*2.5. Lettuce Crop Management and Yield Evaluation*

Small, five-day-old lettuce plants were transplanted on 1st July. All treatments were fertilized equally at planting with N-P-K (14.4-3.6-17.4) at a rate of 833 kg ha−1. The plots were irrigated on 1st July (5 mm), 10th July (5 mm), and 6th August (15 mm). On 7th July, the first set of resin probes was installed in the field. Plot yields (fresh plant matter) were evaluated on 19th August, by weighing the lettuce harvested from one linear meter, repeated three times per treatment.

#### *2.6. Soil Sampling and Analysis*

Following the lettuce harvest, composite soil samples were taken at a depth of 0–20 cm at each of the locations previously occupied by the resin probes. The soil samples were then dried at 70 ◦C, sieved at 2 mm, and used for chemical analyses.

Nitrogen was extracted from the soil with water, using a 1:10 (*w/v*) soil:solution ratio. After an agitation period of 1 h at 200 rpm (19 mm circular orbit), the extract was centrifuged at 3000 rpm (2060× *g*) for 10 min and filtered through a No. 42 Whatman filter paper. Ammonium (NH4 +) and nitrate (NO3 −) concentrations were quantified with a Quikchem 8500 Series 2 system (Lachat Instruments, Loveland, CO, USA), using Quikchem methods 10-107-06-2-B and 12-107-04-1-F, respectively. Total N in the water extract was measured using the same system with prior persulfate oxidation, according to the procedure described by Qualls (1989) [32]. Soluble organic N (SON) was calculated by subtracting N-NO3 − and N-NH4 <sup>+</sup> from total N.

Mehlich III elements were extracted with a 1 g of soil to 30 mL of solution ratio, a 5-min agitation period at 200 rpm (19 mm circular orbit), and filtration through a No. 42 Whatman filter paper [33]. Elements were analyzed by inductively coupled plasma–mass spectrometry (Icap 6500 MK2 radial, ThermoFisher Scientific, USA) within 12 h of extraction to prevent organic compounds from precipitating with metals.

For the active carbon fraction, a 0.02 M KMnO4/0.1 M CaCl2 solution was used according to the procedure described in Weil et al. (2003) and Blair et al. (1995) [34,35]. Total organic carbon and total nitrogen concentrations were determined using a LCN-2000 dry combustion analyzer (Leco Corporation, St. Joseph, MI, USA).
