I would second the idea of liquid fertilizer. Use something organic. A couple of things to consider: First, lettuce and spinach grow very quickly, and you should accommodate that rapid growth cycle by providing them all they need in a readily available form. Granular organic fertilizers generally take too long to break down. That's not to say that they take an unreasonable amount of time to do so, just that these particular plants have such a quick growth cycle that waiting even just a couple weeks for the nitrogen in granular fertilizers to become bioavailable is too long. The second thing to consider is that spring soils are still cool, and the bacteria that fix nitrogen and convert it from one form to another are not fully active yet. What's more, nitrogen does not cling to soil particles very well, and thus gets flushed out of soil by spring rains. Putting these factors together makes a pretty strong case for providing extra nitrogen in a quickly available from for lettuce, spinach, and other fast growing spring greens. Fish emulsion works extremely well, or if you're particularly beholden to Miracle Grow, then try their new Organic Choice All Purpose Plant Food. It's high in nitrogen (8-0-0) and is made from beet molasses. I never thought I would recommend a Miracle Grow product, but in fact I have tried it and it works quite well. Walmart and the big box stores usually carry it. See here: http://www.scotts.com/smg/catalog/productTemplate.jsp?proId=prod10280006&itemId=cat70048 -Diggity Here is a link that might be useful: Garden Imperative blog

I agree with everything so far however I am puzzled by the fact that several stated that you planted more every two weeks. Why? I plant a wide row and cut what I need about an inch above the ground . Add a little mulch,or fertilizer to that part of the row and water deep. One row will provide all the spinach we can eat,give away to all our neighbors and some for the chickens. I will be ready to cut in about four weeks and it will give us all we can use until about April. The row is about 28 feet long.I have had as many as six cuttings of the plants. No need to replant.

in my z5.. ground freeze MI. i will yell.. NEVER FERTILIZE AFTER 8/1 ... EVER!!!! made too many mistakes and killed things over the years ... they are plants.. they are not children.. and i swear i have told you this before .. lol ... they do not need to eat.. nor be fed.. presuming you have a decent soil ... and trust me on that.. because i do not fertilize on mineral sand.. which is basically beach sand [no salt issues, its a glacial thing] .. and nothing fails to prosper.. for such .. [of course water management is the BIG!!! problem] .. all that said.. a little of this or that.. shouldnt hurt.. the issue is that your plants be slowing down as the first frosts come.. and definitely by first freeze ... which for you warm zone folks.. i am talking 10/1 ... in MI ... and lets just pretend.. thats like 30 days ... ya know.. with thirty days.. can it even get into the soil.. and processed into the plant.. to do much of any good???.. i wonder ... anyway ... anyway.. doubt you could do much harm with something light and fruity.. like a fine 1976 rose' called.. 12-12-12 ... broadcast VERY sparingly ... and i said broadcast.. i would NOT be putting it in the planting hole.. down 6 inches.. because of the activation time.. and the cold issue ... very frankly.. i would water them in well.. mulch them for winter.. and in early may.. then give them the fert ... safer all around in my world ken ps: one year.. i killed about 50 of my 100 hybrid t roses.. thru fertilization.. never again ... the roses.. nor the fertilizing .. lesson learned ... feed them when they are starting to.. or actually growing.. not when they are supposed to be going to sleep for the year .... and again.. if you are z7 and up .. and winter is 45 minutes in mid january.. we z5'ers.. dont live in your world ..

I love Scotts Organic in the green bag, greens up over night, keeps the soil moist longer, and is loaded with great organics for a lower price than some others. It doesn't burn so can be put on a couple of times a year or if an animal does something brown to the grass, POOF it's gone. I put some on my green tropicals as well as honeysuckle and a few other bloomers and they are happier than ever.I got mine at Home Depot..