Nitrogen Management & Crop Performance

Overview

Nitrogen is the second most crucial element for plants after carbon and plays a key role in plant growth. Conventional agriculture traditionally uses crop removal estimates to determine the amount of synthetic nitrogen fertilizers to apply each crop season. Other nitrogen sources such as soil organic matter, water, and air are often not considered. This approach, combined with the ready availability of inexpensive synthetic nitrogen fertilizers, can result in the overapplication of synthetic fertilizers. Excess nutrients can lead to nutrient leaching and runoff that has caused a number of environmental problems. Modern conventional and organic agricultural practices improve nitrogen use efficiency to reduce environmental risks. Regular soil and plant tissue analyses are used to determine pre-plant and in-season fertilizer application rates, timing, and placement. Conventional agriculture relies on synthetic nitrogen fertilizers to replace the nitrogen removed by the previous crop. Organic nitrogen management considers all organic nitrogen sources, such as soil organic matter, air, water, and type of organic fertilizer, to supply nitrogen needs that are determined by soil and plant tissue analyses. Organic fertilizers have variable nitrogen release rates that can be efficiently managed by implementing the Four R’s (Right rate, Right time, Right source, and Right place (Farzadfaar 2021).

Part 1: Sources of Organic Nitrogen

Organic nitrogen can come from the atmosphere, Soil Organic Matter (SOM), crop residue, irrigation water, manures, compost, organic fertilizer residue, and residual soil nitrates and ammonium (M. D. Lloyd 2020). The nitrogen from crop residues, soil organic matter, and some organic fertilizers such as poultry litter, contain nitrogen that is not immediately available to the plant. Plant secretions and microorganisms are needed to breakdown these complex organic sources of nitrogen in into small peptides, amino acids, or mineralized into plant-available nitrate (NO₃^-) and ammonium (NH₄⁺). Nitrogen is immediately available in the form of amino acids from organic fertilizers made from hydrolyzed soy meal. Nitrogen from the air requires free-living or symbiotic microbes to facilitate nitrogen fixation and incorporation into plant tissues. Some nitrogen such as soil ammonia, nitrate, nitrite, amino acids, and small peptides that remain are in soil, are immediately available to the plant. The key to successful organic nitrogen management depends on knowing total and available nitrogen, crop nitrogen needs, and crop production targets.

Part 2: Modern Models for Plant Nitrogen Nutrition

Conventional agriculture’s prior reliance on inorganic fertilizers assumed that plants take up nitrogen primarily as nitrates (NO₃^-) and ammonium (NH₄⁺) (Figure 1). Nutrients sequestered in soil organic matter had to be mineralized into plant available inorganic nitrogen NO₃^- and NH₄⁺.  Since the efficiency of this process was unclear, emphasis centered on synthetic fertilizers to supply a crop’s nitrogen requirement needs.  Today, both conventional and organic crop management programs recognize the contribution of soil organic matter (Figure 2) (Farzadfaar 2021).

The model includes nitrogen contributions from various sources other than synthetic nitrogen.  In doing so, modern practices reduce the need for nitrogen inputs, reduce loss of nitrogen to the environment, and increase nitrogen use efficiency.  Since organic fertilizers are variable in their nitrogen release, it is important to remember the Four R’s of nitrogen use efficiency:  Right rate, Right time, Right source, and Right placement, to optimize the released nitrogen.  The Four R’s recognizes that excess application of any input, synthetic or organic, contributes to the inorganic nitrogen pool and can lead to nutrient leaching from the soil.

Work Cited

Farzadfaar, S., J. D. Knight, K. A. Congreves. 2021. "Soil organic nitrogen: an overlooked but potentially significant contribution to crop nutrition." Plant Soil 462: 7-23. https://link.springer.com/article/10.1007/s11104-021-04860-w.

Lloyd, M., D. Geissleler, P. Lazicki, J. Muramoto, and R. Smith. 2020. Estimating nitrogen availability in organic annual production: For N budgeting and other purposes. https://ucanr.edu/sites/SFA/files/322312.pdf.

Lloyd, M., P. Lazicki, D. Geisseler, J. Muramoto, R. Smith, E. Smith. n.d. Estimating nitrogen (N) for organic crop production: worksheet. https://ucanr.edu/sites/SFA/files/322313.pdf.