Too Much Manure?

I raised and expanded my garden this year to keep the plants out of the flood that comes every spring. I added soil from a local sand pit, (I know it's risky, but I'm willing to deal with the problems in lieu of 14 cubic yards of dirt delivered for less than $100), then added a truck load of fresh horse manure to the soil, rototilled the manure into the soil and planted tomatoes and peppers using walls of water to keep warm while cold. It has been a cold and dry spring, here in Central Nebraska, but I have watered deeply once a week. The weather broke for a few days, getting quite hot. I took off the walls of water in fear the plants would get too hot in there. Now it's cold again and I have replaced the walls of water, but my plants are turning yellow. Some of the leaves toward the top are turning black down the veins. They were doing just great in the walls of water, but now everything seems to be turning yellow and starting to die. Any suggestions??? Thank you in advance for any advice or suggestions!