Nitrogen is a primary macronutrient, required by plants in larger quantities than any other element except carbon, hydrogen, and oxygen. It is a fundamental component of chlorophyll, essential for photosynthesis, and a building block for all amino acids, which form plant proteins. Although the atmosphere is nearly 78% nitrogen gas (\(\text{N}_2\)), plants cannot directly absorb this inert form. To be useful, atmospheric nitrogen must first be converted, a process that occurs naturally through biological fixation and industrially to create fertilizers. Fertilizer supplies nitrogen in specific chemical forms that plant roots can absorb from the soil.
The Three Essential Forms of Nitrogen in Fertilizer
Commercial fertilizers rely on three distinct chemical forms of nitrogen: Nitrate, Ammonium, and Urea. These forms differ in their electrical charge, mobility in the soil, and immediate availability for plant uptake. Nitrate (\(\text{NO}_3^-\)) is a negatively charged ion, highly water-soluble, and immediately available to plant roots. Because it does not bind to soil particles, this high mobility means nitrate is easily carried away from the root zone through water movement, a process known as leaching.
Ammonium (\(\text{NH}_4^+\)) carries a positive charge. This positive charge allows it to temporarily bind to the negatively charged surfaces of soil particles, which significantly reduces its mobility and susceptibility to leaching compared to nitrate. Plants can absorb ammonium directly, but it provides a more stable, slightly slower-releasing source of nitrogen because of its binding to the soil structure.
Urea (\(\text{CO}(\text{NH}_2)_2\)) is an organic, amide form of nitrogen and is the most common nitrogen fertilizer used globally due to its high nitrogen content. Unlike nitrate and ammonium, urea cannot be used directly by most plants and must first undergo a chemical transformation in the soil. This conversion process releases the nitrogen into a usable form, making urea a slower-acting source than immediately available nitrate or ammonium.
The Role of Soil Microbes in Nitrogen Conversion
The different chemical forms of nitrogen are connected by transformations driven by living organisms in the soil, which determine nutrient availability. The conversion of urea begins almost immediately upon application through hydrolysis. This process is catalyzed by the urease enzyme, which is naturally present in the soil and produced by various microorganisms. Hydrolysis rapidly converts the urea molecule into ammonium (\(\text{NH}_4^+\)) and carbon dioxide.
The conversion temporarily raises the pH of the soil immediately surrounding the fertilizer granule. If the urea is left on the soil surface, this reaction can lead to the loss of nitrogen to the atmosphere as ammonia gas, a process called volatilization. Once ammonium is formed, a second biological process called nitrification begins. Nitrification is a two-step oxidation process carried out by two distinct groups of soil bacteria.
The first step involves Nitrosomonas bacteria, which convert ammonium (\(\text{NH}_4^+\)) into nitrite (\(\text{NO}_2^-\)). Nitrobacter bacteria then oxidize the nitrite into the final product, nitrate (\(\text{NO}_3^-\)). This microbial conversion pathway is temperature-dependent and dictates the rate at which a fertilizer transitions from the stable ammonium form to the highly mobile nitrate form.
Decoding Fertilizer Labels and Release Rates
Understanding the forms of nitrogen in a fertilizer requires interpreting the guaranteed analysis printed on the product label. All fertilizers display three numbers, known as the N-P-K ratio, which represent the percentage by weight of Nitrogen (N), Phosphate (\(\text{P}_2\text{O}_5\)), and Potash (\(\text{K}_2\text{O}\)). The first number represents the percentage of elemental nitrogen present in the product.
Quick-Release Components
A detailed label specifies the sources of nitrogen, differentiating between quick-release and slow-release components. Quick-release nitrogen is water-soluble and provides nutrients immediately upon application, often consisting of simple nitrate or non-coated urea. This rapid availability supports immediate plant growth but increases the risk of nutrient loss through leaching or volatilization shortly after application.
Slow-Release Components
Slow-release or controlled-release nitrogen sources are designed to provide a steady supply of nutrients over an extended period. These are often created by coating water-soluble nitrogen forms, like urea, with a polymer or sulfur shell that breaks down gradually. Alternatively, they may be complex organic sources that require extensive microbial activity to mineralize. This controlled breakdown ensures consistent nutrient delivery, which is more efficient for the plant and reduces environmental risks.