I know I could forget about it, or just count on the compost I've added to the soil to take care of that deficit, but I have this thing about trying to follow directions exactly.

Thanks,
Ginny

You could fertilize at planting time if you want but it is necessary? Maybe, maybe not. I don't fertilize them, I just rely on compost and organic matter for carrots and other root crops. I'd wait until the carrots are about one quarter to half grown before thinking about fertilizer. By that time you'll be able to tell by their growth as to whether or not they need it.

Rodney

Sounds good in theory but doesn't work in practice. It only leads to over-fertilization, wasted money, and ground water contamination from run-off of the excess.

Have you had your soil tested professionally? Without that you can't know what you need to add or how much. Your soil may already contain more than enough potassium as well as everything else the plants need.

Dave

I had my soil tested years ago. I would like to do it again but it is a bit of a chore. I would think adding the right amount of fertilizer when I plant the seeds should be ok. I always fertilize when I plant anything, and then usually forget to do it again.

I also always add store bought compost to the holes when I plant anything. I have my own compost piles but can't turn them very often.

Ginny

I haven't done a soil test and hope to do it this fall. just one more task :(

Unless the test were to show I was deficient in potassium, I'd think I would still want to use a 5-10-10 organic equivalent fertilizer when growing carrots since that is recommended on every single site I read.

I can't believe that all these sites suggest fertilizing if it isn't necessary in most cases. These are university sites, not fertilizer product sites.

Ginny

Best advice: get local advice. Soil testing, local county extension, other experienced LOCAL gardeners. Glib's point about western soils may, or may not apply to your microclimate soil. Soil is slow stuff, I try to take small steps with fertilizer.
KateQ

So many people telling you to get a soil test. That's often a good idea, but not such a do-or-die thing. It is good to know your approximate pH; everything else follows that.

One problem with relying on composted material to provide a specific thing like potassium is that the stuff's specific nutrient levels are extremely varied due to what was used to make it. Hardwood ash is also extremely variable and very alkaline.

Since gardening is long-term and you are only being cautious, you might add rock minerals such as greensand. It has very slow (eon-scale!) K release, especially if your pH is high, and you may need a lot but a lot won't generally cause any problems.

Gardening is also local, so take all that with a grain of salt (poor source of K).