The mean minimum temperature and relative humidity during potato growing seasons exhibited more variation over the five years as compared to mean maximum temperature and relative humidity at the experimental site. Changes in available NPK status of soil under different nitrogen treatments were also observed. The initial available nutrient content at the experimental site was found to be 209 kg N, 43.6 kg P2O5 and 207 kg K2O ha− 1. Every year, variable doses of N (0, 75,150, 225 or 300 kg ha− 1) were applied as per treatment along with uniform dose of 80 kg P2O5 and 100 kg K2O ha− 1. At the end of experiment, the available N content in different treatments was found to be in direct proportion to the N dose, that is, the treatment applied with highest amount of N fertilizer also exhibited maximum available soil N. The P2O5 and K2O contents were found to be the highest in control without N application.
Based on the mean data over years, highest tuber yield of 21.21 t ha− 1 was observed in the treatment with 225 kg N ha− 1 (Table 3). This treatment exhibited 6.16% higher yield over the recommended dose of 150 kg N ha− 1. Application of 300 kg N ha− 1 showed reduced yield as compared to the recommended dose (150 kg N ha− 1) which might be due to lodging and more tender plants prone to disease incidence. The excessive N application stimulates more growth of shoot than tuber resulting deterioration of canopy structure (Sommerfeld and Knutson, 1965). However, some other workers observed substantially delayed leaf senescence along with increased leaf area duration and tuber yield because of excessive N (MacKerron and Heilbronn, 1985) which is thought to be due to maintenance of photosynthetically active leaves for longer duration as well as formation of new young leaves than with lower or no N supply (Millard and Marshall, 1986). In our study, significant variation among treatments was observed for N dose, year and year × nitrogen dose interaction. There was gradual increase in tuber yield from 0 kg N /ha to 225 kg N ha− 1.
Table 3
Tuber yield of Kufri Surya under varying nitrogen levels over years
Nitrogen levels
(kg ha− 1)
|
Total tuber yield (t ha− 1)
|
2013-14
(Y1)
|
2014-15
(Y2)
|
2015-16
(Y3)
|
2016-17
(Y4)
|
2017-18
(Y5)
|
Mean
|
0
|
18.66
|
13.66
|
9.17
|
11.40
|
10.68
|
12.71
|
75
|
21.53
|
15.83
|
11.59
|
13.81
|
12.86
|
15.12
|
150
|
24.54
|
16.24
|
18.78
|
21.00
|
19.36
|
19.98
|
225
|
25.37
|
16.90
|
20.29
|
22.51
|
20.99
|
21.21
|
300
|
25.89
|
18.90
|
12.51
|
15.28
|
14.91
|
17.50
|
Mean
|
23.20
|
16.31
|
14.47
|
16.80
|
15.76
|
|
SE(d)
|
0.79
|
0.42
|
0.92
|
0.84
|
0.62
|
|
C.D.(0.05)
|
1.73
|
0.93
|
2.03
|
1.84
|
1.38
|
|
C.V. (%)
|
4.84
|
3.68
|
8.99
|
7.04
|
5.60
|
|
Pooled analysis:
Factors
|
SE (+m)
|
C.D. (0.05)
|
Year (Y)
|
0.254
|
0.718
|
Nitrogen dose (N)
|
0.254
|
0.718
|
Y × N
|
0.568
|
1.605
|
Several workers have reported increase in total tuber yield and size of potato tubers with higher dose of nitrogen (Sanderson and White, 1987; Zabihi-e-Mahmoodabadet al., 2011). There is a decrease in harvest index in response to increasing N fertilization which of course is statistically non-significant. This may be because of the fact that increasing N fertilization enhanced partitioning of assimilates to the shoots rather than to the tubers (Biemond and Vos, 1992). Heavier application of Nitrogen over the recommended dose significantly reduces tuber specific gravity and dry matter content which might be due to the influence of N on gibberellin biosynthesis having direct influence on plant growth and dry matter accumulation (Cole, 1975; Kleinkopf et al.1981; Zelalem et al., 2009) although findings of other workers contradict it (Roberts and Cheng, 1988).
Millard and Marshall (1986) reported that tuber yield improvement due to N fertilization could be attributed to increased radiation interception during the first part of the season and lower rates of decline in photosynthetic efficiency of the canopy during the later part. High dose of nitrogen increases days to flowering and physiological maturity, plant height, and dry matter production of different plant parts (Zelalem et al, 2009; Israel et al, 2012 and Alam et al, 2007).
Effect of N dose on plant height, shoots per plant, grade wise tuber yield and dry matter content was also noticed. Significant variation among the N treatments was observed with respect to plant height and shoots per plant (Table 4). Application of 225 kg N resulted in maximum plant height and shoots per plant which of course was at par with 300 kg N. There was no significant variation in tuber yield among N treatments between 0-25g category to 25 -50g category but in rest other N treatments maximum tuber yields was realized in > 75 g category. Of course, tuber number is not an important attribute limiting yield (Sharma and Arora, 1987; De la Morena et al., 1994; Lynch and Rowberry, 1997) because of the inverse relationship between tuber number and average tuber weight (De La Morena et al., 1994). The number of tubers increases due tonitrogen fertilization through an increase in stolon number due to more Gibberellins biosynthesis (Kumar and Wareing, 1972; Kanzikwera et al., 2001) although there are also reports on absence of strong relationship between N dose and tuber number in potato (Sharma and Arora, 1987; De la Morena et al., 1994). Several workers have highlighted significant increment in tuber number in response to N application (Ifenkwe et al., 1974; Zabihi-e-Mahmoodabad, 2011; Israel et al., 2012). The tuber dry matter content did not vary significantly among the N treatments.
Table 4: Effect of Nitrogen dose on plant height, shoots / plant, grade-wise tuber yield and dry matter content
Nitrogen levels
(kg ha-1)
|
Plant height (cm)
|
Shoots/
plant
|
Grade-wise yield of tubers (%)
|
% Tuber Dry Matter Content
|
0-25g
|
25-50g
|
50-75g
|
>75g
|
0
|
43.88
|
3.10
|
12.08
|
30.06
|
29.87
|
28.00
|
18.27
|
75
|
46.68
|
3.30
|
11.12
|
26.67
|
29.70
|
32.50
|
18.74
|
150
|
48.48
|
3.33
|
7.23
|
23.30
|
31.46
|
38.02
|
18.78
|
225
|
52.00
|
3.70
|
6.62
|
23.20
|
25.15
|
45.07
|
18.46
|
300
|
49.80
|
3.60
|
8.85
|
27.57
|
28.71
|
34.94
|
18.52
|
SE(d)
|
1.55
|
0.17
|
0.47
|
3.31
|
1.09
|
1.41
|
0.18
|
C.D. (0.05)
|
3.41
|
0.36
|
NS
|
7.28
|
2.30
|
3.12
|
NS
|
C.V. (%)
|
4.55
|
6.84
|
7.10
|
17.84
|
5.15
|
5.65
|
1.38
|
Continuous supply of nitrogen promotes shoot and root growth but reduces tuberization in potato (Gunasena and Harris, 1969; Wilcox and Hoff, 1970; Ivins and Bremner, 1969). The number of bigger size tubers also increase because of nitrogen application resulting in higher yields (Patricia and Bansal, 1999). Presence of photosynthetically active leaves for longer duration and formation of new leaves confer positive influence on total and marketable tuber yield (Krishnippa, 1989). Adequate nitrogen helps in obtaining a LAI of 4 to 6, which is required for high tuber yields (Marschner, 1995). High harvest index does not exhibit positive correlation with high yield always (Gawronska et al., 1984) which implies that a cultivar with poor harvest index may not necessarily be a low yielder.
In this study, potato yield decreased with increasing GDD and was the highest (23.2 t ha− 1) in the year 2013-14 with lowest GDD (9120C) irrespective of fertilizer dose given to the crop (Table 5). Growth and duration of crop in relation to temperature can be estimated by using accumulated heat units in terms of growing degree-days (GDD) (Kingra and Kaur, 2012; Meena et al., 2013). Lower GDD and lower BSH (397 hrs) combination leading to lowest HTU (3888.9) increased the tuber yield during 2013-14 with the highest HUE (371.3g0C days− 1 hrs− 1) and HTUE (6.0) which is in conformity with (Ruttanaprasert et al., 2013).
Table 5: BSH, GDD, HTU during cropping periods in different years and respective HTUE and HUE of potato cv. Kufri Surya.
Parameter
|
Year of study
|
2013-14
|
2014-15
|
2015-16
|
2016-17
|
2017-18
|
BSH(hrs)
|
397.0
|
550.7
|
414.8
|
537.0
|
688.4
|
GDD(0C)
|
912.0
|
934.4
|
1044.5
|
985.0
|
1216.1
|
HTU
|
3888.9
|
6228.0
|
5656.3
|
6898.4
|
8329.9
|
HTUE
|
6.0
|
2.6
|
2.6
|
2.4
|
1.9
|
HUE
(g0C days-1 hrs-1)
|
371.3
|
257.0
|
204.8
|
251.0
|
191.1
|
Economics of nitrogen dosage was calculated based on the tuber yield varying from 12.71 t ha− 1 in Control to 21.21 t ha− 1 in the treatment with 225 kg N ha− 1 (Table 6). In a recent study, 225kg N ha− 1 has been found to be the best dose for tuber productivity (Yadav et al., 2024). However,they revealed that potato yield parameters are negatively affected by the nitrogen dose above 225 kg ha− 1. The seed tuber requirement was 20 q ha− 1 in different treatments costing Rs. 2500 per quintal or Rs.50000 ha− 1. Considering the selling price of potato at Rs.8000 per ton, the highest gross return was found to be Rs.1,69,696 with 225 kg N ha− 1 treatment which exhibited net return of Rs.52946 ha− 1 and B:C ratio of 1.45. Next best treatment was found to be 150 kg N ha− 1 exhibiting net return of Rs. 44,472 and B: C ratio of 1.39. Net return was negative in Control (no N) which clearly shows that potato should not be cultivated without application of N fertilizer.
Table 6: Economics of nitrogen application based on mean yield data
Nitrogen levels
(kg ha-1)
|
Tuber yield (t ha-1)
|
Cost of cultivation (Rs/ha)
|
Sale price (Rs t-1)
|
Gross return
(Rs
ha-1)
|
Net returns* (Rs ha-1)
|
B:C ratio
|
Seed tuber 20 q ha-1 @ Rs.2500 q-1
|
Fertilizer
N:P2O5:K2O @150:80:100 (kg ha-1)
|
Cultivation
(Tillage +
Labour)
|
Total
|
0
|
12.71
|
50000
|
8700
|
53000
|
111700
|
8000
|
101712
|
-9988
|
0.91
|
75
|
15.12
|
50000
|
10050
|
53500
|
113550
|
8000
|
120992
|
7442
|
1.07
|
150
|
19.98
|
50000
|
11400
|
54000
|
115400
|
8000
|
159872
|
44472
|
1.39
|
225
|
21.21
|
50000
|
12750
|
54000
|
116750
|
8000
|
169696
|
52946
|
1.45
|
300
|
17.50
|
50000
|
14100
|
53500
|
117600
|
8000
|
139984
|
22384
|
1.19
|