Using Flow Rate to Apply a Target Amount of N
By far the easiest way to apply a specific rate of lagoon nutrients is to use a flow meter and control valve. It is possible, however, to apply a target rate of nutrients using the pump output or gravity flow rate method of measuring flow, although it is awkward and imprecise.
This method entails running the lagoon pump during only the last part of each check or irrigation set. Injecting the lagoon water into the irrigation water for only part of the total run time not only allows you to run the pump at a high enough flow rate to prevent pipeline plugging, but injecting into the last part of the set actually improves application uniformity of the nutrients. One disadvantage of this method is that personnel will need to be available to turn the pump on and off. It can also be challenging to time the applications so that the mixed water arrives at the field and ends at the right time if it takes a long time for the lagoon water to travel from the mixing point to the field.
This method can also be used in conjunction with a flow meter if the necessary flow rate needed is less than that at which it is safe to run the lagoon pump or would result in the velocity of the water being too low to keep solids from settling out in the lagoon transfer pipeline.
You will need to have an estimate of the pond output gpm. Resources for determining the flow rate from a pond by depth are here:
Resources for using hand calculations or tables to determine how long to run the lagoon pump on each area to achieve a target nitrogen application when the flow rate is not adjustable are here:
To determine target run times using an Excel based program, go here:
Remember that the lagoon nutrients must be mixed with the fresh water during only the last portion of the irrigation set or the application will not be uniformly distributed across the field.