Differences in fertiliser applications are one of the things which should be considered when setting up the blocks within an analysis. The "What is a Block" Knowledgebase article may be helpful here.
However, sometimes you may have situations where a single fertiliser application is different from the others, and so it may be necessary to model application to "part" of a block.
While in geospatial terms this cannot be done; the application of the nutrients can be added to the software in such a way as to allow the model to understand the nutrient implications.
For example, keeping things simple, if 100kg of DAP is applied to one quarter of the block, then in OverseerFM the application should be added as 25kg of DAP.
Similarly, if the application was 100kg of DAP to half of the block, the application would be modelled as 50kg applied.
This way the Overseer management block receives the same quantity of nutrients as actually takes place, bearing in mind that OverseerFM presumes good management practise for fertiliser application is followed.