The appearance of your plants should be a good indicator of where you are standing with your nitrogen and its availability. It's one of the easiest nutrient deficiencies to spot. I manure my gardens from my chicken coop and also work the rotten hay bedding into it. I've never had to add anything else and there is so much organic material in my soil now that the once heavy clay can be worked with your hands.

I have great soil and still fertilize (organic granular) before planting, to meet the needs of the crop. Corn gets more, lettuce gets less. I only use fish emulsion/kelp if a plant is in distress.

But if you really can't test effectively for N, do you have to blindly keep adding it each year?

N dissipates quickly into the air which is why you can't properly test for it. So yes, depending on the need of the specific crop, you need to add it but not overdo it.

It is all the other nutrients you don't need to add regularly, especially P.

Dave

Dave is right, N is a very volatile nutrient, over fertilizing with P and K is ok, the plants will only take what they need and leave the rest.

Here is a link that might be useful: Gardening

If you could keep all the nutrients in the plants you use for compost, you would not need anything else. But as it is,
compost has too high P and K, because half the N volatilizes. Manure is a bit better because it starts with relatively high N.

99% of the time, if you have put in a lot of organic matter in the past, you only need to add N. That is why urea (46/0/0) is such a good friend of the mature garden. Also, be aware of who needs N. Carrots, parsnip, beans and peas need nothing. Most veggies need a single shot just about now (or in early spring in the case of garlic, basically, at the beginning of active growth). Only a few (cabbage, collard, corn)need multiple applications.

Sounds like maybe I should be focussed more on blood meal or alfalfa meal than anything with NPK since my garden is more "mature" now.

Thanks for all the great info!