How a fertilizer blend is calculated

A blend is a set of products applied together so that their nutrients add up to what the crop needs. Each product carries a fixed analysis: urea is 46 percent nitrogen, MAP is 11 percent nitrogen and 52 percent phosphate, muriate of potash is 60 percent potash. To hit 150 kg of nitrogen, 60 kg of phosphate and 60 kg of potash per hectare you need 100 kg of potash for the K₂O, 115 kg of MAP for the P₂O₅, and then only 298 kg of urea, because the MAP already brought 13 kg of nitrogen with it.

That credit is the whole point of calculating instead of guessing. Every compound product brings a second or third nutrient along, and a blend that ignores it overspends on the one you priced first. The calculator tries every combination of up to four of the products you tick and shows the ones that come closest, flagging any nutrient that lands short or well over.

Credits before you buy

Manure, a legume in the rotation and anything already applied this season all count against the seasonal need. The worksheet takes each of them and hands the remainder to the targets. Availability matters here: only part of the nitrogen in manure reaches the crop in the first year, so the worksheet starts from typical extension figures and lets you overwrite them with your lab's.

What nitrogen really costs in a compound

When a blend includes MAP or DAP, part of what you pay for nitrogen is really paying for phosphate. The calculator values the P₂O₅ and K₂O in each blend at the price of the straight products, subtracts that, and shows the cost of the nitrogen that is left. It is the fair way to compare a compound against straight urea.

From one blend to a prescription map

A blend is one rate for the whole field. Turning it into a prescription map is one step in GeoPard, and the map can take two shapes. A flat rate writes the blend you picked across the whole field as a file your terminal reads; use it where the field is even or where you want the blend on the ground this week. A variable rate map lets each zone carry its own rate: an equation over the soil test and yield layers works out the nutrient each zone lacks most, sets the rate zone by zone, converts it to product with the same analyses used here, and exports the map. Fields are rarely uniform, one zone short of potash and the next fine on potash and short of phosphate, so this is where the blend starts paying for itself.

Either way the products, analyses and prices come straight from the blend you ranked here. The Copy as GeoPard equation button on any blend gives you the variable-rate equation with your products already filled in.