Hydro,
If all of them are brown, just use the cultivator or hoe to destroy them and start over. Plant only two, and wait 10 seconds. If they stay green, then use that spacing for the rest. If they turn brown, then plant one more a little farther out from the first two, and wait..repeat until they stay green. I did this, and saved myself a lot of seeds early on. In game distance, they need about 3 feet between each carrot. Onions need 2 feet, Barley and Flax need 6 inches...it varies by plant type.