Well rotted manure is a good source of organic Nitrogen as is Liquid fish fertilizer. Zucchini love manure, I mix the manure with leaf mulch just to provide some good structure to the soil as well. Not to mention micronutrients.
Tomatoe plants love liquid fish fertilizer. I give my tomatoe plants (in pots this year) a blast of it (1/2 cup with 4 litres (1 gallon) of water) once a week all growing season and they always look great and give me really high yields. Liquid seaweed fertilzer is also really good for tomatoe plants. Or you can buy some kelp meal and add that your the soil when you plant the tomatoe plants.
-------some time later----------
Just had an idea. Last year, I did some mustards and some squash in a few pots. I built the potting soil out of mushroom manure, soil, worm castings and some kelp meal. Even with the constant watering, the plants did really well and did not suffer from any nutrient deficiencies.
With oranic bound nutrients, the nutrients themselves are part of the humus and not in solution (floating around as ions in the soil) and therefore are not leached out of the soil during watering. What happens is, the plants release chemical (acids) that dissociate the nutrients from the humus when the plant needs them.
Worked great last year. Try it next year with your zucchini and don't forget that liquid fertilizer. It can only help.