This talk is intended to touch on the 'nuts and bolts' of fertigating strawberries and not the nutritional aspects of fertilizing the crop. Extension small fruit specialists are the best sources of information concerning the fertility recommendations for strawberries in your states. Several important facts are necessary to assist growers in setting up their fertigation systems. These are: 1) the flow rate of the drip tape, 2) the total number of feet of drip tape used, 3) the total GPM, 4) the amount of nutrient needed per acre, 5) the concentration of the nutrient in the fertilizer, and 6) the dilution ratio or flow rate of the injector. Solid-state or mechanical devices are usually used to control irrigation timing and fertilizer injection. Several key maintenance factors can ensure accurate operation of your fertigation system during the cropping season.

The tape flow rate is unique for each type of tape and will usually be found stamped on the tape itself and/or on the tape spool. This figure is valid only within the operating range of pressures for a drip system, will vary slightly with the lay of the field, and is necessary to determine the total water demand of the field. For example: .45 tape = 0 .45 gpm/100 ft of tape, .27 tape = 0.27 gpm/100 ft. Often this volume is expressed in 1000-ft. units, i.e. 2.7 or 4.5 gpm/1000 ft.

The number of feet of tape used can be obtained by measuring the length of rectangularly shaped fields and multiplying by the number of beds. For an irregularly shaped field, you can calculate this number by using the row spacing figure and the number of acres planted. Don't forget to account for the percentage of drive rows if you use them. Some common row spacing figures are: 7200 bed feet/acre for 6-ft. spacing, 7900 bed feet/acre for 5.5-ft. spacing, and 6700 bed feet/acre for 6.5-ft. spacing. The number of feet of tape used and the tape flow rate are used to calculate the total water demand of the field.

The total GPM is your field water demand. This is an important figure in that it will tell how you must zone your field. It can also be used to determine the injection period for certain injectors. If your total GPM (field demand) exceeds your well flow rate or pumping capacity, you must zone your field to match your well or pumping capacities. For example, if a field demand of 45 gpm must be satisfied by a 25 gpm well, the field should be divided into three zones requiring 15 gpm each. Although, two zones of 22.5 gpm could be used, this doesn't give much of a cushion to accommodate fluctuations in the flow rate.

The amount and concentration of nutrient is necessary to determine the length of injector time or volume of fertilizer solution to be injected. The amount of nutrient needed per acre for strawberries will be obtained from your small fruit specialists. For N, this figure is in the range of 0.75 to 1.0 lb./A/day. The concentration of nutrient in a fertilizer is given in the fertilizer formula, i.e. 7-0-7 contains 7% Nitrogen and 7% potash. The specific gravity (weight/volume) for liquid fertilizers is necessary to calculate the amount of nutrient present in a given volume of fertilizer solution. This figure can be obtained in the product information about a fertilizer product or from the formulator. If you are assisting farmers in making their own fertilizer solutions, you will need the concentration of nutrient in each ingredient and their solubilities in cold water. These numbers can be found in Knott's Vegetable Growers' Handbook or gotten from a fertilizer formulator.

There are several types of injectors used to introduce the fertilizer solution into the irrigation water. The different types of injectors have different specifications used to calculate the amount of fertilizer applied. Positive displacement (piston pump) injectors and venturi (suction) injectors have flow rates. With positive displacement injectors the stroke rate and stroke length can be adjusted to vary the amount solution injected in a given time. This type of injector needs a safety shutoff to turn off the pump if water stops flowing. Otherwise, concentrated fertilizer solution can be injected into the irrigation system resulting in severe crop damage. Adjusting the difference between the upstream and downstream pressures will vary the flow rate of venturi injectors. Venturi injectors often are equipped with flow meters. Proportioners (water pistons) use a large piston that is activated by the irrigation water flow to move a small piston. The small piston draws up the fertilizer solution to be injected into the irrigation water. The piston sizes are proportional; therefore there is a ratio between the volumes pumped by each piston. This is the dilution ratio, i.e. 1:100, 1:150, 1:50, etc.

Controllers are microcomputers or electro-mechanical timers which can turn off and on various irrigation and fertigation components so that the grower doesn't have to be physically present for each operation. They range from sophisticated computers capable of running multiple injection rates and irrigation rates for many crops on thousands of acres to simple devices based on clockworks. Inexpensive turf irrigation controllers that will operate the drip program and irrigation programs simultaneously are often sufficient for fertigation of most strawberry operations. Turf controllers and irrigation computers can be operated by low voltage controller circuits or small, pressurized water lines.

Biological materials and chemical precipitates can accumulate in your lines. This will adversely affect the flow of water and distribution of fertilizer by your system. Periodic flushing will avoid this problem. In areas with poor water quality, flush caps will allow flushing each time the system operates. Screen, disc, or other types of membrane filters for well water filtration should cleaned regularly according to the manufacturers' instructions. Media filters for use with surface water sources should be back-flushed regularly or have an automatic back-flushing component. Drip systems are low-pressure systems; the rated pressures should not be exceeded. This will shorten the lifespan of the components and create blowouts. Also, the calculated flow and injection rates will be incorrect when the system is operated out of the stated operating range. The fill time of each zone should be determined by injecting a concentrated solution of dye and timing how long it takes for the color to appear at the most distant emitter of that zone. The system should operate for the fill time before injecting fertilizer and be allowed to operate for the fill time after finishing a fertilizer injection. This will ensure accurate distribution of the fertilizer and avoid fertilizer remaining in the drip lines when the system shuts down. This could lead to increased precipitation problems.

These fairly simple guidelines will help growers begin to use fertigation systems as a management tool to improve their strawberry operations. Adherence to the maintenance tips will avoid many of the pitfalls in operating any drip fertigation/irrigation systems. Individualization of any system to local soil types and growing conditions will be necessary for optimum efficiency of operation.