The above table should be self explanatory. The last column (Over?) shows if the crop will take a yield penalty if you over-fertilise it.

Why is all of this important?
If you play with infinite money, you won't need to care. However, for the cost conscious farmer, the various fertilisers in game have different costs and spreading efficiencies. The organic fertilisers (slurry, manure and digestate), as bi-products of animal husbandry or BGA sales, cost a lot less to spread than the mineral fertilisers. They apply a flat +60 Nitrogen value.
Solid or liquid fertilisers cost more to spray, but the amount of to spray will be automatically customised to match the crop you're growing.
Last, but not least, oilseed radish will add a flat 40 nitrogen to the ground after you cultivate it, at the cost of one (long) growth cycle of 10 hours + seeding and cultivating time.

Here are a couple of pointers that work for me:

• Before seeding, check your base nitrogen levels. Add organic fertiliser as long as it doesn't kick you above optimal rates
  
• Soybeans are undemanding, and won't take a yield hit whatever you do in your field. It's a good default crop
  
• Oilseed radish is probably worth it for the more Nitrogen-hungry crops, in particular on fields with lots of Loamy ground. For this to work, it is important to realize that you have to cultivate the radish in as a separate action. A seeding machine that also cultivates will ignore the bonus.
  
• For Loam, Potatoes are interesting in the sense that they demand less base nitrogen than many other crops and add 40 to the ground after harvest. Provided you're already equipped with potato machinery of course.
  
• You'll want to watch overfertliziation with Oat and Sunflowers.