Nitrogen Cycle - What You Need to Know about Mineralization

 

Nitrogen (N) fertilizer is one of the greatest input costs a producer incurs throughout the growing season, and can have one of the largest impacts on crop yield/quality. It is a plant essential macro-nutrient which means it’s function in the plant cannot be replaced by any other element, is essential for the plant to complete its life cycle, and acquired by the plant in larger quantities than most other nutrients.

 

 

Nitrogen application

Nitrogen should be applied in a way that promotes fertilizer recovery, maximizes economical return, and lessens environmental impact. Once N fertilizer is applied to the environment, it is susceptible to multiple transformations that can render N unavailable to the plant. Therefore, it is important to understand different N transformations in the soil to maximize crop recovery of applied N and increase fertilizer use efficiency.

 

Nitrogen in the Soil

 Nitrogen can go through many transformations in the soil (Fig. 1). Nitrogen exists in the soil in both organic and inorganic forms. Organic N makes up the largest (>90%) pool of N in the soil and requires transformation before becoming plant available. Inorganic N in the form of ammonium (NH4+) and nitrate (NO3-) are the typical plant available forms of N in the soil, accounting for approximately 10% of the total soil N. The first topic to be discussed in the N cycle is mineralization.

Figure 1: Top dress Urea apllication

 

Mineralization

The N mineralization process (carried out by microbes) converts organic N into inorganic N, with the ability to supply large amounts of plant available N to crops throughout the growing season. This organic-N pool in the soil is made up of organic matter, crop residues, manure, and other organic compounds. Mineralization is a two-step process where organic-N is first converted to ammonia and then reacts with water to turn into ammonium.

The quantity of ammonium in the soil is important because it is plant available and can be attached to soil colloids making it more stable than other forms of N.

 

Mineralization Factors

The quantity of soil N that becomes available to crops throughout the growing season is dependent upon the factors that influence the rate of mineralization. One of the key limitations in developing site-specific N fertilizer recommendations for agricultural crops is the ability to predict how much soil N will be available to the plant at different growth stages.

 

What Influences Mineralization?

Being a biological process, mineralization is influenced by several variables including C/N ration of residue, organic matter content of the soil, temperature, moisture content, pH, and soil aeration. Temperature plays an important role in the rate of N mineralization, with mineralization generally increasing as soil temperature increases. This interaction is why N mineralization rates typically peak in the summer and are low in the winter months.

 

The Goal of Nitrogen Management

The goal of N management is to maximize N efficiency by capitalizing on all N available to crops whether it is from the soil, atmosphere, or through the use of fertilizers. Producers should take into consideration previous crop, organic matter content of the soil, manure applications, and all other factors contributing to the supply of N being mineralized when developing a N management plan.

 

The Takeaway

To learn more about the nitrogen cycle and how to properly manage the nutrients in your soil, contact your local Simplot Grower Solutions Crop Advisor.