*2.2. Experimental Design*

The experimental field was roughly 0.2 ha in size (A = L × W = 180 m × 10.8 m). The experimental design consisted of 12 treatments with four replicates (i.e., 48 plots). Each replicate consisted of three beds, each 15 m by 0.9 m. The drip line had drippers with a 0.6 L h−<sup>1</sup> discharge and 25 cm spacing between the drippers. As mentioned previously, this setup was selected based on our previous experiments that examined drip discharge and spacing for optimal water productivity [14]. The main findings from the previous study demonstrated that a combination of sprinkler irrigation for germination followed by low discharge drip irrigation could provide potato yields similar to those reported for sprinkler irrigation only, without harming tuber size and quality. During the two years of the previous study, the 80% water dose (i.e., 100% was approximately 620 mm) was sufficient for optimal potato growth in conjunction with water and fertilizer savings (fertigation rate

was 100 mg N L<sup>−</sup>1). Accordingly, the current experiment examined the effect of nitrogen dosage (i.e., 0, 50, and 100%) in conjunction with the water dose (i.e., 40, 60, 80, and 100%) on potato growth and yield. The treatments were marked as following: WX%FX%, where W and F refer to water irrigation and fertigation, respectively, and X is a % value (e.g., W80%F50%: 80% irrigation water dose and 50% fertigation dose). The treatment blocks were randomly distributed over the experimental fields. The representative chemical composition of the irrigation water in Yotvata is shown in Table 1.

Potato tuber (cv. Hermes) sowing took place on 28 October 2014. This date is day zero after sowing (i.e., DAS = 0). Tubers were distributed at a density of five tubers per meter at a depth of 20 cm. The spacing was 1 m between beds and the height of each bed was 40 cm with a trapezoid-like shape—created during placement of the shallow subsurface drip irrigation laterals at 5 cm depth (DAS = 2). The cumulative dose of irrigation water for germination was about 100 mm and was provided by sprinkler irrigation at three-day intervals. The actual cumulative irrigation dose, supplied by drip irrigation for each treatment, is shown in Table 2.

In the Kibbutz Yotvata fields, the main nitrogen sources are fertigation and soil amendment with composted cattle manure, produced from the kibbutz's dairy barns. The representative composition of this composted cattle manure is given in Table 3. It was applied at 40 m3 ha−<sup>1</sup> before sowing. Consequently, this was the main nitrogen source for the F0% treatments. The fertigation with nitrogen fertilizer, 100 mg N L−<sup>1</sup> of "Arava" liquid fertilizer (ICL Ltd., Haifa, Israel), was supplied with the irrigation water. The nitrogen amount for each irrigation regime was proportional to the irrigation dose. However, since a continuous irrigation regime was employed, the available nitrogen concentration in the soil solution was comparable. The Arava fertilizer consists of NH4NO3 and KNO3 as the nitrogen sources. Moreover, it was adjusted to be used with the irrigation water that is common to the Arava region (<1% Cl− and an acidic pH). It is important to mention that the fertigation started with initiation of drip irrigation (DAS = 26) and halted at the foliage top-kill (DAS = 130). Following the foliage top-kill, irrigation (without fertigation) was continued for three weeks, for the purpose of peel formation, until the harvest (DAS = 152).
