*3.2. Red Onion Experimental Conditions*

The experiment was conducted in triplicate for 3 months in the field (until the bulb is fully ripe) in alkaline sandy-loam soil with a pH of 8.5. The soils contained 3.09% organic matter, 0.17% nitrogen, 110 g·kg−<sup>1</sup> CaCO3 and 0.334 g·kg−<sup>1</sup> SO4. In each parcel, 30 uniform seedlings of red onion/m squared were transplanted. The soil was divided into parcels of 10 m squared. In each parcel, 30 uniform seedlings of red onion/m2 were transplanted. Pads of sulphur bentonite (SB, 90%/10%) or SB with two percentages of orange residue (SBOR) conventionally called high (with a greater amount of orange residue, SBORHP) and low (with a lower amount of orange residue, SBORLP) or SBOP with olive pomace (OP) were used at a concentration of 16 gm−<sup>2</sup> (corresponding to 476 kg S ha−1), the dose generally used to lower the pH and to replenish the S. Sulphur was the major component of the fertilizers. The industrial process and the formulation of the pads are covered by an industrial secret drawn up in the agreement signed with Steel Belt System in 2015. Nonamended soils were used as the control CTR (Table 3). OP was used only at a low concentration, because previous experiments (data not shown) demonstrated a toxicity of OP on crops increasing its concentration (preliminary experiments carried out in a greenhouse, data not shown). SB and SB with OP or OR produced onions with greater bulb sizes with respect to the control (data not shown).


**Table 3.** Experimental design.

During the experiment, all onion plants were regularly irrigated to maintain 70% of their field capacity. At harvest time (3 months), onions were collected and stored at −20 ◦C for chemical and biological determinations.

#### *3.3. Extraction and Determination of Total Anthocyanins*

The assessment of the total anthocyanin content was carried out by the pH differential method according to the Official Methods of Analysis of AOAC International, as previously described by Muscolo et al. [17]. Absorbance was measured using a 1800 UV-Vis Spectrophotometer (Shimadzu, Kyoto, Japan) at 510 and 700 nm in buffers at pH 1.0 and 4.5. Values were expressed as mg cyanidin-3-glucoside equivalent g−<sup>1</sup> dry weight (DW) using 26,900 as the molar extinction coefficient.
