Introduction

Strawberries are produced on 5000–6000 acres each year in Florida and the 1992-93 season had a crop value of $103 million (Freie and Pugh 1994). Although the major strawberry production area is in Hillsborough county, there are numerous other production areas throughout the state, notably Alachua, Bradford, Dade, Manatee, Palm Beach, and Polk counties (Hochmuth 1988).

Production costs for strawberries average $14,000 per acre; however, fertilizer accounts for only 2% of the total costs (Smith and Taylor 1993). Although fertilizer comprises only a small fraction of strawberry production costs, proper fertilization is important for maximizing yield and fruit quality, and for minimizing potential negative impact to the environment caused by leaching or runoff of excess fertilizer.

Fertilization Recommendations

Strawberry P and K fertilization recommendations are based on the calibrated Mehlich-1 soil test results and vary according to the soil-test level of P and K (Table 1). N requirements are 150 lb N per acre for the total season (200 days) (Hochmuth and Hanlon (in press)). Growers should avoid the temptation to apply more N because excess N can increase the amount of malformed fruits (Albregts and Howard 1982) and can lead to excessive vegetative plant growth and reduced yields. Rates of K higher than recommended reduced average strawberry fruit size and yield (Albregts et al. l991; Albregts et al. l994).

Drip Irrigation

Drip irrigation can result in up to 50% reduction in water use for strawberries, compared to overhead sprinklers (Myers and Locascio 1972). Nearly all strawberries in Florida now are irrigated with drip irrigation systems. Although drip irrigation can improve irrigation efficiency, care must be exercised to operate the system properly so optimum amounts of water are applied. Inadequate irrigation can reduce yields and over-irrigation can leach N and K. Irrigation amounts should be scheduled to meet the strawberry crop evapotranspiration needs which range from 800 gal per acre per day in October to 3000 gal per acre per day in April (Clark 1993).

Fertigation

Applying fertilizer through the drip irrigation system (fertigation) offers the potential for increasing efficiency of application of leachable nutrients such as N and K (Locascio and Myers l975; Locascio and Martin l985). With fertigation, N and K can be applied in small amounts corresponding to crop N and K requirements. Current recommendations call for incorporating all the P and micronutrients and up to 30 lb per acre each N and K₂O in the beds preplant. (Figure 1) The remaining N and K can be injected through the drip irrigation system.

N and K can be injected on a daily or weekly basis. Schedules for N and K injection were presented for strawberries (Hochmuth and Clark 1991); however, recent research has made it possible to reduce strawberry fertilization rates (Hochmuth et al. 1994; Albregts et al. 1994). Fertilization rates can be reduced from 1.0 lb N and K₂O each per acre per day. Revised fertigation recommendations are presented in Table 2.

Petiole Sap-Testing

Timely plant tissue analysis for N and K is important for optimizing N and K injection and for aid in making decisions regarding amounts of N and K to inject. Using research data from Dover AREC and commercial strawberry farms, extension scientists developed petiole fresh sap-testing procedures. The petiole sap sufficiency ranges for N and K are presented in Table 3. There are several sap-testing kits available for determining sap N and K concentrations. (Figure 2) The brochure describing the procedures available from the county Extension offices, is entitled "Guide for Plant Petiole Sap-Testing for Vegetable Crops."

Whole-Leaf Testing

Petiole sap analyses should be backed up by routine analyses of whole leaves by a tissue testing laboratory. (Figure 3) Presently, sap testing is limited to N and K because either test kits do not exist or procedures have not been developed for other nutrients. Sufficiency ranges for nutrients in whole strawberry leaves are presented in Table 4.

Checklist for Successful Strawberry Drip Fertigation

• Practice calibrated soil testing to determine lime and fertilizer requirements.

• Select correct rates of fertilizer to apply to soil before bedding and mulching. Preplant (broadcast) N and K₂O should not exceed 30 lb each per acre.

• Practice careful irrigation management to avoid over-irrigation which leaches N and K from the root zone.

• Inject N and K in daily or weekly injections at rates not exceeding 0.60 lb each N and K₂O per acre per day except for 0.75 lb each N and K₂O per acre per day in February and March.

• Monitor petiole sap N and K concentrations and use that information to adjust N and K injection schedules or rates.

• Back up petiole sap-testing with laboratory analyses of whole leaves.

Literature

Albregts, E. E., G. J. Hochmuth, C. K. Chandler, and J. A. Cornell. 1994. K requirements for drip irrigated strawberry. Unpublished data.

Albregts, E. E., and C. M. Howard. 1982. Effect of fertilizer rate on number of malformed strawberry fruit. Proc. Fla. State Hort. Soc. 95:323–324.

Albregts, E. E., C. M. Howard, and C. K. Chandler. 1991. Strawberry responses to K rate on a fine sand soil. HortScience 26:135–138.

Clark, G. A. 1993. Water requirements for drip-irrigated strawberries in south central Florida. Gulf Coast Res. and Ed. Center Res. Report BRA 1993–2.

Freie, R. L., and N. L. Pugh. 1994. Florida Agricultural Statistics. Vegetable Summary. Fla. Dept. Agric. Consumer Serv. Tallahassee, FL.

Hochmuth, G. J., ed. 1988. Strawberry production guide for Florida. Circ. 142C. Gainesville: University of Florida Institute of Food and Agricultural Sciences.

Hochmuth, G. J., and G. A. Clark. l991. Fertilizer application and management for micro- (or drip) irrigated vegetables in Florida. SS-VEC-45. Gainesville: University of Florida Institute of Food and Agricultural Sciences.

Hochmuth, G. J., and E. A. Hanlon. In Press. Commercial vegetable crop nutrient requirements. Circ. 806. Gainesville: University of Florida Institute of Food and Agricultural Sciences.

Hochmuth, G. J., D. N. Maynard, C. S. Vavrina, and E. A. Hanlon. 1991. Plant tissue analysis and interpretation for vegetable crops in Florida. SS-VEC-42. Gainesville: University of Florida Institute of Food and Agricultural Sciences.

Hochmuth, G. J., E. E. Albregts, C. K. Chandler, and J. A. Cornell. 1994. N requirements for drip-irrigated strawberry. Unpublished data.

Kidder, G., E. A. Hanlon, and G. J. Hochmuth. 1989. IFAS standardized fertilization recommendations for vegetable crops. SS-SOS-907. Gainesville: University of Florida Institute of Food and Agricultural Sciences.

Locascio, S. J., and J. M. Myers. l975. Trickle irrigation and fertilization method for strawberries. Proc. Fla. State Hort. Soc. 88:185–189.

Locascio, S. J., and F. G. Martin. l985. Nitrogen source and application timing for trickle-irrigated strawberries. J. Amer. Soc. Hort. Sci. 110:820–823.

Myers, J. M., and S. J. Locascio. 1972. Efficiency of irrigation methods for strawberries. Proc. Fla. State Hort. Soc. 85:114–117.

Smith, S. A., and T. G. Taylor. 1993. Production cost for selected vegetables in Florida, 1992-1993. Circ. 1121. Gainesville: University of Florida Institute of Food and Agricultural Sciences.

Tables