Fertigation Effects on Nutrient Use Efficiency, Energy Productivity, and Economics of Coconut (Cocos nucifera L.) Cultivation in the Eastern Indo-Gangetic Plains of South Asia

ABSTRACT

Drip fertigation has potential to reduce fertilizer dose in comparison to surface application in coconut (Cocos nucifera L.) by increasing its’ use efficiency in the Eastern Gangetic plains of South Asia. A long-term field experiment (2007–2013) was conducted on Inceptisol to evaluate 100% of the soil test-based recommended fertilizer dose (RDF; N:P₂O₅:K₂O applied at 500:250:750 g palm⁻¹ year⁻¹) in soil with ring basin irrigation as conventional, 100%, 75%, 50%, 25%, and 0% of the RDF through drips. The fertigation level of 75% RDF resulted in 32%, 19%, 39%, and 16% higher copra yield, energy productivity, net return, and benefit-cost ratio, respectively, over conventional. Increasing trend of nutrient use efficiency was observed with decreasing dose of N, P and K. Use efficiencies decreased from 50.4 kg copra kg⁻¹ N, 230.7 kg copra kg⁻¹ P, and 40.5 kg copra kg ⁻¹ K (with 25% of the RDF through drip) to 6.1 kg copra kg⁻¹ N, 27.8 kg copra kg⁻¹ P, and 4.9 kg copra kg⁻¹ K (with 100% RDF through ring basin irrigation). Fertigation with 75% RDF in drip had the highest copra yield (3.19 t ha⁻¹), energy output (206082 MJ ha⁻¹), energy productivity (187 g copra MJ⁻¹), net returns (US $ 3073 ha⁻¹), and benefit–cost ratio (2.36) compared to others.

KEYWORDS:

• Fertigation
• nutrient use efficiency
• nutrient balance
• energy balance
• energy efficiency
• profitability

Introduction

The coconut is eulogized as “Kalpavriksha” – the tree of life due to the multifarious uses of all of its plant parts, from roots to leaves to fruits. It is also ecologically important in sustaining several fragile island ecosystems. It is believed to be a crop of future in view of its carbon sequestration potential in the context of climate change (Bhagya et al., 2017). The crop has spread to more than 94 countries, mainly in coastal areas between latitudes 20° N and 20° S of the equator giving employment opportunities and livelihood options to 64 million families across the world and demand for coconut products are expected to increase in the years to come (APCC, 2017; Rodrigues et al., 2018). The crop is particularly important in the Eastern Indo-Gangetic Plains (EIGP) of South Asia, where it is climatically suitable and has potential to grow under limited water and nutrient conditions (Bandyopadhyay et al., 2019). However, it is a paradox that the small landholders of coconut are now in a dilemma facing with the challenge of lower productivity, mainly due to lack of irrigation (Surendran, 2019) and appropriate nutrient management (Maheswarappa et al., 2014). The scarcity and higher cost of labor, energy and water, and the method of fertilizer and irrigation application affects the efficiency of these inputs which are deterrents in adopting traditional irrigation systems. In a field study at Kerala, India, higher net energy production was reported in coconut-based systems over the areca nut-based system and among them, coconut + areca nut + pepper + banana system was the highest energy producer followed by coconut + areca nut + pepper + nutmeg + vanilla (John et al., 2010).

All agricultural inputs and operations require energy, the majority of which comes from nonrenewable and increasingly costly fossil fuels (Aghaalikhani et al., 2013). Increasing energy use efficiency in South Asian agriculture is crucial for environmental and economic sustainability (Aravindakshan et al., 2015). To reduce the need of an exhaustible source of energy, drip irrigation is gaining importance in the EIGP of India (Bandyopadhyay et al., 2019). Fertigation is one of the most advanced and efficient methods of fertilizer application which ensures the application of fertilizers directly to the root zone of the crop throughout the cropping period. The amount of nutrient loss through leaching can be as low as 10% in fertigation whereas, it is 50% in the traditional system as reported by Solaimalai et al. (2005). The advantage of fertigation over the conventional method of fertilizer application was emphasized by several workers (Mmolawa and Or, 2000; Mohammad, 2004a, 2004b; Shigure et al., 1999). Jayakumar et al. (2017) observed that in coconut palms, the drip fertigation at 100% RDF with 100 µm black polythene mulch favored above ground and below ground microclimate for better production and profitability in comparison to conventional application of fertilizer and irrigation on the ring basin without mulching. Rodrigues et al. (2018) reported that the benefit-cost ratio declined as sustainability increased in the coconut-based farming system. A study from the Philippines by Banzon (1984) showed the possible greater energy harvest from coconut sap than from coconut oil. Very meager information is available on the effect of fertigation on nutrient use efficiency, energy analysis, and economics of coconut cultivation. We studied yield, water use efficiency (WUE) and soil and plant nutrition in our earlier paper (Bandyopadhyay et al., 2019). We reported that the fertilizer requirement could be reduced by up to 25% and WUE increased by more than three times in fertigation method through drips over conventional system of irrigation and fertilizer application. Our frequent interactions with coconut farmers had revealed that coconut production can be increased and adopted by farmers if the coconut crop is also nutrient- and energy-use efficient and profitable compared to other competitive crops. Hence, we collected and analyzed data on energy- and nutrient-use efficiencies and prices of inputs and outputs and performed nutrients, energetics, and economic analysis of coconut production to recommend for the large-scale coconut plantations for the farming community in the EIGP of South Asia.

Materials and Methods

A long-term experiment was carried out in 27 years old coconut plantation variety DXT spaced at 7.5 m × 7.5 m under All India Coordinated Research Project on Palms, in the Horticultural Research Station, Mondouri of Bidhan Chandra Krishi Vishwavidyalaya, West Bengal, India during 2007–2008 to 2012–2013. Prior to this experiment, the orchard of the experimental site was maintained with standard management practices with conventional system of ring irrigation method and broadcasting of fertilizer. The average copra yield in the established orchard was 2.39 ± 0.07 t ha⁻¹ year ⁻¹ since 15^th year of the plantation (Anonymous, 2005). The experimental site comes under the subtropical humid climate, situated at 23°30^/13^//N Lat. and 89°20^/32^//E long with an average altitude of 9.75 meters above sea level. The monthly rainfall of the experimental period is presented in Figure 1 and evaporation in Figure 2. The monthly evaporation was higher during summer months and started decreasing after the onset of monsoon. The mean monthly maximum temperature ranged from 24.9°C in January to 36.7°C in April while the mean monthly minimum temperature ranged from 11.0°C in January to 26.7°C in June. The mean monthly maximum relative humidity over the period ranged from 89.0% in April to 94.4% in January while the mean monthly minimum relative humidity ranged from 41.0% in March to 72.8% in June (Table 1).

Table 1. Monthly average temperature and relative humidity at the experimental site in West Bengal, India (2007–2013)

	Maximum temperature (^0C)	Minimum temperature (^0C)	Maximum relative humidity (%)	Minimum relative humidity (%)
January	24.94	11.04	94.37	52.71
February	28.88	14.36	92.24	46.72
March	34.25	20.39	89.76	41.00
April	36.74	24.73	89.04	46.76
May	35.93	25.85	89.79	60.93
June	34.25	26.69	92.38	72.78
July	32.63	26.54	94.63	78.74
August	32.58	26.39	95.42	79.00
September	32.94	25.99	95.69	77.32
October	32.71	23.43	93.34	66.85
November	30.38	17.91	90.32	55.04
December	26.14	12.57	92.50	55.52

Figure 1. Monthly rainfall pattern at the experimental site in West Bengal, India

Figure 2. Monthly evaporation at the experimental site in West Bengal, India

The soil of the experimental site is well-drained clay loam with pH 6.6, organic carbon 0.6%, available N 264 kg ha⁻¹, available P 82.3 kg ha⁻¹, and available K 288 kg ha⁻¹. The experiment was laid out in Randomized Complete Block Design with four replications and six treatments: T1 – Control (no fertilizers) but irrigated through drip, T2 – 25% of the RDF through drip system, T3 – 50% of the RDF through drip system, T4 – 75% of the RDF through drip system, T5 – 100% of the RDF through drip system and T6 – conventional method, i.e., 100% of the RDF in soil with ring basin irrigation at IW/CPE = 1, where IW is the amount of irrigation water in terms of depth, and CPE is the cumulative pan evaporation which was observed as the sum of daily evaporation from standard U.S.W.B. open pan. Progressive total of evaporation after accounting for rainfall was maintained and 50 mm irrigation was scheduled on attaining the 50 mm CPE. Drip irrigation scheduling was based on the replenishment of the loss of evapotranspiration which takes place between two irrigation application dates. The departmental recommendation of Directorate of Agriculture, Government of West Bengal on the soil test basis was considered as RDF (N: P₂O₅: K₂O applied at 500:250:750 g palm⁻¹ year⁻¹) (Anonymous, 2008). The nuts were harvested periodically at maturity throughout the year. Copra yield was computed based on the copra content in the nut in each treatment.

Irrigation Water Requirement

The effective rainfall (Re) was calculated 24 h after rainfall, following the balance sheet method (Bandyopadhyay and Mallick, 2003) and was deducted time to time from cumulative pan evaporation (CPE).

The drip irrigation system consisted of a pump, control unit (hydrocyclone, mesh filter, fertilizer tank, flow meter, valves, pressure regulator, and manometers) and pipelines (main, manifold, and driplines). One lateral line was provided for each treatment with a valve to control the nutrient application. Four emitters at the rate of 6 l hr⁻¹ discharge rate were placed in between 60 and 90 cm away from the base of the palm on four sides. Irrigation was applied through drip with emitter flow rates 6.0 L h⁻¹ under 0.1 MPa operation pressure. Volume of water (V) in each irrigation event after N number of days for the area (A) was calculated as follows (Kumar and Dey, 2011)

(1) $\mathrm{V}=\sum \left(\mathrm{E}\mathrm{T}\mathrm{c}\times \mathrm{A}\times \mathrm{N}--\mathrm{R}\mathrm{e}\times \mathrm{A}\right)$$\mathrm{V}=\sum \left(\mathrm{E}\mathrm{T}\mathrm{c}\times \mathrm{A}\times \mathrm{N}--\mathrm{R}\mathrm{e}\times \mathrm{A}\right)$(1)

ETc (mm) was computed by multiplying pan evaporation (Epan) with pan coefficient (Kp) and crop coefficient (Kc) values as follows:

(2) $\mathrm{E}\mathrm{T}\mathrm{c}=\mathrm{E}\mathrm{T}\mathrm{p}\mathrm{a}\mathrm{n}\times \mathrm{K}\mathrm{c}\times \mathrm{K}\mathrm{p}$$\mathrm{E}\mathrm{T}\mathrm{c}=\mathrm{E}\mathrm{T}\mathrm{p}\mathrm{a}\mathrm{n}\times \mathrm{K}\mathrm{c}\times \mathrm{K}\mathrm{p}$(2)

Kp was considered a value of 0.85 for relative humidity as high (more than 70%), wind speed as light (<2 m s⁻¹) and windward side distance of 1000 m from the pan placed at short green cropped area (Allen et al., 1998). The Kc values, 0.78, 0.73, 0.63, 0.62, 0.60, 0.58, and 0.56 were considered for the month of November, December, January, February, March, April, and May, respectively, to calculate ETc (Jayakumar et al., 1988). Applied irrigation water through ring basin and drip irrigation methods were given in Table 2.

Table 2. Monthly pan evaporation, effective rainfall and irrigation water applied through drip and ring basin method during November-May, 2007–08 to 2012–13

Month	Year
	2007–2008	2008–2009	2009–2010	2010–2011	2011–2012	2012–2013
Pan evaporation (mm)
Nov.	46.7	57.0	57.1	50.3	44.5	44.7
Dec.	38.9	31.0	37.9	33.4	30.1	29.8
Jan.	45.0	37.3	40.1	38.0	32.8	35.3
Feb.	55.4	71.1	63.4	60.7	60.6	55.8
March	92.5	104.7	126.6	106.7	110.7	109.3
April	129.5	111.6	158.1	68.8	144.6	125.8
May	139.1	148.1	120.1	150.4	135.8	115.9
Total	547.1	560.8	603.3	508.3	559.1	516.6
Effective rainfall (mm)
Nov.	91.4	111	0	0.7	8	50.2
Dec.	7.6	0	0	19.5	0	7.3
Jan.	60.5	0	0	0	57	1.9
Feb.	27.8	0	7.2	1	11.8	9
March	16.2	67.7	0.5	62.5	0	0
April	32.2	0.2	47.5	69.4	36	113
May	120	130	113.1	148	93.5	114.8
Total	355.7	308.9	168.3	301.1	206.3	296.2
Ring basin irrigation at IW/CPE = 1 (mm)
Nov.	0.0	0.0	57.1	49.6	36.5	0.0
Dec.	0.0	0.0	37.9	13.9	30.1	17.0
Jan.	0.0	14.3	40.1	38.0	0.0	33.4
Feb.	0.0	71.1	56.2	59.7	24.6	46.8
March	75.0	37.0	126.1	44.2	110.7	109.3
April	97.3	111.4	110.6	0.0	108.6	12.8
May	19.1	18.1	7.0	1.8	42.3	1.1
Total	191.4	251.9	435.0	207.2	352.8	220.4
Drip irrigation at ETc (mm)
Nov.	0.0	0.0	37.9	32.9	24.2	0.0
Dec.	0.0	0.0	23.5	8.6	18.7	10.5
Jan.	0.0	7.7	21.5	20.3	0.0	17.9
Feb.	0.0	37.5	29.6	31.5	13.0	24.7
March	38.3	18.9	64.3	22.5	56.5	55.7
April	48.0	54.9	54.5	0.0	53.5	6.3
May	9.1	8.6	3.3	0.9	20.1	0.5
Total	95.3	127.5	234.6	116.7	186.0	115.7

Fertilizer Schedule

Fertigation was done to the crop through the drips at 21 days interval in ten splits using urea (46% N), diammonium phosphate-DAP (18% N, 46% P₂O₅) and muriate of potash-MoP (60% K₂O) as source of N, P, and K, respectively, from November to May. Both DAP and MoP were soaked overnight and filtered before application. All the nutrients were applied separately. A venturi was used to inject the fertilizer solution to pass through the screen filter. Plastic drippers were washed immediately after fertigation with DAP to avoid clogging. Two equal splits of recommended fertilizer were applied on soil through ring basin method in conventional treatments with the onset of monsoon during May–June and with the retreat of monsoon during September–October. Cultural practices like weeding, plant protection measures and other cultural practices were done as per the recommendations of the results of AICRP on Palms, Bidhan Chandra Krishi Viswavidyalaya (BCKV).

Nutrient Use Efficiency

The nut yield obtained from each harvest was weighed and the yield of all pickings was added and expressed in kg ha⁻¹. Nutrient use efficiency (NUE) for each nutrient was calculated separately for applied N, P, and K fertilizers. The N, P, and K contents of applied fertilizers were considered to calculate nutrient use efficiency, which is yield increase per unit nutrient applied (Doberman, 2007).

(3) $\mathrm{N}\mathrm{U}\mathrm{E}=\left(\mathrm{Y}\mathrm{t}-\mathrm{Y}0\right)/\mathrm{n}$$\mathrm{N}\mathrm{U}\mathrm{E}=\left(\mathrm{Y}\mathrm{t}-\mathrm{Y}0\right)/\mathrm{n}$(3)

Where, Yt is the yield of treated plot (kg ha⁻¹), Y0 is the yield of the treatment where no nutrient was applied (kg ha⁻¹), and ‘n’ is the amount of nutrient applied (kg ha⁻¹).

Energy Use Efficiency and Energy Productivity

To study the energy inputs and outputs, a complete inventory of all inputs (fertilizers, seeds, plant protection chemicals, drip materials, fuels, human labor, etc.) and outputs of copra, and nut yield was recorded (Tables 3 and 4). The energy value was determined based on energy inputs and energy production. Inputs and outputs were converted from physical to energy unit measures through published conversion coefficients (Banzon, 1984; Biswas et al., 2006; Sarkar, 1997) (Table 3). Average annual energy use efficiency (EUE = energy output/energy input) and energy productivity (EP = yield/energy input) were also calculated.

Table 3. Price and energy equivalents for different inputs and outputs used in coconut cultivation in an experiment in West Bengal, India

Source of energy	Unit	Equivalent energy (MJ)	Price (US $)*
Input
Human labor (man)	H	1.96	3.74
Diesel	L	56.31	0.99
Fertilizer
Fertilizer N	Kg	60.00	0.36
Fertilizer P	Kg	11.10	0.22
Fertilizer K	Kg	6.70	0.23
Farm yard manure	kg (dry mass)	0.30	0.01
Chemicals
Superior chemicals	Kg	120.00	6.51
Inferior chemicals	Kg	10.00	1.44
Plastic/polyethylene	Kg	90.00	5.37
Water for irrigation	m^3	1.02	0.01
Electricity	kWh	3.60	0.09
Farm machinery	H	13.60	1.42
Output
Copra	Kg	37.57	1.18
Husk	Kg	16.69	0.11
Shell	Kg	23.01	0.13

1US {{footpara}}amp;#x00A0;= IRs 47.73

Table 4. Input requirement for various treatments

	Treatment
Item	T1	T2	T3	T4	T5	T6
Farm yard manure (t ha^−1)	12	12	12	12	12	12
Fertilizer -N (kg ha^−1)	0	22	44	67	89	89
Fertilizer -P (kg ha^−1)	0	22	10	15	19	19
Fertilizer -K (kg ha^−1)	0	22	55	83	111	111
Carbofuran 3 G (kg ha^−1)	35	35	35	35	35	35
Carbendazim 50% WP (kg ha^−1)	17.5	17.5	17.5	17.5	17.5	17.5
Monocrotophos 36% SL (l ha^−1)	3	3	3	3	3	3
Diesel (l ha^−1)	18	18	18	18	18	18
Plastic drip materials and irrigation pipes (kg ha^−1)	241	241	241	241	241	89
Irrigation (m3)	1460	1460	1460	1460	1460	2765
Electricity (kw ha^−1)	787	787	787	787	787	369
Farm machinery (h ha^−1)	6.5	6.5	6.5	6.5	6.5	6.5
Labor before harvest (8-h days ha^−1)	268	303	308	311	314	293
Labor for harvest and processing (8-h days t^−1 copra)	16.2	16.2	16.2	16.2	16.2	16.2

*T₁- Control (no fertilizers) and irrigated through drip; T₂- 25% of the recommended dose of fertilizer (RDF) through drip system; T₃- 50% of the RDF through drip system; T₄- 75% of the RDF through drip system; T₅- 100% of the RDF through drip system; T₆- 100% of the RDF in soil with ring basin irrigation as conventional method

Economic Analysis

Working out the production cost of coconut from experimental plots was considered inaccurate. Family labor at the mean wage rate of hired labor was included in the cost calculations, thus ignoring possible opportunity costs. The cost of harvesting and processing also depends on the amount of yield. Therefore, cost per unit yield for harvest and processing was calculated using measured mean yield in combination with the published standard costs for harvesting and processing (Biswas et al., 2006) (Tables 3 and 4). Net return or profit was calculated by subtracting production cost from the gross value of the product, including by-product (leaf, shell, and coir) value or gross return. Prices used for inputs and harvest products were average prices observed during the experimental period (Table 3). The benefit-cost ratio (BCR) was calculated by dividing the net return by the production cost

Statistical Analysis

All the collected data were analyzed statistically by the analysis of variance (ANOVA) technique using the SAS Windows Version 9.3 (SAS Institute, Cary, NC). Treatments were compared by computing the “F-test”. The significant differences between treatments were compared pare wise by the least-square difference at 5% level of probability. The nonsignificant treatment differences were denoted as NS.

Results and Discussion

The efficient management of water is of utmost importance for sustaining and enhancing agricultural production. Thus, the importance of scientific water management and the need to adopt advanced techniques such as drip irrigation to enhance productivity and water-use efficiency became imperative (Jayakumar et al., 2015). Drip irrigation has added advantages because it can also be used to apply any water-soluble fertilizer or chemical in precise amounts, as and when required to match the plant needs or any other agronomic management (Patel and Rajput, 2011). In our earlier study, we observed that drip fertigation can reduce 25% fertilizer requirement along with 3.2 times higher water use efficiency in comparison to surface irrigated fertilizer application method in coconut (Bandyopadhyay et al., 2019). In the current study, we investigated the effect of fertigation on nutrient and energy use efficiency, and economics of coconut cultivation to provide the nutrient and energy efficient and profitable recommendations for coconut cultivation in the EIGP of South Asia.

Nutrient Use Efficiency

Copra yield was highest with 75% RDF application through the drip (3.19 t ha⁻¹ year⁻¹) followed by 100% RDF application through the drip (3.12 t ha⁻¹ year⁻¹) (Table 5). The data presented in Table 6 revealed that drip fertigation was more efficient nutrient user than conventional ring basin method; the nutrient use efficiency had an increasing trend with decreasing dose of fertilizer. Nitrogen use efficiency ranged from 50.4 kg copra kg⁻¹ N with 25% of the RDF through the drip to 6.1 kg copra kg⁻¹ N with 100% RDF with ring basin irrigation as the conventional method. Similarly, the P use efficiency also increased with decreasing P application, ranging from 230.7 kg copra kg⁻¹ P in 25% of the RDF through the drip system to 27.8 kg copra kg⁻¹ P in 100% RDF with ring basin irrigation. This indicates that lower the N or P application better is their utilization. The same trend was also observed in case of K use efficiency, which increased with the decreasing application of K ranging from 40.5 kg copra kg ⁻¹ K to 4.9 kg copra kg ⁻¹ K. This indicates that the plant might have utilized K more efficiently with a lower dose of its application. The higher nutrient use efficiency with lower nutrient dose under drip fertigation may be due to a slow but steady supply of nutrients directly to the root zone (Table 6). The 100% RDF with ring basin method had the lowest nutrient use efficiency among the methods studied. From the Table 6, it was clear that the nutrient use efficiency was inversely proportional to nutrient application rate and it showed an increasing trend over the years due to inherent soil fertility status. Subramanian et al. (2012) reported that an increase in fertigation level will result in increased levels of soil nutrient status. The use efficiency of N, P, and K with 75% of RDF through drip was 76%, 77%, and 83%, which was higher than 100% RDF through ring basin indicating the efficiency of drip method over surface application of fertilizer. Use efficiency of N, P, and K in T4 was 36%, 36%, and 38% which was higher than 100% RDF through drip indicating the scope of reduction of fertilizer load without reducing nutrient use efficiency and productivity.

Table 5. Copra, shell and husk production (t ha⁻¹ year⁻¹) in coconut as influenced by fertigation levels (pooled over 2007–2013)

Treatments	Shell	Husk	Copra yield
T1	1.86	2.10	1.87
T2	2.87	2.39	2.99
T3	2.93	3.74	3.08
T4	2.76	3.22	3.19
T5	2.99	3.54	3.12
T6	2.51	2.97	2.41
SEm ±	0.09	0.13	0.01
LSD (P = .05)	0.29	0.42	0.03

*T₁- Control (no fertilizers) and irrigated through drip; T₂- 25% of the recommended dose of fertilizer (RDF) through drip system; T₃- 50% of the RDF through drip system; T₄- 75% of the RDF through drip system; T₅- 100% of the RDF through drip system; T₆- 100% of the RDF in soil with ring basin irrigation as conventional method

Table 6. Nitrogen use efficiency (a), phosphorus use efficiency (b) and potassium use efficiency (c), in coconut as influenced by fertigation levels

Treatment	2007–08	2008–09	2009–10	2010–11	2011–12	2012–13	Pooled
a. Nitrogen use efficiency (kg copra kg ^−1 of N)
T2	44.1	48.2	51.8	50.9	51.8	55.4	50.4
T3	21.8	23.6	25.9	28.6	30.6	33.1	27.2
T4	14.9	17.3	20.1	22.1	22.1	23.3	19.8
T5	10.8	12.8	13.5	15.1	15.8	16.9	14.1
T6	3.9	5.3	5.9	7.2	7.2	7.1	6.1
SEm±	0.4	0.6	0.8	1.0	1.0	0.9	0.8
LSD(P = .05)	1.3	1.8	2.4	3.0	3.0	2.8	2.3
b. Phosphorus use efficiency (kg copra kg ^−1 of P)
T2	201.8	220.4	236.8	232.7	236.8	253.3	230.7
T3	99.9	108.1	118.4	130.8	140.0	151.4	124.6
T4	68.0	78.9	92.0	100.9	100.9	106.4	90.6
T5	49.4	58.7	61.8	69.0	72.1	77.2	64.4
T6	18.0	24.2	26.8	33.0	33.0	32.4	27.8
SEm±	3.0	2.1	3.3	3.9	2.9	4.3	3.3
LSD(P = .05)	9.2	6.4	9.8	11.9	8.7	13.0	9.8
c. Potassium use efficiency (kg copra kg^−1 of K)
T2	35.4	38.7	41.6	40.8	41.6	44.5	40.5
T3	17.5	19.0	20.8	22.9	24.6	26.6	21.9
T4	11.9	13.9	16.1	17.7	17.7	18.7	15.9
T5	8.7	10.3	10.8	12.1	12.6	13.6	11.3
T6	3.2	4.2	4.7	5.8	5.8	5.7	4.9
SEm±	0.4	0.5	0.6	0.7	0.6	0.6	0.6
LSD(P = .05)	1.1	1.5	2.0	2.1	1.8	1.9	1.7

*T₁- Control (no fertilizers) and irrigated through drip; T₂- 25% of the recommended dose of fertilizer (RDF) through drip system; T₃- 50% of the RDF through drip system; T₄- 75% of the RDF through drip system; T₅- 100% of the RDF through drip system; T₆- 100% of the RDF in soil with ring basin irrigation as conventional method

Fertigation enables the application of fertilizer uniformly and more efficiently as has also been observed by Patel and Rajput (2000). They reported that the judicial application of nutrients through the drip fertigation method helps build up nutrient content in the experimental field. Hence, nutrient availability in the soil can be maintained at a higher level with fertigation as compared to soil application of fertilizers. Silber et al. (2003) reported enhanced upward movement of absorbed nutrients by mass flow in lettuce when irrigation frequency was enhanced. Fertigation system retains the applied water and the nutrients in the plant root zone as has also been reported by Fares and Alva (2000). Basavaraju et al. (2014) also reported that the application of fertilizers in split doses through drip irrigation minimizes leaching losses of N and K and fixation of P in the soil. Papadopoulos (1988) also reported that the uptake of major plant nutrients, e.g. N, P, and K is higher with fertigation than with conventional methods.

Energy Analysis

It was observed that the treatments with higher energy input also recorded higher energy output in the drip fertigation system (Table 7). Energy analysis of various levels of fertigation in coconut revealed that total energy input in different fertigation treatments ranged from 36281 MJ ha⁻¹ to 43605MJ ha⁻¹. The basis of variation of the energy inputs were plastic used for drip system, irrigation input, fertilizer, and man days requirement in different treatments. The lowest energy input was recorded in conventional system of ring basin method (29210 MJ ha⁻¹). This method of irrigation had even lower energy input than in T1, in which irrigation was applied through drip. The higher energy input in T1 was due to high energy requirement of drip materials. Among the drip fertigation treatments, it was the amount of fertilizer and associated human labor requirement, which influenced the energy input ranging from 38840 MJ ha⁻¹ in drip fertigation with 25% RDF to 43605 MJ ha⁻¹ in drip fertigation with 100% RDF. Fertigation with various fertilizer input and irrigation through drip improved energy output (192618 ~ 206082 MJ ha⁻¹) over no fertilizer application but irrigation applied through drip (120356 MJ ha⁻¹) or through ring basin method with 100% RDF (155424 MJ ha⁻¹). Conventional basin irrigation with 100% RDF showed the highest energy use efficiency (5.32) and energy productivity (82.51 g copra MJ⁻¹) but with the lowest energy output (155424 MJ ha⁻¹). However, drip fertigation with 75% RDF (T4) was more efficient in energy use than drip irrigation with 100% RDF (T5) due to equivalent energy output and productivity between treatments (Table 7). Treatment T4 signifies the reduction of 25% of energy input in the form of fertilizers having the maximum output of energy without reducing the coconut yield. Fertigation of 75% of RDF through drip irrigation attributed the energy savings to reductions in fertilizer as well as fuel and labor for application and irrigation management over conventional method. Fertilizers constituted the largest proportion of energy inputs. Thus, this drip fertigation in coconut also resulted in increase in energy use efficiency and energy productivity. Drip irrigation reduced the evaporative demand and reduced the irrigation requirement and increased energy use efficiency by enhancing water use efficiency, gaining an economic advantage while also reducing environmental burdens (Gathala et al., 2019; Levidowa et al., 2014).

Table 7. Energy analysis of coconut as influenced by fertigation levels

Treatment	Energy input (MJ ha^−1)	Energy output (MJ ha^−1)	Energy use efficiency	Energy productivity (g copra MJ^−1)
T1	36281	120356	3.32	51.54
T2	38840	192618	4.96	76.98
T3	40359	198621	4.92	76.32
T4	42003	206082	4.91	75.95
T5	43605	201570	4.62	71.55
T6	29210	155424	5.32	82.51
SEm±	943	1642	0.07	2.22
LSD (P = .05)	2844	5007	0.20	6.70

*T₁- Control (no fertilizers) and irrigated through drip; T₂- 25% of the recommended dose of fertilizer (RDF) through drip system; T₃- 50% of the RDF through drip system; T₄- 75% of the RDF through drip system; T₅- 100% of the RDF through drip system; T₆- 100% of the RDF in soil with ring basin irrigation as conventional method

Economic Analysis

Economic analysis showed a higher initial cost of drip installation contributing to higher production cost, and different fertilizer application rates also contributed to the varied cost of cultivation among treatments. Data presented in Table 8 revealed that 75% of the RDF through the drip system recorded a significantly higher BCR of 2.36 with maximum net returns of US 3073 USD ha⁻¹ followed by 50% RDF with drip (2.30). The lowest BCR was recorded with no fertilizers (1.25) with the lowest net return of US 1421 USD ha⁻¹. BCR in drip irrigation at 100% ETc was 43% higher than basin irrigation at IW/CPE ratio of 1 in sandy clay loam soil (Nampoothiri, 2018). Madhava Chandran and Surendran (2016) also reported that in coconut higher economic returns and profitability could be achieved by drip fertigation with polythene mulching irrespective of variation in nutrient dose. However, Jayakumar et al. (2017) observed 100% RDF fertigation and mulching was the most profitable option. The productive and remunerative advantage of nut production in drip irrigated coconut was due to sustained moisture and nutrient supply to the active root zone through two or three directional flow regimes rather than vertical in conventional ring basin method (Nampoothiri, 2018).

Table 8. Economic analysis of various treatments for Coconut (US $ ha⁻¹)

Treatment	Cost of Cultivation	Gross Return	Net return	Benefit cost ratio (BCR)
T1	1134	2555	1421	1.25
T2	1252	4089	2837	2.27
T3	1277	4216	2940	2.30
T4	1301	4375	3073	2.36
T5	1320	4279	2958	2.24
T6	1084	3299	2215	2.04
SEm±	24.6	95.1	77.2	0.04
LSD (P = .05)	75.7	292.9	237.9	0.13

*T₁- Control (no fertilizers) and irrigated through drip; T₂- 25% of the recommended dose of fertilizer (RDF) through drip system; T₃- 50% of the RDF through drip system; T₄- 75% of the RDF through drip system; T₅- 100% of the RDF through drip system; T₆- 100% of the RDF in soil with ring basin irrigation as conventional method

Conclusions

This study showed that though in some drip fertigation treatments there was a reduction in yield, there was saving in terms of quantity of water used. Further, drip fertigation along with different doses of fertilizers resulted in higher productivity with a saving of chemical fertilizers, besides ensuring the higher use efficiency of nutrients and the production of energy in coconut. It can be concluded that fertigation with 75% of RDF can result in higher energy output (206082 MJ ha⁻¹), higher copra yield (31.94 T ha⁻¹), more profit (US 3073 USD ha⁻¹), and higher benefit-cost ratio (2.36). The six years study on different levels of fertigation led to the findings that coconut performed well with drip fertigation in respect of NUE, energy production and its use efficiency, and profitability as compared to no fertilizer. The study clearly suggests that the coconut farmers can accrue benefits in terms of higher yield and profitability with efficient utilization of energy and water with reduced quantity (25%) of fertilizers. These have benefits in terms of savings of input expenditure, water, as well as environmental pollution through adoption of drip fertigation with 75% of recommended dose of N:P₂O₅:K₂O (500:250:750 g palm ⁻¹ year ⁻¹). Such water and nutrient savings have the potential to reduce extra chemical load into the environment by saving of 25% of chemical fertilizer (an external input) and may usher boom for vertical and horizontal expansion of coconut in the EIGP of South Asia.

Acknowledgments

Authors are thankful to the Director of Research, BCKV, Department of Agriculture, Government of West Bengal and the PC cell, AICRP on Palms, ICAR-CPCRI, Kasaragod, Kerala, for providing necessary support to conduct this long-term research project and anonymous reviewers for their valuable suggestions for improvement of the manuscript.

Additional information

Funding

This work was supported by the Department of Agriculture, Government of West Bengal [AICRP_PALM]; ICAR-CPCRI [AICRP ON PLAMS].