CARIBBEAN FOOD CROPS SOCIETY. Thirty First. Annual Meeting Barbados. Vol.XXXI

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1 CARIBBEAN FOOD CROPS SOCIETY 31 Thirty First Annual Meeting 1995 Barbados Vol.XXXI

2 WATER USE AND YIELD OF BASIL AS INFLUENCED BY DRIP IRRIGATION LEVELS AND MULCHING Manuel C. Palada, Stafford MA. Crossman and Jacqueline A. Kowalski Agricultural Experiment Station, University of the Virgin Islands St Croix, US Virgin islands ABSTRACT Basil (Ocimum basilicum L.) is one of the popular herbs with culinary importance in the Virgin Islands. Local herb growers traditionally water their crops with sprinkler cans and garden hoses. This method results in inefficient use of water, a scarce resource in the Virgin Islands. A field experiment was conducted in 1994 to determine the minimum requirement and efficient use of irrigation water for basil. Basil was grown in plots with and without mulch and drip-irrigated at various levels to maintain soil moisture tensions of 20,40 and 60kPa. A black fabric weed barrier ground cover was used in mulched plots. A rainfed plot without mulch was included as a control. Total fresh basil yield was highest (42.3 t/ha) in plots with no mulch and irrigated to maintain soil moisture at 40 kpa. Yield from this treatment was significantly different from other treatments except treatments irrigated at a soil moisture of 20 kpa with and without mulch. Similar results were obtained with leaf fresh yield. Yields from all drip-irrigated plots were significantly higher than the control. Total plant dry matter yield (5.79 t/ha) was highest in the treatment irrigated at 40 kpa without mulch, but was not significantly different from other treatments except the control. Water use was highest in the treatment irrigated at a soil moisture of 20 kpa with no mulch and lowest in the mulched plots irrigated at a soil moisture of 60 kpa. Mulching reduced water use of basil by 50%. Water use efficiency was highest when basil was irrigated to a soil moisture of 60 kpa combined with mulching. For this treatment water cost to produce a kilogram of fresh basil was US$1.97 compared with $5.76 when basil was irrigated at soil moisture of 20 kpa without mulch. This study shows that basil can be grown with minimum irrigation water equivalent to soil moisture of 60 kpa with mulch. At this level, water use is reduced and efficiency is improved. INTRODUCTION Basil (Ocimum basilicum L.) is one of the major culinary herbs with economic importance in the Virgin Islands. It is an annual herb that can be easily grown from seed every year. Basil ranks as one of the top selling fresh herbs in the local market with a retail price ranging from US$5 to $8/kg. Herb growers can therefore make substantial returns from a small land area. Although most of the locally produced basil is sold in local markets, there is potential for an export market since most of the herbs consumed in the mainland US are imported. 143

3 Technical information for growing herbs in the Virgin Islands is inadequate and it is one of the major reasons why few growers are engaged in basil production. More research should be undertaken to improve field production of basil because of its economic importance. Although basil can be easily grown in the tropical climate of the Virgin Islands, field production is constrained by factors related to environment and management practices. Limited water supply for crop production is a major physical constraint. Without supplemental irrigation, commercial production of high value crops such as basil is not feasible. Even with limited water availability, traditional practices of growing herbs are inefficient and do not conserve water use. Information on improved crop management practices for culinary herb production is very limited. Literature on planting methods, fertilizer application, water requirement, weed, insect and disease control is scanty. There is very little information on the minimum water requirement of herbs, although soil moisture availability has a strong influence on plant growth and yield performance. Research with varying rates of drip irrigation showed increases in growth and total yield of essential oil from herbs with increasing rates of water application (Simon, 1987). A similar study conducted one year later during a season with high rainfall showed no benefit from drip irrigation. The interaction of soil moisture and soil fertility affects the accumulation of plant products, indirectly influencing plant growth in aromatic and medicinal plants (Bernath, 1986; Franz, I983;Penke, 1978). Experiments comparing sprinklers with drip irrigation on herbs at the University of the Virgin Islands-Agricultural Experiment Station demonstrated a more efficient water use and increased yields under drip irrigation (Collingwood et al., 1991). Integration of mulch with drip irrigation resulted in additional increases in water use efficiency and yield of basil (Palada et al., 1992). The minimum water requirement for growing various herbs needs to be established. Studies on determining irrigation water requirements of herbs are important in the Virgin Islands where water is usually a costly input in crop production. This experiment was conducted to determine the minimum water requirement of basil under field conditions and to study the effects of mulching on the yield and economic returns of basil under drip irrigation. MATERIALS AND METHODS The experiment was conducted at the Agricultural Experiment Station, University of the Virgin Islands in St Croix (Lat 'N and Long. 64 n 48'W). The soil is Fredensborg loamy, fine carbonatic, isohyperthermic, shallow, typic calciustolls (Lugo-Lopez and Rivera, 1980). The average annual rainfall is 1,016 mm, but évapotranspiration exceeds rainfall 10 months of the year resulting in a negative water balance. The experiment was established using a randomized complete block design with four replications. The treatments consisted of three irrigation levels corresponding to soil moisture tensions of 20,40 and 60 kpa. Half of the treatments were mulched using a black fabric, water-permeable weed barrier ground 144

4 cover. A rainfed control (no mulch) was included as a treatment. The drip irrigation system consisted of main and submain lines made of 1.27-cm polyhose and 15-mm Hardie tape with laser drilled orifices 20 cm apart as laterals. Basil seedlings were grown in the greenhouse from 20 March to 24 April Seedlings were transplanted on 25 April in three-row plots. Each plot measured 1.0x2.1 m. Plants were spaced 30 cm between rows and 30 cm within rows. One week after planting, plants were fertilized with granular forms of Ν, Ρ and Κ at the rate of 50,100 and 100 kg/ha, respectively. The plots with no mulch were hand-weeded regularly to eliminate weed competition. Soil tensiometers (IiTometer Co., Riverside, CA) were installed at 15 cm depth in each treatment on two replications to monitor soil moisture tension. The tensiometers were read daily and readings were used to turn on the irrigation water when soil moisture tension exceeded the specified level. A flow-meter and a timer were installed in the irrigation system for each treatment. Water use was determined from weekly flow-meter readings. Total irrigation water use was determined over a period of 8 wk. Data on rainfall and pan evaporation were collected from a USDA weather station on land adjacent to the experiment station. The plants (excluding roots) were harvested five times on 6 June, 13 July, 17 August, 22 September and 7 November. Yield samples were taken from 2-m long sections of the middle rows. Data were collected on plant height, plant fresh and dry weights and leaf fresh and dry weights Statistical analysis of data was performed using analysis of variance by a SAS Program (Statistical Analysis System). Differences among treatment means were separated using the Duncan's Multiple Range Test. RESULTS AND DISCUSSION Rainfall and évapotranspiration. As shown in Figure 1, total rainfall in 1994 was about 50% below normal (633 mm). This rainfall was one of the lowest and resulted in one of the driest years in St Croix. The island experienced a severe drought. Evapotranspiration exceeded rainfall in all months for which ET data were available. Total rainfall received during the growing period for the trial (25 April to 7 November ) was only 374 mm. Most of the rainfall occurred in August to November. Because of low rainfall during the growing season, drip irrigation was frequently applied to all irrigated treatments. Irrigation and water use Total water use increased as soil moisture tension decreased (Table 1). For each irrigation regime water use in plots with no mulch was greater than plots with mulch. Highest water use (65.4 L/plot) was observed in plots under an irrigation regime of 20 kpa with no mulch. Except for the rainfed treatment, the lowest water use (17.6 L/plot) was recorded in 145

5 plots with an irrigation regime of 60 kpa with mulch (Table 1 ). Plants in this treatment also had the most efficient use of irrigation water. At irrigation levels set at 60kPa under mulch, water use was only 467L (WUE) to produce a kilogram of dry basil (Table 1). To produce the same yield of basil, water use was 1,362 L under an irrigation level set at 20 kpa without mulch. In terms of water cost efficiency (WCE), it cost only US$1.97 to produce a kilogram of dry basil under an irrigation regime of 60 kpa with mulch compared with $5.76 at 20 kpa without mulch (Table 1). These data would suggest that even under conditions of rainfall shortage, basil can be efficiently produced with minimum irrigation water set at 60 kpa coupled with mulching. Table 1 Estimated irrigation water use and efficiency of basil at three levels of irrigation with and without mulch. UVI/AES, 1994 Irrigation Mulch Total water Total water WUE 1 WCE 2 level (kpa) use (L/plot) use (m-vba) (LTkg d.wt.) (US$/kg) 20 No , No , No , Yes , Yes , Yes , Rainfed No , WUE=Water use efficiency in liters per kg dry wt produced; 2 WCE=Water cost efficiency = cost of water ($) to produce a kg dry wt; computed based on 1 mm rainfall = 4,047 L Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec M o n t h Figure 1 Rainfall and pan evaporation at UVI/AES,

6 Fresh and diy matter yield Both fresh and diy matter yields of basil were significantly affected by levels of drip irrigation (Table 2). Highest fresh basil yield of 42.t/ha was obtained from irrigation level of 40 kpa without mulch. This yield however, was not significantly greater than treatments under an irrigation regime of 20 kpa with and without mulch. Under mulch, highest yields were observed in treatments with an irrigation regime of 20 kpa, however, this yield was not significantly higher than treatments with 40 and 60 kpa, All drip-irrigated treatments significantly out yielded the control (rainfed). Basil dry matter yield was not significantly affected by drip irrigation level or mulching (Table 2). However, treatments with irrigation levels of 20 and 40 kpa under mulched had a significantly greater dry matter yield than the control (rainfed). Highest dry matter yield (5.79 t/ha) was obtained from plots irrigated at 40 kpa without mulch (Table 2), Based on fresh basil yield, the data would indicate that without mulch, optimum drip irrigation level is at a soil moisture regime maintained at 40 kpa. With mulch, the minimum water requirement for optimum yield is at irrigation level that maintains soil moisture tension at 60 kpa. Table 2 Plant height, total plant fresh and dry matter yield of basil grown at three levels of irrigation with and without mulch Irrigation Mulch Plant height Fresh yield Dry yield level (kpa) (cm) (t/ha) (t/ha) 20 No 44.0 ab 36.3 b 4.86 ab 40 No 46.8 a 42.3 a 5.79 a 60 No 39.3 be 27.2 b 3.98 abc 20 Yes 37.4 cd 33.3 ab 4.26 abc 40 Yes 36.9 cd 28.6 b 3.77 abc 60 Yes 37,4 cd 26.1 b 3.76 abc Rainfed No 34.1 d 14.5 c 2.68 c Averge plant height and total yield from 5 harvests Means within each column followed by common letters are not significantly different by Duncan's Multiple Range Test, P=0.05 Economic returns and water use efficiency The gross return was highest from no-mulch basil under an irrigation regime of 40 kpa followed by treatment under 20 kpa (Table 3). However, irrigation water costs were higher from these treatments compared with mulched treatments. This resulted in lower returns to irrigation water (Table 3). The highest return to irrigation water was obtained from mulched 147

7 plots under an irrigation regime of 60 kpa. For this treatment every dollar spent in irrigation water resulted in economic return of $28.13, This treatment would therefore give the most efficient and profitable use of supplemental irrigation water. Table 3 Yield and economic returns from basil grown at three levels of irrigation with and without mulch Irrigation Mulch Total water Total water WUE 1 WCE 2 level (kpa) use (L/plot) use (m 3 /ha) (L/kg d.wt.) (US$/kg) 20 No 36, ,192 28, No 42, ,096 16, No 27, ,880 12,060 18,07 20 Yes 33, ,232 14, Yes 28, ,888 9, Yes 26, ,032 7, Rainfed No 14, , Fresh basil at retail price of US$8.00/kg; irrigation water cost at $4.23/m 3 SUMMARY AND CONCLUSIONS This study indicated that the optimum irrigation rate for basil in the Virgin Islands is at soil moisture tension maintained at 40 kpa with no mulch. With mulch, the minimum irrigation level is reduced to a soil moisture regime maintained at 60 kpa. Under low rainfall, mulching is beneficial for basil production by reducing total water use and increasing water use efficiency. ACKNOWLEDGEMENTS This research was supported in part by Hatch Regional Project No. S-247 and Hatch Project No

8 REFERENCES Bemath, J Production ecology of secondary plant products. In: Craker, L.E. and Simon, J (eds.) Herbs, spices and medicinal plants: recent advances in botany, horticulture and pharmacology. New York, USA: Onyx Press pp Collingwood, C.D., Crossman, S.M.A, and Navarro, A.A Response of selected herbs to improved production practices. Proc. 26th Annual Meeting of the Caribbean Food Crops Society. Roseau, Dominica. Franz, C Nutrient and water requirement for medicinal and aromatic plants. Acta Hort. 132: Lugo-Lopez, M.A. and Rivera, L.H Updated taxonomic classification of the soil of the U.S. Virgin Islands. J. Agric. Univ. Puerto Rico 64: Palada, M.C., S.M.A. Crossman and Collingwood, C.D Effect of organic and synthetic mulches on yield of basil under drip irrigation, HortScience 27:99 (abst.). Penke, M Influence of irrigation on the contents of effective substances in medicinal plants. Acta. Hort. 73: Simon, J.E Developing herbs as cash crops in the United States. Proc. of the First National Herb Growing and Marketing Conference. West Lafayette, Indiana, USA, July