A 2023 perspective on inorganic nitrogen in soilless culture

When using conventional (i.e., inorganic) fertilizer alone, nitrogen (N) is commonly delivered as a singular component of a complete water-soluble or controlled release ‘specialty’ fertilizer that commonly contains micronutrients. These specialty fertilizers are predominantly used for specialty crops due to labor savings, ability to manage crop growth, and affordability when producing a high market-value individual specialty crop or that perennially bears high-value, fresh fruit. While fertilizer formulation is loosely based on Liebig's law of the minimum, with the most abundant and limiting mineral nutrient typically assumed to be N, all mineral nutrients, including N, are commonly applied in excess to overcome deliberate, inherent system inefficiencies. The resulting N use efficiency (NUE), i.e., the proportion of applied N used by the crop, averages between 30 and 60% with values as low as 5% observed; this NUE being similar or greater than conventional agronomic crops (≤40% in the United States) even though specialty crop fertilizer cost more dollars per kg when compared to agronomic fertilizers. This inefficiency is exacerbated by excess application of high quality, fresh water to a minimally reactive, porous organic media that has minimal specific surface area compared to mineral soils. Though mineral nutrients are delivered throughout production via fertigation or polymer technology, water application and subsequent preferential flow can leach N and other pore water mineral nutrients from initially low water content substrates as fast as applied. The remaining N in the substrate undergoes in situ biochemical processes before or after being minimally sorbed to cation exchange sites, taken up actively or passively by crop roots, or converted to inert or reactive N. There exists a large body of literature on N in soilless substrate production; however, research has focused on N formulation to control substrate pH, rate optimization for a specific specialty crop, environmental fate when crops are produced in containers with a focus on irrigation scheduling, and most recently the effect of microbiome on N transformations. Little research has looked comprehensively at gaseous emissions, mediated by microbial and physical processes, allowing for greater accounting of N fate. Additionally, research on the effect of N placement, formulation, and delivery method on potential climate impact, novel N delivery methods utilizing today’s material science and nanotechnology, and a focus on altering substrate physiochemical composition to adjust N efficiency is in its infancy or does not exist. These high-cost research endeavors and solutions must be investigated now to aid in future widespread adoption of more efficient N practices in the face of rises in fertilizer demand and costs along with increasing environmental regulatory pressure to minimize N loss to the water- and air-sheds and appease consciences consumers.