A 2023 perspective on inorganic nitrogen in soilless culture

Authors: Owen Jr, James - Jim

Submitted to: Acta Horticulturae
Publication Type: Review Article
Publication Acceptance Date: 6/19/2023
Publication Date: 2/26/2024
Citation: Owen Jr, J.S. 2024. A 2023 perspective on inorganic nitrogen in soilless culture. Acta Horticulturae. 1389:403-408. https://doi.org/10.17660/ActaHortic.2024.1389.47.
DOI: https://doi.org/10.17660/ActaHortic.2024.1389.47

Interpretive Summary: Soilless media, blended without compost or meal, has little or no free inorganic nitrogen (N), is relatively non-reactive, and highly porous making water content and solute transport key to mineral nutrient availability. Furthermore, conventional soilless media such as peat, bark, or rock-wool typically cannot supply sufficient macro-or micro-nutrients to support crop growth. Thus, synthetic sources of inorganic N, commonly delivered as a singular component of a complete water-soluble (WSF; including liquid fertilizer) or controlled release ‘specialty’ fertilizer (CRF) also contains micronutrients, is applied in excess due to the fear of deleterious effects on crop production or appearance and subsequent economic loss. Furthermore, N must be supplied continually to not become crop limiting. The result is that numerous forms of N are added to the growing media to aid in establishment, growth, and to hold-over plants for sale. This costly proposition of excessive N application can reduce profits, hinder consumer perception of sustainability, and negatively impact surrounding surface waters and consequently the surrounding ecosystem. However, synthetic N fertilizers do allow for specified ratio of N to other macro-nutrients, namely phosphorus, to provide a balanced mineral nutrient formulation. The increasing demand of N coupled with growth of soilless culture market (Blok et al. 2021) and crop production N inefficiencies warrants further research into basic science of understanding N fate in existing and novel soilless culture systems, revising antiquated N recommendations for a given crop and production type and novel methods to deliver N. Initial basic science can slowly progress to discover avenues for better formulation and delivery; however, questioning current guidelines and best management practices and trialing lower N rates in a production setting can make an immediate impact when coupled with proper irrigation. This paper outlines current N use trends during production; adding commentary of avenues that are unexplored and that could serve as future opportunities.

Technical Abstract: 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.