Variable rate fertilizer is variable rate application with a nutrient product in the machine. The target, in pounds of product per acre, changes as the spreader, the toolbar, or the planter moves. Nitrogen, phosphorus, and potassium products are the ones most often varied. Each product is its own pass, or its own bin, with its own map.
The product rate is not the same thing as the nutrient rate. A ton of 11-52-0 does not contain a ton of phosphorus. The guaranteed analysis on the bag or the invoice says how much of each nutrient is in the product. A prescription written in pounds of product has to match the product in the bin. A prescription written in pounds of nutrient has to be converted before the controller sees it.
Phosphorus and potassium
Phosphorus and potassium bind to soil particles. They move very little with water compared with nitrate. A pound applied on a hill largely stays on that hill. Because of that, maps for these nutrients are often built from soil tests, from how much the crop has been removing, or from both. A soil test reports a concentration. Crop removal is an estimate of what harvest took away, usually from a yield map and a removal factor for the crop. The prescription is a set of product rates laid on those patterns.
The pass itself is commonly a dry spinner or air-boom spreader, loaded with a graded product such as MAP, DAP, potash, or a blend. A blend is more than one product mixed. The controller varies the blend as a whole unless the machine has separate bins and separate meters.
Nitrogen
Nitrogen behaves differently. In the nitrate form it moves with water. It can leach below the root zone, convert to a gas in saturated soil, or leave the surface if a urea form sits on the ground without incorporation. The amount of nitrogen a soil will mineralize from organic matter also changes with the soil and with the season. A soil test for nitrate is a snapshot, not a stable map of the field the way a phosphorus test is.
Nitrogen prescriptions are built from yield patterns, from zones, from canopy sensors, and from the form of the product, more often than from a single grid of soil nitrogen. Anhydrous ammonia, urea, UAN solution, and manure are all nitrogen sources, and they are not interchangeable in the file. The controller meters the product. The nutrient math has to have been done for that product already.
One map, one product
A field can need all three nutrients and still have three files. The phosphorus map follows the soil test. The potassium map may follow a different pattern. The nitrogen map may follow yield or a sensor. Loading one file for all three bins applies one pattern to every product. The 4Rs separate those choices into source, rate, time, and place. When the pass happens is the time. The map is the place and the rate, for the source that is actually in the machine.