Nitrogen, a fundamental element, plays an important role in the growth and development of all living organisms, including plants. In the environment, nitrogen exists in various forms, such as nitrates (NO3-) and nitrites (NO2-). Nitrates combine nitrogen with three oxygen atoms, while nitrites contain nitrogen with two. Both are naturally present in soil, water, and air. Understanding how plants interact with these forms is important for supporting healthy plant life.
Why Plants Need Nitrogen
Nitrogen is an essential macronutrient for plants, required in large quantities for their growth. It serves as a primary building block for components within plant cells. Nitrogen is a key element in amino acids, the units that construct proteins. Proteins are important for cell structure, enzyme functions, and metabolic processes like photosynthesis and respiration.
Nitrogen is also a component of nucleic acids, such as DNA and RNA, which carry genetic information and are involved in cell division. It is part of chlorophyll, the green pigment responsible for capturing sunlight during photosynthesis. Nitrogen is also part of adenosine triphosphate (ATP), the primary energy currency within plant cells, facilitating energy transfer for cellular activities.
How Plants Take Up Nitrogen
Plants primarily absorb nitrogen from the soil in two inorganic forms: nitrate (NO3-) and ammonium (NH4+). Nitrate is the most common form available in well-aerated agricultural soils. Plant roots possess specialized transport proteins that actively facilitate the uptake of these ions from the soil solution.
While nitrites (NO2-) are present in soil as an intermediate step in the nitrogen cycle, they are generally toxic to plants and not the preferred form for absorption. In healthy soil, nitrites are quickly converted to less harmful nitrate by soil microorganisms. If nitrite is absorbed, it is rapidly converted to ammonium, as it is a highly reactive compound that can interfere with cellular processes. Plants have internal mechanisms, such as specific enzymes, to manage any absorbed nitrite and convert it to safer forms for assimilation.
Nitrogen Forms in Soil
Nitrogen in soil exists in various forms, constantly transforming through the nitrogen cycle. Most soil nitrogen is initially in organic forms, unavailable for direct plant uptake. Soil microbes convert this organic nitrogen into plant-available inorganic forms through mineralization. During mineralization, decomposers break down organic matter, releasing ammonium (NH4+).
Once ammonium is present, specialized soil bacteria carry out nitrification, a two-step process. First, ammonium converts to nitrite (NO2-), then nitrite rapidly converts into nitrate (NO3-). Nitrate is mobile in soil because it is negatively charged, making it susceptible to leaching with water movement. Ammonium, being positively charged, tends to bind to soil particles, making it less mobile than nitrate.
Environmental Factors Affecting Nitrogen Uptake
Several environmental factors influence a plant’s ability to absorb nitrogen from the soil. Soil pH, a measure of acidity or alkalinity, affects nitrogen availability and microbial activity. Nitrate is most readily available in soils with a pH typically between 6.0 and 7.5. Nitrification, the conversion of ammonium to nitrate, is most rapid in soils with a pH between 7 and 8, slowing below pH 5.
Soil temperature also plays a role in nitrogen uptake and availability. Warmer soil temperatures generally increase nitrogen availability and enhance plant uptake rates. Microbial processes like mineralization and nitrification are more rapid in warm soil, typically between 67-86°F (16-35°C). These processes slow when soil temperatures drop below 41°F (5°C).
Oxygen levels in the soil are another important factor. Nitrification is an aerobic process, requiring oxygen. In soils with low oxygen, such as waterlogged conditions, nitrifying bacteria activity is inhibited, leading to ammonium accumulation and reduced nitrate availability. Oxygen-deficient conditions can also impair root growth and nutrient absorption. Other nutrients like phosphorus and potassium can influence nitrogen uptake. Water availability also facilitates nitrogen movement to plant roots for uptake.