I just got a book called "1001 Gardening Secrets," and already it's worth its weight in gold. It has tips on everything--fertilizing, pest control, planting, problems--just about anything you could want to know is in there. Our tomato plants are growing tomatoes (YAY! And may I say, this would be my first successful tomato crop, assuming it actually ends up producing anything real) which excited me to no end, but then I was taking a closer look at them and was dismayed to see the bottom leaves all yellow and curling up on one another.
I looked in the book, and lo and behold! discovered several possibilities. I'm going to try all the solutions, hopefully they will work. First I added fresh compost around the bottom of the plant. Then, in case they don't have enough nitrogen, I added fresh coffee grounds. In case they're low on magnesium, I mixed 2 tbs. Epsom salts (which we had from a science project) into a gallon of water, mixed well, and added to the plants, also spraying it onto the leaves. Finally, they could be low in iron, so I am now steeping rusty nails in a bucket of water, which tomorrow I will use to water them. Yes, rusty nails! You can even "plant" rusty nails into the soil next to plants to add iron....
I also learned that next year when I plant tomatoes I should wrap the root ball will banana peels because tomatoes and roses LOVE banana peels....honestly, there are so many good tips that maybe I'll put one in now and then. By the way, on the coffee ground front it also said that if you have leftover coffee, it's great to use for plant water once it's cold--but I never have leftover coffee, so I'll just use the grounds!