I'll admit that at first sight filling your tractor with oil and planning your nitrogen applications don't appear to have much in common but in one simple way they are very similar. Before you do both, you need to know how much you've already got in the tank or in the soil, so you know exactly how much more you need to put in.
When it comes to nitrogen applications, rates should only be decided once you have fully assessed the requirements of your crops. As well as thinking about inputs, this also means making an informed judgement on how much nitrogen will be supplied by your soils.