MSU Extension Floriculture & Greenhouse Crop Production

Water-soluble fertilizers are the most common fertilizer choice for growers in greenhouse production. When looking at a fertilizer label, the first and usually largest number represents nitrogen. However, that number only refers to the percentage of total nitrogen in the fertilizer and not the form of nitrogen, of which there are three. To find this information, you must look at the guaranteed analysis on the label (Tables 1 and 2). Nitrogen is an essential nutrient that plants need in relatively large amounts, and because it can influence the pH of your substrate over time, it's important to understand the different forms of nitrogen in water-soluble fertilizers to produce high-quality crops.

Forms of nitrogen in water-soluble fertilizers

Nitrogen in water-soluble fertilizers can exist in different forms: nitrate, ammoniacal nitrogen and urea. Each form of nitrogen has different characteristics that affect how it reacts in the substrate and how plants uptake and assimilate it.

Nitrate

Nitrate is the most mobile form of nitrogen and easily dissolves in water. When applied to the substrate, it moves readily with water and does not attach to substrate particles. This mobility can have advantages in some situations but can also result in nitrate leaching and loss. Generally, nitrate is the form of nitrogen most often used in plug and liner production as well as finished crops. Nitrate assimilation is regulated by the plant, reducing the occurrence of toxicity.

For this reason, nitrate-nitrogen is usually the most abundant form in greenhouse fertilizers, especially in dark-weather feeds. Fertilizers with a high percentage of nitrate-nitrogen are labeled as having potential basicity (Table 1) because the uptake of nitrate by the plant increases the substrate pH. Fertilizers with potential basicity are often used to fertilize micronutrient-efficient crops like marigolds and geranium. Due to the production process, nitrate-nitrogen tends to be more expensive than the other forms listed in this article.

Ammoniacal nitrogen

Ammonium, labeled as "ammoniacal nitrogen" on fertilizer labels, is a readily available form of nitrogen. While ammonium is water-soluble, it readily attaches to organic matter particles, which helps prevent it from being leached. Fertilizers with a moderate to high percentage of ammoniacal nitrogen are labeled as having a potential acidity (Table 2) because they decrease the substrate pH. Fertilizers with potential acidity are often used to fertilize micronutrient-inefficient crops like calibrachoa and petunia. Growers should consider the growing environment when deciding if ammoniacal nitrogen suits their crops since ammonium toxicity is problematic in cool and dark environments.

There is a perception that using a fertilizer with a high percentage of ammoniacal nitrogen can stimulate growth. However, research performed at North Carolina State University in the early 2000s showed that the form(s) of nitrogen did not influence plant size. Instead, it was the relatively high levels of phosphorus present in fertilizers with a high percentage of ammoniacal nitrogen that stimulated growth. Therefore, for more compact growth provide less phosphorus, and for more elongated and lush growth, provide more phosphorus, independent of the form of nitrogen.

Urea

Urea is another form of nitrogen commonly found in water-soluble fertilizers. Urea-based fertilizers are typically more affordable than other forms of nitrogen. Urea-nitrogen and ammoniacal-nitrogen are almost identical in how the plant responds and utilizes nitrogen. Therefore, all the characteristics and risks mentioned for ammoniacal nitrogen also apply to urea-nitrogen. The main difference between urea-nitrogen and ammoniacal-nitrogen occurs before the conversion of urea to ammonium.

Urea dissolves in water and moves with substrate water, like nitrates. Before plants can utilize nitrogen in urea, it must convert to both ammonia and ammonium forms. Urea can be lost through leaching if it does not convert to ammonia, and ammonia can be lost to volatilization if it does not convert to ammonium. Under warm-growing conditions, this conversion can happen in as little as a few hours.

So, what form of nitrogen should I use?

The choice of nitrogen form in your greenhouse water-soluble fertilizer depends on various factors including the crops grown, water alkalinity and environmental conditions. In most cases, at least two forms of nitrogen are simultaneously used. Some growers initially utilize ammoniacal or urea-based nitrogen because of their affordability. However, a predominantly nitrate-based fertilizer is usually advised to prevent ammonium toxicity and a decrease in substrate pH over time. Excessive ammoniacal nitrogen content, exceeding 30%, can potentially lead to ammonium toxicity in plants from late fall through early spring. Therefore, several factors should be considered when determining the nitrogen forms and their percentages to apply to greenhouse crops including specific crop responses, environmental conditions, water alkalinity and the potential risks associated with each form of nitrogen. Consult with a university extension educator or greenhouse fertilizer supplier to help you make more informed fertilizer decisions.