This is a very sensitive subject…
By definition is to use a variable rate of the input, in order to put the desirable quantity on a specific place.
This post aims to find some web help from someone that has some experience with this subject on extensive dryland crops.
Besides all the data collection, which consisted on soil sampling (pH, P, K, N, o.m.), productivity maps and altimetry, the following steps are the critical ones, that consists on analyzing the data and take a management decision.
I’m aiming to apply, on a variable rate basis, cover fertilizer (nitrogen) and deep fertilizer (NPK).
On nitrogen cover fertilizer, the decision process is easier, because the crop is already fully installed, and we only have to change the rate according to the different development aspect of crop spots – increase the rate on the best potential parts and decrease it on the worse ones.
On deep fertilizer the decision taking is quite more complex, because it will somehow influence the availability of nutrients for the whole crop cycle.
The bibliography available talks more about the data collection, almost says nothing regarding the decision process and is not conclusive regarding the cause-effect between the variable rate fertilizer application and the productivity.