Need more expert advice please

Thanks for all your responses to my questions. They really help! My next question is I am now finding on my basil, shiso, arugula, and some of my other micros only the cotelydons are yellowing even before they mature. Does anyone have any thoughts on why this is occuring? Thanks

Leaves turning yellow are usually due to three things: Lack of water, lack of nutrition/root problems, or lack of sunlight.

Assuming you've already checked the water issue, I'd take a look at the roots of the plants. Are they compacted? Are they healthy looking? What sort of soil are you using or is this a hydroponics system? If the former, add com fertilizer/compost tea. If the later, try adjusting your nutrient solution.

Otherwise, your plants need more light.

Nitrogen deficiency?

Add some nettles tea, comfrey tea, urine. Mixture of bone meal and fish meal will provide a long term solution.

What's the substrate you are growing these in?

Another question! Do you have photos?

Tom OHern wrote:Leaves turning yellow are usually due to three things: Lack of water, lack of nutrition/root problems, or lack of sunlight

I find too much water is pretty common with seedlings.
I don't know anything about micro greens though...