May 19, 2016Use linear bed feet for calculating fertigation
Soil test recommendations usually assume fertilizer is spread evenly across a field. However, with plasticulture and fertigation, growers can direct nutrients more efficiently. Headlands and drive alleys are typically not fertilized after plants are in the ground. Fertilizer can be applied in a band while shaping beds or laying plastic, and also dissolved through irrigation water and delivered by drip tape to the base of the plants.
In these systems, it is helpful to be able to calculate the fertilizer rate per linear bed foot based on the fertilizer rate per acre. To do so, you need to find out how many linear bed feet per acre you are working with. To calculate linear bed foot per acre, divide 43,560 (square feet per acre) by the distance in feet between the centers of your beds. For example, if you have 5-foot bed spacing, then you have 8,712 linear bed foot per acre. The fertilizer rate per linear bed foot can then be calculated by dividing the fertilizer rate per acre by the linear bed foot per acre.
The table below does these calculations for you based on your bed spacing and the fertilizer rate per acre. Find your bed spacing in the left column and follow it across to the cell that corresponds to your recommended fertilizer rate in pounds per acre. If your soil test recommends 200 pounds per acre, use the 100 pounds per acre column and multiply by two. If your soil test recommends 130 pounds per acre, estimate the number between the 120 and 140 pounds per acre columns, or divide 130 by your linear bed foot. Divide that number by the percentage of actual fertilizer in the fertilizer formulation to determine rate per linear bed foot. Multiply the rate per linear bed foot by the length (in feet) of plastic you have laid to determine how many bags of fertilizer you will need.
For example, a soil test calls for 80 pounds per acre of actual nitrogen (a typical amount for a vine crop), and you are planting five 100-foot rows of melons with 5-foot bed spacing. Find your bed spacing on the left column, and follow it across to the column that shows 80 pounds per acre. The cell where those meet is 0.0092 pounds per linear bed foot. Divide that by the percentage of nitrogen in the fertilizer formulation and multiply by the footage of rows you are using. If using 46-0-0, that would be 0.0092 divided by 0.46 = 0.02 pounds per linear bed foot. Multiply that by 500, because you have five 100-foot beds and you will find 10 pounds of 46-0-0 will fertilize those beds at the 80 pounds nitrogen per acre rate.
A more common practice is to broadcast a third of the total nitrogen up front, and split the rest of the nitrogen applications through the drip over the rest of the season. In the previous example, that would be about 25 pounds nitrogen per acre broadcast and incorporated and the remaining 55 pounds nitrogen per acre applied through the drip. If you are set up to band fertilizer during bed-shaping or plastic-laying, you could use the table again to determine how much fertilizer is needed per linear bed foot. With five 100-foot beds spaced 5 feet apart, that would be about 3 pounds of 46-0-0 at the 25 pounds per acre rate.
The remaining 55 pounds of nitrogen that need to be delivered through irrigation water are split across the seven to eight weeks after transplanting. That is about 7 pounds per acre per week. Divide 7 by 8,712 from the table to determine the rate per linear bed foot. Divide that number by the percentage of nitrogen in 46-0-0 and multiply by 500 because you have five 100-foot beds. You will find that you will need to dissolve and deliver about 1 pound of 46-0-0 per week to the five beds.
Source: Michigan State University Extension