Unlocking Soil Health: Why Timing Your Nitrogen Matters More Than You Think

By Bob Gunzenhauser, Director of Agronomy

There has been a great deal of discussion about soil health in agricultural management circles in the last few years.  Terms like “regenerative agriculture” have become more commonly used for practices that would tend to lead towards an overall improvement of soil health.  However, there is still some consensus to be made about exactly what soil health and regenerative agriculture is and how to measure it. 

Indeed, to many conventional farmers, the discussion about soil health can be clouded around the uncertainty of profitability. Farmers typically have management programs that work for them from operations, logistics and profitability standpoints. Asking them to make changes to these time-tested approaches for the sake of a perceived new buzzword and obscured outcome is difficult. 

However, research is starting to reveal that perhaps our conventional approaches are not all that healthy for our soils, and by utilizing this research and making incremental changes to existing programs, farmers can start to profit from this deeper understanding of soil health and microbiology. 

One area of potential improvement is through proper nitrogen management. While farmers know the benefit of sufficient nitrogen application, they may not always see the effects the timing, number of applications, or method have on microbial communities that may provide benefits. 

A single application of nitrogen to the soil will lead to a rapid increase of ammonium and/or nitrate forms of inorganic nitrogen, depending on the source. This sudden change in soil nitrogen balance can create osmotic stress and toxicity for microbes and suppress certain groups, like fungi. In contrast, multiple applications of inorganic nitrogen fertilizer create less stress to the microbial community (lower nitrogen spikes) and reduces stress. 

In terms of microbial composition, large single applications shift the taxa of microbes towards those that thrive in high-nitrogen environments, especially nitrate-reducing bacteria. Multiple-application programs can have more nitrogen-fixing bacteria and ammonia-oxidizing archaea. 

It is well documented that a single application of ammonia nitrogen fertilizer creates a rapid pH shift upon application, first sharply increasing pH levels then lowering it over time. Multiple applications of nitrogen reduce the magnitude of these shifts and allow the soil to buffer against rapid changes. 

Cover crops, legumes and manure can play a part in nitrogen management as well by creating environments for microbes to thrive that can enhance mineralization of organic nitrogen to inorganic forms.  This allows nitrogen to be metered into the soil over time. Of course, the concern with this approach solely for corn production is that organic-to-inorganic nitrogen release is highly dependent on soil temperature and moisture and may not exactly align to the rapid nitrogen uptake demands of corn in mid to late vegetative growth stages, especially when corn is planted earlier. 

Soil health indicators that may be useful to measure existing and changing conditions in the soil from new practices can include soil respiration tests (Solvita), microbial mass and soil bacteria to fungi ratio (MicroBiometer), and soil DNA analysis (BiomeMakers, Pattern Ag, Miraterra (formerly Trace Genomics)). 

Practical Implications – What Can Be Done? 

Understanding that multiple nitrogen applications should lead to benefits to the soil microbial communities by reducing single-application shocks, the next question is how to achieve this through operations and logistics of a large, modern agricultural operation?

Equipment exists to allow for multiple applications, and fertigation technology on irrigation applicators further allows for easy nitrogen application when the crop needs irrigation. For most US Midwest Corn Belt farmers, anhydrous ammonia is the lowest cost nitrogen source available. Most often, this is applied as a single application for the corn crop’s entire N needs, or perhaps as a majority need application with the balance applied later. Anhydrous ammonia is rarely applied in lower and more frequent amounts due to applicator draft and expense. 

Buffering the anhydrous application with a carbon source, like humic acid, can provide some benefits to soil microbial health. However, increasing the number of applications with reduced individual rates, often using other forms of nitrogen fertilizer like UAN, urea or ammonium sulfate, can provide longer term benefits. These applications must be balanced with the cost and logistics timing of application, of course. 

Incorporating manure, legumes and cover crops into the cropping plan will allow for mineralized nitrogen to become available and potentially reduce the overall need for inorganic nitrogen fertilizer. Indeed, increasing the biological activity of the soil should improve overall soil organic matter mineralization and allow for increased crop nitrogen uptake as needed. 

Conclusions

Improving soil health should have benefits to farmers for success. Without understanding the biological activity in the soil, farmers are not to be blamed for taking the shortest path to raise a profitable crop. Instead, invested with the proper knowledge of these hidden workers in the soil, farmers can make proper management decisions that not only improve their bottom line but leave the soil in a better condition for future generations.