EQUIPMENT

Five Easy Steps to Calibrating a Center Pivot for Chemigation 

Chemigation is the addition of any chemical to the water used for
irrigation. In the past, this practice has been called
fertigation for adding fertilizer, herbigation when herbicides
were added, fungigation for fungicides, etc. Now it is just
called chemigation.

Chemigation is a very efficient and effective irrigation
management tool when used properly. In fact it is recognized as a
best management practice (BMP) for irrigated agriculture. When
chemigation is used with an irrigation system, the irrigation
water delivery system and the chemical injection equipment must
conform to state laws regarding backflow prevention. In addition,
when pesticides are added to irrigation water, the pesticide
label must state that it can be used for chemigation and be
applied through center pivot irrigation systems.

Center pivot systems are used on nearly 75% of the irrigated land
in North Dakota. With the cost of pesticides increasing every
year, it makes a lot of sense to properly calibrate a center
pivot irrigation system for chemigation. Below are five easy
steps to follow to ensure that a center pivot chemigation system
is properly calibrated.

1. Calibrate the injector pump.

   Determine the injection rate of the chemical injection pump
   for a particular setting of the injection rate control knob.
   This must be done with the irrigation system running so the
   injection pump is working against the water pipeline
   pressure. Do this by letting the injection pump draw from a
   calibrated container on the suction side of the pump.
   Determine the time in minutes to pump 1 gallon of liquid,
   then use this equation to determine the injection pump rate
   in gallons per hour:

   Injector Pump Rate (gal/hr) = (60)/(minutes to pump 1 gallon) 

2. Determine the number of hours it will take to make one
   complete revolution around the field at the speed the center
   pivot will be operated and the total number of acres the
   pivot covers: 
   Time for one complete revolution = hours
   Total area the center pivot covers = acres

3. Determine the total gallons to be injected.

   Multiply the injection pump rate (step 1) by the total hours
   to cover the field (step 2). Use the following equation:      

   Total Gallons Injected = (gal/hr) x (hours to cover field) 

4. Determine the amount of chemical required to cover the field.
   Multiply the field acreage by the chemical rate as specified
   for the particular chemical and crop. For nitrogen it would
   be the pounds N per acre and for pesticides it would be the
   rate that is recommended on the label for the particular
   crop. Use the following equation:

   Total Chemical Volume = field acres (step 2) x (chemical
   volume/acre)

5. Add the total chemical (step 4) to the injection supply tank
   and then add water to the supply tank until you have the
   necessary total volume to be injected (step 3).

   When working with many pesticides and dry chemicals, make
   sure you have a method to agitate the injector supply tank to
   keep the chemicals in solution. Many chemicals will settle
   out if not agitated.

(Submitted by Thomas F. Scherer Extension Agricultural Engineer
Irrigation/Water Quality)