One of the goals of adding adjuvants to agricultural spray solutions is to enhance the droplet size characteristics of this spray. Droplet size, in turn, has an influence in the deposited spray quality, in addition to the drift and losses of spray to off-target places. The aim of this research was to evaluate the effect of adding adjuvants to two types of water from different sources on the droplet size characteristics. Two types of adjuvants were employed in the tests: the active substance content of the first adjuvant was a 50% aqueous solution of sodium salt of alkylbenzenesulfonic acid—10% (HY), whereas the second was from rapeseed oil (natural origin)—85% (OL). Both adjuvants were tested in two concentrations: the first was with the concentration recommended by the manufacturer (100%), whereas the second concentration was 50% of the recommended dose. Two types of water from different sources were used in the tests: the first was from a village in the eastern part of Poland (WS), whereas the second was tap water from the city of Lublin, Poland (WUP). Dv0.1, Dv0.5, or volume median diameter (VMD), Dv0.9, Sauter mean diameter (SMD), relative span (RS), and the droplet size distribution were measured and calculated as characteristics of the droplet size. Results showed that the source of water affected the influence of adding adjuvant to the spray solution. Water from the WS source with adjuvant resulted in a numerical decrease in the Dv0.5 values in the percentage of droplet size range below 150 µm, whereas water from the WUP source resulted in an increase in these values (except when adding the HY adjuvant at 50% concentration). Adjuvant concentration significantly (p < 0.05) influenced the features of Dv0.9, SMD, and RS. Adding the OL adjuvant type numerically decreased the percentage of droplet size below 150 µm, and the Dv0.5 values, but only when the WS water source was used.
