Section control turns pieces of an applicator off so they do not put product on ground outside the area being applied. The pieces are sections of a spray boom, groups of nozzles, rows on a planter, or portions of a spreader. When a section crosses a boundary, a waterway, or ground that was already applied, that section stops. The sections that are still inside the field keep applying.
It is often mentioned in the same breath as variable rate application. They are different functions. Variable rate changes the amount. Section control changes whether a section is on. A flat rate machine can have section control. A variable rate machine can have it too. One does not require the other.
What the controller is watching
Each section has a known width and a known position relative to the GPS receiver. The display keeps a coverage map of where product has already been commanded. When the predicted path of a section lies outside the field boundary, or on top of coverage, the controller shuts that section off. When the section is back over untreated ground inside the boundary, it opens again.
Point rows are the usual place this shows. The boom enters the headland at an angle. The outer section reaches covered ground first and drops off, then the next section, then the next, until the whole boom is out. Without section control the outer nozzles would keep spraying the headland that was already done. The same logic covers a terrace, a well, or an internal boundary the operator has drawn.
Rate stays a separate command
On the sections that remain open, the rate target is whatever the pass was given. If the pass is flat, that target is one number. If the pass is variable, the target comes from the prescription or the sensor, and it can change while sections are also switching. The as-applied map shows both: gaps and narrowed swaths where sections were off, and color changes where the rate changed.
Shutoff is not instant. A nozzle or a clutch takes a moment, and product in the line can still come out after the command. Controllers let an operator set a lead-in and a lead-out, a short distance that anticipates the delay. Those settings change where the gap falls. They do not change the rate on the ground that stays covered.
Overlap and sections
On a curve, the outside of a boom travels farther than the inside, so a boom with no section control piles extra product on the inside of the turn. Smaller sections, down to individual nozzles, make those overlaps smaller. That is still section control, or nozzle control, operating on coverage. It is not the controller looking up a new target rate for the inside of the turn. The target was already set. The section decides whether to deliver it.