop quality.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu