Question
Visiting garden centers and home improvement centers we seem to be able to find mainly high phosphores fertilizers (15-30-15, 10-15-10) or balanced fertilizers.

We are raising annual plants from seed indoor and are now fertilizing with a weak solution (100 ppm, or 1/2 teaspoon of soulble 15-30-15 per gallon of water). We try to fertilize every 2nd or 3rd watering after first true leaves have formed. Some of the seedlings show puckering of the leaves.. is this generally a sign of too much fertilizer ? if so, how much do you recommend for seedlings (we thought we were conservative). Note: the growing medium has no fertilizer (soilless).

Also, we have started dahlias indoor to get them growing before we can plant outdoor in may. The dahlias are between 6-12" high now. Should we start to fertilize them (also planted in soilless medium with no fertilizer). Affirmatively how much fertilizer should they get ?

Outdoor, once we plant the annuals (mainly coleus, impatiens and salvia) would it be better to put down a slow release (oscomote 14-14-14) fertilizer (12% potassium/nitrogen in slow release form) or should we apply soluble fertilizers weekly ? (prefer the slow release)..

I understand that most flowering annuals prefer a lower nitrogen fertilizer, like 5-10-10, but except for a 7-10-12 tomato fertilizer (liquid, quick release) we have not been able to find these. Would the slow release form of oscomote act like a low nitrogen fertilizer due to fact that it is applied over longer time ?  

Answer
normally the back of the fertilizer container will give a recommendation. I like Peters 20-20-20. 1/4 teaspoon per gallon of water. i would use less than that for seedlings every other week. i would not fertilze until 2 weeks after they 1st germinate. the soilless mix (like promix) should have enough nutrients to get them going. too much fertilizer would show up as burning (whitening) of the foliage. many new annuals germinated will have cup leaves. i would go more by the color of the foliage. green w/not too much yellowing.
same for the dahlias.
use the slow release once a month. i don't like liquids for ground use. you may want to try plantone or gardentone by Espoma as a good slow release fertilizer. osmocote is a good product provided you follow the directios.
high nitrogen fertilizers are more like turf fertilizer 36-10-4 etc.
use up your fertilizer buy mixing it at 1/4 teaspoon instead of 1/2.
hope i was of some help.