INFLUENCE OF DRIP IRRIGATION ON GROWTH, NUT CHARACTERS AND YIELD OF COCONUT IN LITTORAL SANDY SOIL

A field experiment was conducted at Central Plantation Crops Research Institute, Kasaragod, India for six years (1993-1999) to study the impact of drip irrigation levels and mulching on coconut in littoral sandy soil. The treatments consisted of three levels of drip irrigation (66, 100, and 133 % of open pan evaporation (E0)) along with basin irrigation (100 % of E0) and rainfed control as main plot treatments and mulching with coconut leaves and no mulching as sub plot treatments. The experimental results revealed that annual leaf production and leaf nutrient status of coconut was better in the irrigated treatments compared to the rainfed control. The drip irrigation at 66 per cent of E0 (27 litres of water per palm per day during December-January months and 32 litres of water per palm per day during February-May months) resulted in water saving and the nut yield was on par with 100 per cent and 133 per cent of E0 through drip and 100 per cent of E0 through basin irrigation and differed significantly compared to rainfed control. The nut characters like nut weight, husk weight, copra thickness, copra content and copra yield was 1 Corresponding Author: Senior Scientist (Agronomy), CPCRI (RS), Kayangulam – 690 533, Kerala, India. Email: maheshcpcri@yahoo.com


INTRODUCTION
Coconut (Cocos nucifera L.) is a high value commercial crop grown in 92 countries with a total area coverage of 11.91 million ha producing 54130 million nuts annually during the year 1999. India, Indonesia, Philippines and Sri Lanka are the four major global players, which together contribute 78 per cent of the world production. With an area of 1.91 million ha, India's share to the global coconut area is 16 per cent. India is now the leading coconut producing country in the world with a production of 15000 million nuts and percentage share of 27.6 followed by Indonesia and Philippines. In productivity too India ranks in the forefront with a productivity of 7821 nuts per ha whereas the world productivity is as low as 4294 nuts per ha (Rethinam, 2001).
In India, Andhra Pradesh stands in the forefront with the productivity of 19575 nuts per ha while in Kerala it is as low as 6188 nuts per ha (Rethinam, 2001) mainly because of the fact that, it is being grown as rain dependent crop and prevalence of root (wilt) disease. Though Kerala falls under heavy rainfall zone the variability of rainfall coupled with inadequate irrigation resources and poor water management results in mild to severe stress on coconut palms between the months of December and May resulting in lower productivity. Coconut grown in drought-prone areas is often subjected to periodic moisture deficit during the dry season (Carr, 1992).
The littoral sandy soil, which occurs along the coastal length of Kerala, is the second largest soil type where coconut is predominantly grown. These coastal sandy soils are poor in fertility and water holding capacity, get heated up quickly and the heat remains during the full day time affecting water and nutrient uptake. Coconut palms which are grown in these littoral sandy soils experience stress during summer which makes the palms to yield much below their potential. Hence, it is imperative to provide irrigation during the non rainy season. Unlike other crops, coconut produces flower primordia round the year and hence adequate moisture should be available in the soil throughout the year. As the sandy soils contain mainly sand particles (99.1%), their infiltration rate is very high and hence the system of irrigation must be chosen very carefully. An ideal irrigation system will be the one in which water is supplied at the same rate at which it is absorbed by the palms and drip irrigation fits well to these conditions. The importance of irrigating coconut for a sustained yield has been emphasized (Abeywardena, 1971, Varadan and Madhava Chandran, 1991, Keller et al.,1992and Dhanapal et al., 2000b. Among the irrigation systems, drip irrigation is gaining importance as it maintains the soil moisture availability and air balance in the root zone of coconut near field capacity throughout the dry season and saves irrigation water (Vidhana Arachchi, 1998). Dorota and Forrest (1996) reported that drip irrigation wets only a limited portion of the potential soil-root volume which would be adequate for most plants to perform well along with minimum evaporation loss of water. The efficacy of irrigation can be increased by mulching the irrigated area and its favourable effect on soil temperature regulation (Varadan and Rao, 1983), soil moisture conservation and soil temperature reduction (Maheswarappa et al., 1998) has been established.
Keeping in view the above facts, a field trial was initiated at Central Plantation Crops Research Institute, Kasaragod, Kerala (India) in littoral sandy soil with the objective to study the influence of drip irrigation levels combined with mulching with coconut leaves on growth, nut characters and yield of West Coast Tall coconut variety.

Experimental site
The experiment was conducted at Central Plantation Crops Research Institute (CPCRI), Kasaragod which is situated at 12° 30` N latitude and 75º 00` E longitude at an elevation of 10.7 m above mean sea level.
In a normal year the experimental area receives an annual rainfall for about five months from June to October and one or two summer showers. The average rainfall received in the area is 3400 mm, out of which, 86 per cent is received during the four monsoon months (June-September) and the period from December to middle May remains rainless. The maximum temperature ranges between 28.8ºC and 33.1ºC and minimum temperature varies between 19.4ºC and 24.4ºC. The relative humidity ranges between 81 per cent and 94 per cent and the maximum open pan evaporation is recorded during the months of March to May (5.0 to 5.3 mm per day).
The soil of the experimental field was classified as littoral sand (Quartzipsamments) with a mechanical composition of 95.8 per cent coarse sand, 3.3 per cent fine sand, 0.2 per cent silt and 0.7 per cent clay. The field capacity of the soil was 4.2 per cent and wilting point was 0.44 per cent with a bulk density of 1.66 kg m -3 .

Experimental details
The experiment was conducted during the non rainy seasons (December to May) of 1993 to 1999, in a coconut garden planted in 1972 with West Coast Tall variety. The split plot design involving irrigation treatments in the main plot and mulching treatments in subplots replicated four times was adopted for the study. The number of palms per main plot treatment were four and for sub-plot treatment there were two palms. The unit plot was laid out in rows and guard rows between treatments were maintained.

Treatment details
Main plot treatments  Table 1. The coconut palms were planted with the spacing of 7.5 m X 7.5 m and supplied with 500:320:1200 g N, P 2 O 5 and K 2 O per palm per year in the form of urea (46 % N), mussoorie phos (20 % P 2 O 5 ) and muriate of potash (60 % K 2 O) applied in two splits, 1/3 rd during April-May and 2/3 rd during September-October. The drip irrigation system consisted of an overhead water tank and the outlet was connected with water filter along with main pipe line. From the main pipeline, the laterals of 16 mm OD LDPE (Low density poly ethylene) pipes of convenient length were laid with end cap. At the base of each palm six emitters were placed one meter away from the bole at equidistance with the help of 4 mm LDPE microtubes. The water from the emitters was allowed to drip at the rate of 2 litres per hour up to the 30 cm depth by putting the emitters in 30 cm 3 pits with the help of conduit pipe.

Sub-plot treatments
The annual leaf production per palm was recorded during the experimental period every year. Coconut leaf samples were collected from the index leaf (14 th leaf) during 1999 and analyzed for N, P and K content by adopting standard procedures (Jackson, 1973). Nut yield from each palm was recorded separately during each harvest every year. Nut characters were studied from representative samples during three seasons of 1998 and 1999 and average was worked out.
The data recorded on various characters were subjected to Fisher's method of analysis of variance and interpretation of data was done as per the procedure given by Gomez and Gomez (1984).

Growth characters
The mean annual leaf production ranged from 8 to 12 leaves per palm among the different treatments ( Table 2). The irrigated treatments irrespective of the method and quantity of water applied, produced similar results (11.5 to 12 leaves) which varied significantly from the unirrigated control (8 leaves). Irrigating a palm results in production of more number of leaves which may be attributed to adequate supply of water and inturn, nutrients. Coconut palm in general produces one inflorescence/bunch in each leaf axils and thus higher leaf production will directly contribute towards increased nut yield. Abeywardena (1979) and Venkitaswamy et al. (1997) also reported increase in leaf production under irrigated condition.

Leaf nutrient status
The leaf analysis for the major nutrients viz., N, P and K indicated that N and K contents were influenced by the irrigation treatments, but P content was unaffected (Table 2). N and K contents were found to be significantly higher in the irrigated treatments compared to rainfed control. This clearly indicates that water is the key factor which affects nutrient uptake in littoral sandy soils. Nitrogen and potassium contents in 14 th leaf were found to be significantly higher in irrigated treatments. Roots intercept more nutrient ions when grown in soil with adequate moisture than in dry soil because root growth is more extensive. Mass flow of soil water to supply the transpiration stream, transports most of the nitrates to the roots (Tisdale et al., 1985). Higher uptake of K under adequate soil moisture condition was due to the increased solubility and better availability of the nutrient. Under rainfed condition, the nutrient uptake was significantly lower and this might be due to the fact that cells of the absorption zone of coconut roots become inactive by suberization and dehydration during dry weather, thus affecting the nutrient and water absorption process (Vidhana Arachchi, 1996). Better uptake of nutrients under irrigated condition might be because of more number of main and fine roots development compared to rainfed control (Dhanapal et al., 2000a). Higher root activity in olive trees under drip irrigation also has been reported by Fernandez et al. (1991). In young arecanut palm also, higher number of main roots and feeder roots under drip irrigation method has been reported (Sujatha and Abdul Haris, 2000).

Nut Yield
Pooled data on nut yield for six years (1993)(1994)(1995)(1996)(1997)(1998)(1999) indicated that there was no significant difference among different levels of drip irrigation and between the drip and basin irrigation (Table 3). Nut yield under irrigated treatments (66 to 72 nuts palm -1 year -1 ) were on par with each other but were significantly superior to the rainfed control (27.8 nuts palm -1 year -1 ). Influence of irrigation on nut yield could be seen from second year after imposing treatments and nut yield starts stabilized from fourth year onwards (Fig. 1). During first two years the yields were on par among the treatments and the yields under rainfed palms started declining drastically from second year onwards of treatment imposition.
Nut yield under irrigated treatments were significantly higher compared to rainfed control, clearly indicating that there was good response for irrigation in littoral sandy soil. However, the response for the higher quantity of water applied was less conspicuous, the soil being porous and poor in organic matter. Therefore, the treatment where 66 per cent of E 0 was applied through drip could produce the same effect as that of 100 and 133 per cent E 0 through drip and 100 per cent of E 0 through hose irrigation in coconut basin. Increase in nut yield was mainly attributed to production of more leaves and better uptake of nutrients under irrigated treatments. Increase in growth characters and better availability of moisture resulted in increase in photosynthetic rate, stomatal conductance and transpiration rate under irrigated palms (Anon., 1997). Rajagopal et al. (1989) also reported greater stomatal resistance and epicuticular wax content and reduced transpiration rate, leaf water potential and reproductive dry matter under severely moisture stressed palms compared to well watered palms. According to Mahindapala (1987) in the dry zone of Sri Lanka, coconut requires 25 to 30 litres of water per day through drip method. Kulandaivelu (1990) has reported that, under Trichy condition of Tamil Nadu (India), water requirement for coconut palm through drip irrigation ranged from 55 litres per day in December months to 115 litres per day in June months. It was also reported that yield of nuts under drip method at 30 and 45 litres day -1 palm -1 was on par with basin irrigation at 600 litre palm -1 week -1 (Varadan and Madhava Chandran, 1991) besides stabilised yield with minimum fluctuation under adequate irrigation (Jose Mathew et al., 1996). Sivanappan and Kottiswaran (1985) reported that the quantity of water to be irrigated for coconut palms by drip irrigation varies from 0.005 to 0.07 m 3 palm -1 day -1 . Water saving in the 66 per cent of E 0 through drip treatment was due to the reason that the water was applied at reduced quantity and thus the deep percolation loss was avoided. Though more water was applied under 100 and 133 per cent E 0 under drip and basin irrigation, it did not contribute towards higher yield, probably because the excess water might have moved beyond the root zone and was not used by the palms. Maheswarappa et al. (1997), while studying the moisture movement under littoral sandy soil, have reported that the vertical movement of water crossed the active root zone of 120 cm on 3 rd and 4 th day after irrigation at 4 litres/h and 2 litres/h discharge rate, respectively. Subramanian et al. (1997) and Kapadiyal et al. (1998) also reported 40-50 per cent saving of water through drip irrigation system compared with surface irrigation. With the adoption of drip irrigation system there could be saving of labour requirement and thus economically it results in higher benefit cost ratio (Dhanapal et al., 2000b). Coastal sandy soils have very low water holding capacity, hence the supply of water according to the daily requirement of the palms through drip system may prove ideal, since it maintains a uniform moisture level throughout the period and economizes the quantity of irrigation water compared with other surface irrigation methods.

Nut characters
The nut characters studied in irrigated palms were significantly higher compared to rainfed control (Table 4). The nut weight recorded under rainfed control was lesser by 49 per cent compared to basin irrigated palms. The copra thickness and copra content were significantly lower under rainfed control compared to irrigated palms which finally resulted in lower copra yield. The copra yield was 399 kg per ha under rainfed condition, where as it was 2087 to 2202 kg per ha under irrigated treatments. It is evident from the table 4 that the nut weight, copra thickness, copra content was significantly higher in irrigated treatments. The copra yield was 499 kg per ha under rainfed condition, where as it was 2087 to 2202 kg per ha under irrigated treatments. Increase in nut characters under irrigated treatments was due to better growth, nutrient uptake by the palms. Abeywardena (1979) also reported increase in nut size and copra yield due to irrigation.

INFLUENCE OF SOIL MULCH ON GROWTH AND YIELD
The dried coconut leaves which are available in the garden could be used for mulching purpose in coconut garden. The mulching treatment did not bring about any significant difference in annual leaf production, leaf nutrient content and nut characters. However, application of dried coconut leaves as mulch resulted in a significant improvement in yield at the rate of 14 nuts palm -1 year -1 over the 'no mulch' treatment (Table 3). The data on the interaction (Table 3) reveal that there was no yield improvement with mulching compared to no mulching in unirrigated treatment indicating that unless there was enough moisture in the soil, the effect of mulch was negligible on nut production. The yield increase over control in the irrigated treatments under 'no mulch' was only 76 to 121 per cent while in the mulched treatments the difference was 167 to 230 per cent (Fig. 2). This clearly shows the advantages of mulching with irrigation in increasing the nut yield.
Mulching with dried coconut leaves was found to have positive response to yield parameter, though growth characters were not significantly affected. Uthaiah et al. (1993) have reported that the growth of coconut seedlings was better under drip irrigation coupled with coir pith mulching treatment. It is a proven fact that mulching influences soil moisture and soil temperature to a greater extent. Under "irrigation with mulch" conditions, soil moisture availability increases and soil temperature reduces Rao, 1983 andMaheswarappa et al., 1998) which are the two most important physical factors which influence various processes such as evapotranspiration, growth, development and biological activity (Lal, 1974). However, mulching under rainfed condition did not produce any significant impact because the sandy soils have low water holding capacity and perhaps could not retain the moisture during non-rainy season.

CONCLUSIONS
The investigations have clearly shown the importance of irrigation and mulching in coconut cultivation in littoral sandy soil under humid tropical conditions. Irrigation through drip method at all the rates result in better nut yield and copra yield compared to rainfed control treatment, the most economic one being drip irrigation at 66 per cent of E 0 in terms of water saving with six emitters placed at equidistance (27 litres of water per palm per day during December-January months and 32 litres of water per palm per day during February-May months). Mulching with coconut leaves also resulted in better growth, increase in nut yield and copra yield showing a positive interaction with irrigation.

ACKNOWLEDGEMENTS.
Keller, J., Sivanappan, A. K. & Varadan, K. M. 1992 Table 4. Influence of irrigation and mulching on nut characters of coconut in littoral sandy soil (Pooled data of two years).