Establishing Critical Leaf Nutrient Concentrations and Identification of Yield Limiting Nutrients for Precise Nutrient Prescriptions of Oil Palm (Elaeis guineensis Jacq) Plantations

Abstract

African oil palm (Elaeis guineensis Jacq.) is a bulk feeder of nutrients. In this study, we aimed at devising strategies for efficient nutrient management in the oil palm plantations of the Krishna River basin located in Andhra Pradesh, India by assessing soil fertility status, establishing optimal leaf nutrient concentrations and identifying yield restrictive nutrients. In total, 67 oil palm plantations were surveyed from this area in 2020, soil samples were collected and analysed for different soil properties, including pH, EC, SOC, available P, K, S, exchangeable Ca and Mg, and hot water-soluble boron (HWB) in surface (from 0–20 cm depth), subsurface (from 20–40 cm depth) and deep (from 40–60 cm depth) soil layers. As per DRIS (Diagnosis and Recommendation Integrated System) indices estimated in this study, the order of requirement of nutrients is Nitrogen (N) > B > K > P > Mg for this area. Optimum leaf nutrient concentrations ranged between 2.07–4.29%, 0.13–0.27%, 0.52–0.94%, 0.44–0.76% and 44.97–102.70 mg/kg for N, P, K, Mg and B, respectively. In surveyed plantations, about 15, 6, 16, 9 and 12 percent of leaf samples had less than optimum concentration of N, P, K, Mg and B respectively. Nitrogen and Boron are the major yield limiting factors in this region. Leaf nutrient concentrations need to be maintained at the optimum ranges as estimated above for higher productivity in the Krishna basin area.

1. Introduction

Oil palm (Elaeis guineensis Jacq) is a humid tropical crop native to West Africa. It needs a balanced and ample supply of nutrients for optimal growth and performance [1]. This is the highest oil yielding crop on the globe, with an average oil productivity ranging between 4 to 6 tonnes per hectare in a year. Generally, it is cultivated as a rainfed crop in the majority of countries. However, in India it is largely grown under irrigated conditions only under climates ranging from tropics (wet and dry) to semi arid tropics in central part. However, it is grown on a variety of soils ranging from red sandy loam/sandy clay loam (Alfisols), alluvial silty loam (Inceptisols/Entisols), heavy black soils (Vertisols) and acidic red laterite soils (Oxisols). However, the best suited soils are well drained, deep loamy alluvial soils having good organic matter content and water holding capacity. At present it is cultivated in an area of 0.37 million hectares in India of which the largest proportion of area under cultivation (nearly 50%) is located in Andhra Pradesh state, in eight districts [2]. In general, oil palm is cultivated as a monocrop at a spacing of 9 × 9 in × 9 m equilateral triangular geometry. However, in a few pockets on the east coast, it is being grown under intercropping systems with other perennial crops like cocoa, pepper, ornamental crops, etc. The crop responds well to management which includes irrigation, nutrition and pest management. From various studies, it was identified that potassium, magnesium and boron deficiencies, as well as N and K imbalance, are the most limiting causes of oil palm yields from Indian soils. The limiting nutrients are to be supplemented through external application only. The cost of nutrient management in oil palm accounts to nearly 40 percent of the total [3]. Under irrigated conditions, the economic life cycle of the crop being 25–30 years, balanced nutrition assumes greater significance for maintaining site yield potential, and it can only be achieved with balanced and judicious use of fertilizers.

Calibrated soil and leaf tests [4] are used to give fertilizer recommendations in oil palm. Critical leaf nutrient concentration generally indicates the levels of sufficiency, deficiency and toxicity of a particular leaf nutrient but not the interactions and the balance with other nutrients. Generally, critical nutrient concentration is considered to supply required quantities of nutrients. This does not consider the ratios of nutrients which are very much vital to provide balanced nutrition in any crop. However, in the case of Diagnosis and Recommendation Integrated System (DRIS), this interaction and balance factor is taken care of as the ratios of different nutrients are estimated and, based on that, the DRIS norms are arrived upon. Due to its perennial nature and longer cycles of reproduction, in crops like oil palm, conducting field experiments in each location for arriving at location specific nutrient recommendations is difficult. Therefore, precise nutrient recommendations could be arrived at through an alternate method like DRIS which considers the ratio of nutrients in leaves for achieving higher productivity through balanced nutrient management. For this purpose, only the most influential nutrients of the crop are considered while developing the DRIS norms.

Beaufils [5] developed the DRIS concept, which is widely used, especially to find out the order of limiting nutrients of a crop in a particular location. Mourão Filho [6] opines that DRIS mitigates the distortions in diagnosis due to dilution, concentration, crop age and plant part. This requires information about the nutrient levels both in soil and crop tissue, and constant monitoring of these levels is required for supplying nutrients at recommended levels. Differences in yield levels are to be considered for better accuracy of leaf diagnostics and nutrient management [7]. The DRIS system developed by Beaufils [5] considers ratios of different nutrients and helps us out to identify the most limiting nutrients along with their order of importance. This enables planning of judicious nutrient schedules to meet the nutrient demands and to supply the plants with required nutrient levels only. Narasimha Rao et al. [8] reported that certain nutrient disorders/deficiencies like nitrogen (N)/potassium (K) imbalance, K deficiency, magnesium (Mg) deficiency and boron (B) deficiency are very common in different oil palm plantations of India and are the major constraints upsetting oil palm production in the country.

Behera et al. [9] developed DRIS norms for Surat District of Gujarat in Oil palm and the order of limiting nutrients were reported as K > N > B > P > Mg. Similarly, on the Southern Plateau (Karnataka state), the order of limiting nutrients in oil palm has been reported as K > P > N > B > Mg [10]. This indicates that there is variation in the order of limiting nutrients in different places and so development of DRIS indices and estimating the order of limiting nutrients is required at least at district level, especially in the states where there is a larger area under oil palm cultivation. Adding to it, there are wide variations in yield levels of plantations within the district, which is evident from the survey data collected. So far, no information on soil fertility and leaf nutrient concentrations of oil palm plantations of Krishna District is available, even though the district occupies a major area of oil palm, along with East and West Godavari Districts in the state of Andhra Pradesh. In our opinion, DRIS norms specific to a geographical location are more accurate than the ones which are generalized for a large part like a state or province. Therefore, this investigation was taken up to discover yield restricting nutrients in oil palm plantations of Krishna District of Andhra Pradesh, by evaluating the soil fertility and nutrient concentrations in the representative leaves, to allow efficient nutrient management, while comparing them with already available standards for the state of Andhra Pradesh.

2. Materials and Methods

2.1. Study Area

A field survey of oil palm plantations was conducted in Krishna District of Andhra Pradesh state in the year 2020 to estimate soil fertility and plant nutritional status (in representative leaf) in 67 oil palm plantations. The district is located at latitudes 15°43′ and 17°10′ North and longitudes of 80°00′ and 81°33′ East, with an aerial extent of 8797 km². The sampled area lied between 16°39.1′74′′ N to 16°51.2′79′′ N latitude and 80° 50.1′25′′ E to 80°59.3′09′′ E longitude in Krishna District of Andhra Pradesh in the East Coast of India (Figure 1). The average altitude of the area ranged between 25 to 129 m above mean sea level. The age of oil palm plantations ranged between 5 and 24 years in different locations. Oil palm requires specific climatic conditions for exhibiting its potential. The climate of the area is of the tropical rainy type with a forceful summer season. The average rainfall of the region is 1011.2 mm in 41–60 rainy days. Out of this, 700 mm rainfall is received in the southwest monsoon (June–September) and 241 mm is received in the northeast monsoon (October–December), whilst 6.3 mm rainfall occurs in Winter (January–February) and 64 mm is received in summer (March–May). This amounts to 69.25% in the southwest monsoon, 23.82 % in the northeast monsoon, 0.62 %in winter and 6.31 % in summer. The mean daily maximum and minimum temperatures of the region are 38 °C (in May) and 20 °C (in December/January) respectively. The southwest monsoon sets in June, bringing the temperatures down and more or less uniform during the monsoon period. The relative humidity (RH) is in the order of 80% in the mornings, and it ranges from 70 to more than 80% during the evenings. The most predominant soils of the region are: black cotton soils/deltaic soils, sandy soils and red loamy soils.

2.2. Collection of Soil Samples and Analysis

A total of 67 (×3) soil samples were collected from 0–20 cm (surface), 20–40 cm (subsurface) and 40–60 cm (deep) depths in the palm basins during the survey to assess soil fertility status of oil palm plantations located mainly in Nuzividu and Bapulapadu mandals. To derive one representative sample for the purpose of nutrient analysis, three random samples were collected from palm basins (one sample from one basin) and mixed [11]. These soil samples were processed by drying in shade at room temperature and grinding after removing roots and debris. Then they were passed through a 2 mm sieve before storing for analysis. For estimation of organic carbon (OC), these 2 mm sieved samples were ground once again with pestle and mortar before sieving through a 0.5 mm sieve. These processed and sieved soil samples were used for estimating pH, electrical conductivity (EC), OC, available K, phosphorus (P), exchangeable calcium (Ca) and Mg, available sulphur (S) and hot-water-soluble B (available B). Soil pH and EC were estimated in 1:2 soil water ratio (weight (w)/volume (v)) suspension with half an hour equilibrium time, using a pH meter (Systronics pH meter—361) and EC meter (Systronics conductivity meter—306) respectively [11]. Organic carbon content in the soil was estimated by Walkley and Black [12] method. Neutral normal ammonium acetate solution (NH₄OAc-K) [13] with flame photometry (Systronics model 128) was used for estimating available K in soil. Olsen’s method [14] through spectrophotometry (Shimadzu UV Vis—1800) was used for available P extraction. Exchangeable Ca (Exch. Ca) and Mg (Exch. Mg). were estimated using Atomic absorption spectrometry (Thermo AAS 301) with neutral normal ammonium acetate solution [15] For estimating available S (CaCl₂-S), the turbidity method [16] was used. Azomethine-H reagent [17] with spectrophotometry (Shimadzu UV Vis—1800) was used to estimate hot-water-soluble B (HWB). All the measured soil properties were subjected to descriptive statistics.

2.3. Collection of Leaf Samples and Analysis

In adult oil palm, the 17th leaf (frond) is considered as the representative one for nutrient sampling. We identified the 17th frond and a total of 67 samples were collected from the palms, from where soil samples were also drawn [18]. The collected leaf samples were processed by washing first with tap water for removing soil, dirt, etc., and after that in 0.2% detergent solution in sequence. After that 0.1% normal hydrochloric acid (HCl) solution was used to remove waxy and metallic deposits and finally washed in single and double distilled water. Excessive moisture was removed by pressing with blotting paper. Leaf samples were air dried and then oven dried (at 70 °C for 72 h) before powdering in steel mill. Powdered samples were stored in polythene bottles and these powdered samples were used for estimation of N, P, K, Ca, Mg, S and B contents in the leaves. These samples were digested using 1 g powder in di-acid mixture (9:4 ratio of nitric acid and perchloric acid) for estimation of nutrients (except nitrogen) through standard procedures [11]. Micro-Kjeldahl method was used for estimation of nitrogen. Vanadomolybdate, flame photometer and turbidity methods were used for P, K and S estimations, respectively. An atomic absorption spectrophotometer was used to estimate calcium and magnesium. Boron was estimated using Azomethine-H method by producing dry ash out of 1 g leaf sample heated at 550 to 600 °C in a muffle furnace and extraction in dilute HCl.

2.4. Statistical Analysis and Estimation of DRIS Indices

Data on leaf nutrient concentrations was subjected to descriptive statistical analysis in excel. Estimating the DRIS norms or standards is the first step in DRIS implementation. For estimating DRIS norms, the procedure described by Beaufils [5] was followed. DRIS facilitates to achieve high yields through accurate nutrient management as it identifies the yield constraining factors in nutrition [5]. While estimating DRIS indices, the whole population of oil palm from where sampling was done has been divided into two categories (high yielding and low yielding) based on a cut-off yield level (twenty tonnes per hectare fresh fruit bunches (FFB)). The cut-off yield was decided based on the logic that a high yielding oil palm population reflected desirable conditions for obtaining optimal yields. In oil palm, N, P, K, Mg and B have been identified as the most important nutrients for estimation of DRIS norms in India. In

Table 1. Mean values of nutrient expressions and their descriptive parameters * of low and high yielding populations (n = 67).

Nut./Ratio	Low Yielding				High Yielding				Variance Ratio (Sa/Sb)
	Mean	Var (Sa)	SD	CV	Mean	Var (Sb)	SD	CV
N	2.86	0.54	0.73	25.6	3.18	0.70	0.83	26.10	0.77
P	0.20	0.00	0.03	16.68	0.20	0.00	0.05	25.00	0.00
K	0.70	0.04	0.20	28.24	0.73	0.04	0.14	19.18	1.00
Mg	0.62	0.02	0.14	23.05	0.60	0.01	0.12	20.00	2.00
B	71.00	499.81	22.36	31.49	73.84	46806.3	21.65	29.32	0.01
N/P	14.95	46.75	6.84	45.77	17.89	143.25	11.97	66.91	0.33
N/K	4.55	4.86	2.20	48.42	4.52	1.77	1.33	29.42	2.74
N/Mg	4.94	4.49	2.12	42.87	5.52	4.23	2.06	37.32	1.06
B/N	27.09	147.09	12.13	44.77	25.69	133.79	11.57	45.04	1.10
K/P	3.67	2.98	1.73	46.96	3.90	2.95	1.72	44.10	1.01
Mg/P	3.16	0.60	0.77	24.49	3.25	2.52	1.59	48.92	0.24
B/P	377.02	43694.31	209.03	55.44	402.35	47810.03	218.66	54.35	0.91
K/Mg	1.19	0.25	0.50	41.95	1.25	0.16	0.40	32.00	1.56
B/K	110.56	3201.90	56.59	51.18	109.66	2066.47	45.46	41.46	1.55
B/Mg	123.23	3571.99	59.77	48.87	126.97	1749.90	41.83	32.94	2.04

* Var, Variance; SD, Standard deviation; CV, Coefficient of variation; S(a), Variance of low yielding population; S(b): Variance of high yielding population.

, mean values of nutrient concentrations and their associated coefficients of variation (CV) and variances are presented. The leaf nutrient concentrations (mean values) of high yielding oil palm populations were used as reference values. While selecting the nutrient ratio expression, values having high variance ratio (S_a/S_b) were selected.

The DRIS indices estimated help in ordering nutrients according to their importance. The formulae advocated by Walworth and Sumner [19] were used to calculate the DRIS indices.

N index = [f (N/P) + f (N/K) + f (N/Mg) − f (B/N)]/4

P index = [−f (N/P) − f (K/P) − f (Mg/P) − f (B/P)]/4

K index = [−f (N/K) + f (K/P) + f (K/Mg) − f (B/K)]/4

Mg index = [−f (N/Mg) + f (Mg/P) − f (K/Mg) − f (B/Mg)]/4

B index = [f (B/N) + f (B/P) + f (B/K) + f (B/Mg)]/4

where, f (N/P) = [{(N/P)/(n/p)} − 1] × (1000/CV) when N/P > n/p,

f (N/P) = [1 − {(n/p)/(N/P)}] × (1000/CV) when N/P < n/p.

Note: N/P, N/K … Represent test value and n/p, n/k…. represent reference values (norms) from high yielding populations, CV is the coefficient of variation associated with the DRIS norm.

Further, using appropriate norms and CVs, other functions, viz., f(N/K), f(N/Mg), f(B/N), f(K/P), f(Mg/P), f(B/P), f(K/Mg), f(B/N), f(B/P), f(B/K) and f(B/Mg) were also calculated in the same way. The optimum nutrient ranges in oil palm leaf tissue were determined by using the DRIS technique. Optimum ranges are the values obtained by adopting mean ± 4/3 SD [20,21].

3. Results

3.1. Soil Nutrient Status of Oil Palm Plantations

Descriptive analysis of soil property data indicates a high degree of variability in oil palm growing soils in the study region. In the surface layer of soil (0 to 20 cm), the mean values of soil pH, EC (dS/m), OC (g/kg), Olsen-P (mg/kg), NH₄OAc-K (mg/kg), Exch. Ca (mg/kg), Exch. Mg (mg/kg), CaCl₂-S (mg/ kg) and HWB (mg/kg) were recorded as 7.32 ± 0.08, 0.25 ± 0.02, 0.87 ± 0.03, 101.47 ± 6.95, 566.14 ± 42.97, 4.72 ± 0.24, 2.46 ± 0.27, 60.86 ± 2.6 and 5.98 ± 0.25 respectively. In the same layer, the range of soil pH, EC (dS/m), OC (g/kg), Olsen-P (mg/kg), NH₄OAc-K (mg/kg), Exch. Ca (mg/kg), Exch. Mg (mg/kg), CaCl2-S (mg/ kg) and HWB (mg/kg) were 5.23 to 8.54, 0.08 to 0.89, 0.8 to 14.2, 17.45 to 419.58, 66.64 to 1263.25, 1.18 to 8.77, 0.5 to 4.60, 21.25 to 266.68 and 2.15 to 19.58 respectively. This indicated that the status of all the nutrients is lower than optimal in some plantations and in excess in some plantations. Wide variation was also recorded in different soil layers. Among different properties tested, the CV values were lowest for pH (9.5) and highest for NH₄OAc-K (60.7) (

Table 2. Descriptive statistics * for selected soil properties of different soil layers (n = 67).

Soil Property	Soil Layer	Min	Max	Mean	SD	CV (%)	Skew	Kurt
pH	Surface	5.40	8.28	7.32	0.67	9.15	−1.22	1.04
	Sub-surface	5.47	8.45	7.37	0.70	9.50	−1.29	1.28
	Deep	5.23	8.54	7.41	0.74	9.99	−1.29	1.46
EC, dS/m	Surface	0.13	0.77	0.25	0.13	52.00	2.08	4.93
	Sub-surface	0.08	0.89	0.25	0.15	60.00	2.25	6.05
	Deep	0.10	0.89	0.25	0.13	52.00	2.26	7.96
OC, g/kg	Surface	2.5	14.2	0.87	0.27	31.03	−0.05	−0.43
	Sub-surface	2.0	11.7	0.48	0.22	45.83	0.56	1.10
	Deep	0.80	10.5	0.43	0.21	48.84	0.78	0.65
Olsen-P, mg/kg	Surface	17.45	419.58	101.47	56.86	56.04	2.89	13.94
	Sub-surface	31.80	165.79	86.28	33.27	38.56	0.78	−0.11
	Deep	24.48	218.37	77.75	34.89	44.87	1.15	2.68
NH4OAc-K, mg/kg	Surface	87.58	1263.25	566.14	351.74	62.13	0.42	−1.07
	Sub-surface	86.46	1123.70	530.29	303.78	57.29	0.36	−0.93
	Deep	66.64	1053.35	532.24	333.20	62.60	0.54	−0.63
Exch.Ca, mg/kg	Surface	1.55	8.69	4.72	2.00	42.37	0.38	−0.62
	Sub-surface	1.18	8.22	4.41	1.84	41.72	0.43	−0.55
	Deep	1.22	8.77	4.41	1.90	43.08	0.59	−0.33
Exch.Mg, mg/kg	Surface	0.60	19.0	2.46	1.21	65.84	0.54	2.23
	Sub-surface	0.50	4.60	2.28	0.96	42.11	0.18	−0.33
	Deep	0.50	4.50	2.21	0.92	41.63	0.15	0.15
CaCl2-S, mg/kg	Surface	21.25	128.75	60.86	21.52	35.36	0.72	0.53
	Sub-surface	32.81	266.88	67.19	32.05	47.70	3.79	22.32
	Deep	25.94	153.44	67.31	26.25	39.00	1.08	1.37
HWB, mg/kg	Surface	3.19	14.03	5.98	2.02	33.78	1.68	3.7
	Sub-surface	2.15	12.29	5.47	1.78	32.54	1.31	2.87
	Deep	2.43	19.58	5.50	2.43	44.18	3.50	17.47

* Min., Minimum; Max., Maximum; SD, Standard deviation; CV, Coefficient of variation; Skew., Skewness; Kurt., Kurtosis.

). In similar lines, Behera et al. [9] also reported that 5.35, 0.13 dS m⁻¹, 19.8 g kg⁻¹, 270 mg kg⁻¹, 24.7 mg kg⁻¹, 914 mg kg⁻¹, 203 mg kg⁻¹, 23.2 mg kg⁻¹ and 0.70 mg kg⁻¹ are the mean values for soil pH, EC, OC, NH₄OAc-K, Bray’s-P, exchangeable Ca and Mg, CaCl₂-S and hot-water-soluble B respectively, in surface soils of oil palm plantations of west coastal area of India.

3.2. Nutrient Concentrations in Leaves

Wide variations in leaf nutrient concentrations were recorded in surveyed plantations (

Table 3. Descriptive statistics * for selected leaf nutrients (n = 67).

Nutrient	Min	Max	Mean	SD	CV
N (%)	1.47	4.77	3.01	0.79	26.21
P (%)	0.07	0.36	0.20	0.04	21.04
K (%)	0.26	1.04	0.72	0.19	27.02
Ca (%)	0.84	1.68	1.19	0.18	15.04
Mg (%)	0.31	0.82	0.61	0.13	21.62
S (%)	0.01	0.26	0.17	0.05	29.29
B (mg kg−1)	25.66	115.49	72.32	21.91	30.30

* Min., Minimum; Max., Maximum; SD, Standard deviation; CV, Coefficient of variation.

). These variations warrant variable rate application of nutrients for precise management of required nutrients. The mean values of nutrient concentrations in leaves were 3.01 ± 0.79, 0.20 ± 0.04, 0.72 ± 0.19, 1.19 ± 0.18, 0.61 ± 0.13, 0.17 ± 0.05 and 72.32 ± 21.91 respectively for Nitrogen (N) (%), P (%), K (%), Ca (%), Mg (%), S (%), and B (mg kg⁻¹) respectively. The CV values of leaf nutrient concentrations varied from 15.04 % to 30.30%. The variations in the leaf nutrient concentrations in different plantations could be either due to management practices (nutrient management, water management, mulching, weed control) or inherent soil fertility conditions and also could be due to both. Behera et al. [9,22] also reported wide variations in leaf nutrient concentrations of oil palm in different states of India under different soil types. In West Godavari District of Andhra Pradesh, Behera et al. [9] reported that 65, 31, 35 and 8 percent leaf samples had less than optimum concentration of N, P, K and B respectively.

3.3. Optimum Leaf Nutrient Ranges

Optimum ranges of leaf nutrient concentrations were estimated from the nutrient estimations of high yielding populations as they indicate the levels of sufficiency for nutrient concentrations. As suggested by Bhargava [21], only these optimal ranges were chosen as diagnostic norms. DRIS norms estimated for Nitrogen (N), P, K, Mg and B from leaf samples of different oil palm plantations of Krishna District have been presented in the following table (

Table 4. Critical ranges of leaf nutrients.

Nutrient	Deficient	Low	Optimum	High	Excess
N (%)	<0.96	0.96–2.07	2.07–4.29	4.29–5.40	>5.40
P (%)	<0.06	0.06–0.13	0.13–0.27	0.27–0.34	>0.34
K (%)	<0.31	0.31–0.52	0.52–0.94	0.94–1.15	>1.15
Ca (%)	<0.73	0.73–0.94	0.94–1.36	1.36–1.57	>1.57
S (%)	<0.028	0.028–0.098	0.098–0.238	0.238–0.308	>0.308
B (mg kg−1)	<16.10	16.10–44.97	44.97–102.70	102.70–131.57	>131.57
Mg (%)	<0.28	0.28–0.44	0.44–0.76	0.76–0.92	>0.92

). In general, optimum range for a nutrient indicates that the growth, yield, and quality of the palms are satisfactory at that range. It is also assumed that changes in the nutrient concentration in any specified plant part or region do not change growth or production. In the present study, the optimum ranges of N, P, K, Mg, and B concentrations in oil palm leaf were in the range of 2.07% to 4.29%, 0.13% to 0.27%, 0.52 to 0.94%, 0.44 to 0.76%, and 44.97 to 102.70 mg kg⁻¹, respectively. Considering these optimum ranges, 15%, 6%, 16%, 9%, and 12% leaf samples had less than optimum concentration of N, P, K, Mg, and B, respectively. In Mizoram, optimum leaf nutrient ranges were reported as 1.91 to 2.95%, 0.46 to 0.65%, 0.63 to 1.00%, 0.48 to 0.88% and 9.41 to 31.0 mg kg⁻¹ for N, P, K, Mg and B respectively [22]. Similarly in Gujarat, the optimum leaf nutrient ranges varied between 2.63 to 2.85%, 0.16 to 0.18%, 0.56 to 0.88%, 0.34 to 0.84% and 9.10 to 32.5 mg/kg for N, P, K, Mg and B respectively [9].

3.4. DRIS Norms and DRIS Indices

DRIS norms were estimated as per the procedure given by Beaufils [5], and those norms were employed to arrive at the DRIS indices. The DRIS indices for Krishna District had been −4.615, 3.32, 0.86, 3.915 and −3.485 for Nitrogen (N), P, K, Mg and B respectively and Nitrogen (N) > B > K > P > Mg is the order of importance of nutrients in Krishna District (Figure 2).

The order of importance of nutrients in a region specifies the order of requirement of nutrients in that region. In the Krishna basin, Nitrogen was found to be the most limiting nutrient element followed by B, K, P and Mg. Therefore, priority needs to be given to the management of N followed by B, K, P, and Mg for achieving optimal yields in oil palm. On the basis of DRIS derived optimum ranges, 32%, 9%, 27%, 12%, and 12% of leaf samples had less than optimum concentration of N, P, K, Mg, and B, respectively. Behera et al. [22] reported that the order of requirement of nutrients is B > K > Mg > P > Nitrogen (N) for Mizoram state to grow oil palm. Optimum leaf nutrient ranges as per DRIS norms varied from 1.91% to 2.95%, 0.46% to 0.65%, 0.63% to 1.00%, 0.48% to 0.88%, and 9.41 to 31.0 mg kg¡1 for N, P, K, Mg, and B, respectively.

4. Discussion

4.1. Order of Importance of Nutrients

The present nutrient recommendations for oil palm in India are based mainly on uptake studies conducted at ICAR-IIOPR and no data is available from any factorial experiments. Simple extrapolation of recommendations cannot be done with the existing knowledge. Hence, the existing plantations of Krishna District which cover nearly 30,000 ha of area were surveyed to precisely formulate the nutrient recommendations. Our results indicated larger variation in soil fertility status of existing oil palm plantations in Krishna District of Andhra Pradesh, and the order of importance of nutrients is Nitrogen (N) > B > K > P > Mg. Behera et al. [9,10,22] reported that the order of importance of nutrients as per DRIS norms for Mizoram as B > K > Mg > P > Nitrogen (N), for Gujarat as K > nitrogen (N) > B > P > Mg and for Goa as P > Mg > K > nitrogen (N) > B. This indicates that there are wider variations in the order of importance of nutrients in different locations of the country. Oil palm produces very high dry matter per annum and so requires large quantities of nutrients. Effective nutrient management contributes nearly 50 per cent of FFB production [23]. Nutrient demand being crop specific, soil supplies the majority of nutrients and forms the base for interactions too. Therefore, estimation of variability in soil fertility status provides a clue for nutrient management especially in perennial crops like oil palm. This ensures proper supply of nutrients to achieve higher and sustainable yields.

The surveyed plantations exhibited large variation in their nutrient reserves as well as in leaf nutrient contents. The reasons for this variation could either be intrinsic or extrinsic. Adoption of varied land management practices and application of differential rates of nutrients could be possible reasons for this spatial variability of soil properties in oil palm plantations of India [24]. This wider variability warranted development of DRIS norms which considered the nutrient ratios rather than individual nutrients. In the present area under survey, Nitrogen (N), K and B concentrations showed more deficient palms in comparison with P and Mg. Accordingly, the order of importance also was Nitrogen (N) > B > K > P > Mg. The DRIS derived optimal concentrations act as guiding standards for deriving nutrient recommendations. In the case of oil palm, it is observed that even fertile soils can not endure continuous production of fruit bunches without fertilizer application. Plant-nutrient-climate-soil interactions make the nutrient recommendations more complex. Oil palm is a long duration crop (25 to 30 years) and requires a continuous supply of nutrients. Because, in this crop, it takes nearly 42 months from floral initiation to fruit development, if there is any stress during this period it is reflected only after 20 to 30 months [25]. The most critical periods for stress in fruit development are: 1. Sex determination (8–20 months after leaf initiation); 2. Inflorescence abortion (28-32 months); and 3. Bunch failure (36–38 months) [23]. For producing 2.5 t of oil ha⁻¹ year⁻¹, oil palm requires 162, 30, 217, 36 and 38 kg of N, P, K, Ca and Mg respectively [26]. Therefore, precise estimations are required while deciding the nutrient recommendations. To validate the DRIS derived optimal ranges of leaf nutrients in oil palm, an on-farm evaluation was carried out in farmers’ plantations of West Godavari District for 6 years (2013 to 2019) and found that the results are highly impressive [27].

Besides water, nutrition plays a major role in plant stress and proper nutrition results in healthy, vigorous and stress tolerant plants [28]. Davidson [29] reported that in Malaysia, the yield improvement from 1951–1991 was mostly influenced by improved genotype (51.3%), nutrition management (29.5%), improved milling (9.7%) and other management practices (9.5%). Nutritional status of the plant indicates its health and the intrinsic factors present in the soil influence both native and applied nutrients. Furthermore, the interactions between nutrients at the site of exchange (e.g.; NH4⁺ and K⁺, Ca²⁺ and Mg²⁺) influences the uptake of one nutrient by the concentration of another.

4.2. Comparison with DRIS Indices of West Godavari District

West Godavari is the district adjacent to Krishna. A comparison has been made between the DRIS indices and optimal leaf nutrient concentrations of these two districts (Figure 2). Behera et al. [30] developed the DRIS indices for West Godavari Districts. In West Godavari, the order of importance of nutrients is B > Mg > K > N > P. The optimum concentrations of leaf nutrients were 1.57–2.63% for N, 0.08–0.16% for P, 0.48–0.88% for K, 22.6–60.2 mg kg^–1 for B and 0.25–0.71% for Mg. In the case of Krishna, it was Nitrogen (N) > B > K > P > Mg. Optimum leaf nutrient ranges varied from 2.07–4.29%, 0.13–0.27%, 0.52–0.94%, 44.97–102.70 mg/kg and 0.44–0.76% for N, P, K, B and Mg respectively.

Fairhurst and Hardter [31] reported that for oil palm, N/P, N/K, K/Mg, K/B ratios are more important. Therefore, the survey and analysis conducted in the present study are helpful in developing DRIS norms and identifying the most limiting nutrients in oil palm areas of Krishna District in Andhra Pradesh State. Hence location specific survey is very much essential as the gap between site yield potential and actual yield need to be lessened. In India, Andhra Pradesh is the leading grower of oil palm crop and within the state, West Godavari, East Godavari and Krishna Districts are the highest producers of oil palm. As field experiments are highly expensive, time consuming and not feasible, DRIS norms developed in the present study are highly useful in designing nutrient recommendations.

5. Conclusions

In perennial crops like oil palm which have a long growth cycle with wider spacing, factorial field trials are difficult to conduct in different locations for generating location specific information. For nutrient management, DRIS is an effective potential tool. DRIS has the additional advantage of establishing the ranking of nutrients according to deficiency or excess concentrations, and it considers the ratios of the nutrients rather than individual nutrients. From the soil analysis it is found that the soil properties of oil palm plantations of Krishna District varied widely. Leaf tissue analysis of oil palm plantations enabled development of optimal nutrient ranges by the DRIS approach. DRIS norms and DRIS indices generated were of great help in designing efficient nutrient management schedules for the surveyed area., A positive index of DRIS represents adequate and exceeding levels of the nutrient under consideration, whereas a negative DRIS index points out below a sufficiency level; thus, the nutrient requirement can be ordered comparative to one another. On the basis of DRIS-derived optimum ranges, oil palm plantations of Krishna basin consisted of 15%, 6%, 16%, 9%, and 12% of populations with less than optimum concentration for nitrogen (N), P, K, Mg, and B, respectively. Based on the indices obtained for Krishna District of Andhra Pradesh, the order of importance of nutrients has been placed as N > B > K > P > Mg. This indicates the emphasis that is required on N and B nutrients in oil palm plantations of Krishna District. Large variations were also noticed in DRIS indices and critical leaf nutrient concentrations in adjacent districts (Krishna and West Godavari) which warrant development of location specific standards for precise nutrient management in oil palm plantations. We opine that the optimal leaf nutrient concentrations act as guiding principles for precise and balanced nutrient management in oil palm.