Answer: Your fertilizer sounds OK, as long as you are applying it at the correct dilution. (Seedlings don't need fertilizer until they have their first true leaves anyway.) Were the cucumbers direct seeded? It could be that now that they are popping out of the ground the sun and wind are a bit much for their tender constitutions and they are stressed. If they were transplanted, they could be suffering a little transplant shock. If this is the case you may want to give them a little bit of shelter from the sun and wind for a few hours only during the midday or when the sun is at it's strongest. You shouldn't have to do this for long, only until the seedlings are a little larger and a little stronger. Also, they could be receiving too much or even too little water, sometimes yellowing of the leaves is a symptom of either of these.
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