The sustainable production of food to support the increasing world population is one of humanity’s most pressing challenges. Plasma activated water (PAW), produced using renewable energy, can help fulfil plants’ needs in sustainable agriculture approaches. The design, implementation, and characterization of a digitally manufactured air Plasma-On-Water Reactor (POWR) for the synthesis of nitrate as green nitrogen fertilizer is presented. The interaction of air plasma-generated reactive oxygen and nitrogen species with water produces nitrate (NO3-) and related species, which are the main nitrogen-containing nutrients for plants. The mild conditions of the operation of the POWR opens the possibility to use plastics, particularly through digital manufacturing strategies such as 3D-printing, for its fabrication. A pin-to-plate reactor configuration powered by high-voltage alternating power is chosen due to its simplicity and efficacy. A computational thermal-fluid model is used to evaluate the design and attain expected operational characteristics. The experimental characterization of the POWR encompassed design and operation parameters, namely electrode water spacing, air flow rat- e, and voltage level. A machine learning approach is implemented to extract and quantify characteristic features of the plasma-water interaction, such plasma volume and plasma-water interface area. Experimental results revealed that the nitrate production rate varies linearly with dimensionless plasma volume. The design, fabrication, and characterization methods presented can be adapted to other POWRs and help enable on-demand nitrogen fertilizer production at low environmental and economic cost.