*3.5. Potato Tuber Size Distribution*

In addition to the total harvested yield, marketability and commercial value are of considerable importance in directing farmer decision making. In Israel, a major marketability criterion is tuber width. The results of the cumulative width distribution of all treatments are shown in Figure 5. We used the trade standard of selecting potatoes larger than 50 mm width (diameter); vertical dashed lines in Figure 5. The following cumulative width distributions are based on this standard. The results show that the mean value of tuber widths higher than 50 mm for the three fertigation treatments are 45.16 ± 12.76%, 62.06 ±4.06%, 60.55 ± 3.49% for the F0%, F50%, and F100%, respectively (see Figure 4 for detailed values per fertigation treatment). The F50% treatment was similar to F100% and both were very different from the F0%. With regard to the water doses, no significant differences between treatments were found. Tuber length and mass distribution were also recorded (data not shown) and good correlations were found between tuber length/width and tuber mass/width (mass = 0.0007 width2.9273, R<sup>2</sup> = 0.98; length = 0.7599 width1.1016, R2 = 0.93). Based on this scaling it is reasonable to use one tuber parameter.

**Figure 5.** Cumulative frequency of tuber width according to the treatments. (**a**) F0%; (**b**) F50%; (**c**) F100%.

## *3.6. Potato Tuber Quality*

Representative samples of tubers from each treatment were assessed for other commonly used quality parameters. Tuber color was assessed by spectroscope, where a score of 60 (arbitrary units) and above is considered acceptable for market. No significant differences were found between treatments (mean value 67.0 ± 1.94). Total solids percentage (dry mass basis) should range from 19 to 26%. Our results were a low but acceptable mean of 21.3 ± 0.45%. The Hermes cv. is primarily grown as a chipping potato and so low dextrose values are preferred. Higher dextrose values (>0.1 g L−1) are not considered marketable as high dextrose concentrations leads to dark tubers, an unattractive trait for potato chips. While color and total solids were consistent between treatments, the dextrose concentrations showed some variation (mean 0.008 ± 0.005 g L<sup>−</sup>1), although this was not significant.

Field size may compensate for the total yield. However, the quality of the potato tuber should be a significant parameter in this agricultural practice selection. Considering the large treatment differences in fertigation and irrigation in this study, it is surprising that tuber size and mass distribution was so consistent. Regarding fertigation, the 50% fertigation dose did not result in reduced tuber size. This may be because of the irrigation method used here (i.e., shallow, subsurface and with low discharge rates), which delivers both water and nutrients to the active root zone, and its combination with applied composted manure. Moreover, there were no qualitative differences observed between the treatments.

These insights, in conjunction with the results of water and nitrogen productivity, imply that the additional nitrogen (i.e., the difference between the F50% and F100% treatments), was mainly consumed and allocated to vegetative growth (canopy), and is not reflected in significantly higher tuber yield and/or quality. Moreover, the additional nitrogen also may have been wasted and leached from the root zone, thus leading to economic waste and potential groundwater contamination.
