3.1. Irrigation Needs and Water Requirements of Potato
The mean potato irrigation period for research years 2011–2013 started on 26 June and ended on 15 August and lasted, on average, 51 days (
Table 5). The shortest irrigation period (47 days) was noted in 2013, and the longest (56 days) in 2011. During the irrigation period, 8 single doses were used on average. The three-research-year-average seasonal irrigation norm (sum of single rates) was 61.1 mm, falling within, depending on the precipitation pattern, the range from 50.0 mm in 2011 to 76.5 mm in 2013.
During the irrigation period, the soil matric potential was not allowed to drop below −30 kPa. Due to this potential, optimal soil moisture was maintained. According to Nowacki [
8], when irrigating potatoes, one must not allow too large fluctuations in soil moisture. Optimal soil moisture, which was from 65% to 70% of the field water capacity, increases the use of nutrients by plants and ensures for the proper development of the root system and the above-ground part (haulm).
In the study reported by Rolbiecki, et al. [
29] carried out in 2008–2010 on the same research facility in Kruszyn Krajeński but with other mid-early potato cultivars Vineta and Oman, they applied the seasonal irrigation standard average for three years of 83 mm, depending on the rainfall distribution from 66.5 mm in 2008 to 101 mm in 2010. On the other hand, in experiments with medium-early cultivar potato irrigation [
18] conducted at the same facility in 2005–2007, the seasonal irrigation doses ranged from 40 mm in 2007 to 170 mm in 2005 (depending on the distribution of precipitation), and the average for the three seasons was 120 mm. The low standard of irrigation in 2007 resulted mainly from the large amount and even distribution of precipitation.
Some regions of Poland are characterized by a chronic rainfall deficit [
42]. In Poland, the highest precipitation deficit for early cultivars occurs in the central part of the country, where this experiment was conducted. The amounts of water used for drip irrigation of potatoes in the present research correspond well with Nowak [
43] synthesis, which states that in dry years this deficit ranges from 105 mm to 120 mm.
The analysis of the cumulative decade-long water needs with the drip system of potato cultivar Courage (
Figure 4) shows that the values of these water needs were determined directly according to Hargreaves
DA’s formulas was 202 mm, ranging over the research years from 189 mm in 2011 to 229 mm in 2013 from 1 June to 31 August.
3.2. Potato Yielding
On average for the research period, drip irrigation compared with the control, significantly increased the marketable yield of potato tubers from 22.61 t ha
−1 (O) to 38.53 t ha
−1 (D); the yield increase being 15.92 t ha
−1 (55%) (
Table 6). The greatest yield increase (20.88 t ha
−1 namely 89.34%) was due to irrigation in 2012, and the lowest (8.42 t ha
−1 namely 26.4%) in 2011.
For comparison, in experiments conducted in region of Bydgoszcz with drip irrigation and nitrogen fertigation of the mid-early potato cultivar Vineta, drip irrigation increased the marketable yield of potato tubers from 17.4 t ha
−1 to 36.3 t ha
−1 (109%) [
29]. In another research, Mazurczyk, et al. [
44] with drip irrigation of mid-early potato cultivar Triada carried out also in Poland obtained an increase in tuber yield by 26 t ha
−1 (88%). In the plots with drip irrigation, farmyard manure and nitrogen fertigation, the yields increased from 29.4 t ha
−1 to 55.4 t ha
−1 compared to the control. In other studies, Mazurczyk, et al. [
27] drip irrigation and nitrogen fertigation enabled the authors to obtain the yield of the early potato cultivar Owacja to 30 t ha
−1 on 70th day after planting and about 50 t ha
−1 after the end of cultivation, i.e., 103rd day after planting. A significant increase in the marketable yield of potato tubers of two potato cultivars (very early ‘Monika’ and semi-early ‘Jolana’) in two different regions with reduced rainfall in the Czech Republic during the growing period after the application of drip irrigation has also reported by Elzner, et al. [
7].
According to Elzner, et al. [
7] and Badr, et al. [
28], the increase in the yield of potato tubers depends on specific agri-climatic conditions and ranges from a few percent to multiple increases in yields compared to non-irrigated plots. Nowacki [
8] and Głuska [
10] report that cultivars with high yielding potential and high water requirements are characterized by a higher yield increment. Due to irrigation, in its development, each potato cultivar has a period of the greatest demand for water, which for potatoes occurs in the phase of tuber setting and the phase of rapid weight gain. This period extends over a period of several weeks, depending on the cultivar, and falls most often from June to the end of August. If there is a shortage of rainfall at that time, the yield increase due to irrigation is the greatest.
Nitrogen fertigation, on average in the years of the study, significantly increased the potato yield from 28.85 t ha−1 to 32.28 t ha−1 (an increase by 3.43 t ha−1, i.e., 12%). We recorded a significant interaction between drip irrigation and fertigation in developing of the marketable tuber yield. The highest potato yield of the four experimental treatments was noted on plots D + F and on average in the three-year (2011–2013) of the investigated period was 40.47 t ha−1.
A significant impact of the D on the marketable yield of tubers results from the increase of the average weight of tubers (
Table 7) and their number (
Table 8). Tubers of irrigated plants in the studied period were on average heavier by 22.8 g (37.3%) than those collected from the O plots while the number of tubers per plant increased on average from 8.7 to 14.7.
The results obtained are consistent with Rolbiecki, et al. [
29] who reports that drip irrigation has a significant effect on the increase of the weight of tuber and the number of tubers per plant compared to the control (without irrigation). Nagaz, et al. [
45] reports that water deficiency during tuber initiation and development has an impact on the decrease in tuber yield, which results from the reduction in the number and weight of tubers. Walworth and Carling [
46] also observed a lower number of tubers under non-irrigated conditions.
The second of the examined factors, which is fertigation, also had a significant impact on the weight of tubers and their number. Plants in the D + F treatment were characterized by significantly higher tuber weight compared to the D + P. The tubers of plants collected from the F plots in the studied period were heavier by 6.1 g (8.7%) than those harvested from the treatment P. The number of tubers per plant increased on average from 10.3 to 12.8 after D + F. The largest number of tubers was found in drip irrigated plants, where the D + F was applied. The analysis of the correlation between the tuber yield and the average tuber weight showed a very strong correlation (
r = 0.912), which indicates that the increase in tuber yield was mainly attributed to the increase in tuber weight (
Figure 5).
3.3. Irrigation Water Use Efficiency and Nitrogen Use Efficiency
The results of this study show that, on average, for the treatments of D + F and years, it was high and amounted to 260 kg ha
−1 mm
−1 (
Table 9). In 2011 and 2013, the IWUE index for D + F was higher compared to D + P. This proves that plants in this treatment used water better in conditions of its deficiency or limited amounts in the soil. The heavy rainfall recorded in June and July 2012 was probably the reason for the low IWUE value. According to Nikolaou, et al. [
24] pressurized irrigation systems and appropriate irrigation schedules can increase water productivity (i.e., product yield per unit volume of water consumed by the crop) and reduce the evaporative or system loss of water as opposed to traditional surface irrigation methods. A number of other studies also confirm that D + F is an effective method in increasing the efficiency of water use and potato yields [
7,
12,
13,
27,
28,
29,
30,
47]. According to Guoju, et al. [
23], improving water efficiency is a key factor for the continued increase in crop productivity in arid and semi-arid regions. D treatments gave higher IWUE compared to other irrigation methods, which results from lower water consumption for drip irrigation [
48].
The NUE, on average for fertilization treatments in the three-year study period (2011–2013), amounted to 189 kg ha
−1 in O + P and O + F plots and increased to 321 kg ha
−1 in D + P and D + F plots. Under the conditions of D, this index was 305 kg ha
−1 in the plots D + P and increased to 337 kg ha
−1 in the plots D + F (
Table 10). The significant impact of irrigation treatments of the potato in the efficiency of nitrogen use under Nubaria region west of Nile Delta of Egypt (arid climate region) is reported by Badr, et al. [
28]. The authors obtained the highest value, 176 kg yield kg
−1 N being in the full drip irrigation treatment (W
1.0 = 100% of crop evapotranspiration) while the lowest value, 55 kg yield kg
−1 N in the most severe water deficit treatment (W
0.4 = 40% of crop evapotranspiration).