Yellowing leaves is commonly caused by too much water. Light green leaves indicate a lack of nitrogen. Light green tomato leaves for instance, will turn a darker green within a week after a good dose of fertilizer.

No, all the leaves won't re-green after adding nitrogen.

That said, please post images to that we can just what sort of yellowing your leaves exhibit. Until then, all responses are just guesses.

Here's the zucchini plant with the yellowing leaves. It is in a 2x4 raised bed that gets about 5 hours of direct sun and 2 hours of dappled sun/bright shade every day. I know that isn't ideal but my yard is extremely small and this is the sunniest spot.

That yellow leaf is finished. A balanced fertilizer will help with the new growth. Have you fertilized much already? Not sure where you are located but be careful with the watering. Can't be avoided here in the east though.......lots of rain.

Yes, nitrogen is one of the few nutrients where symptoms reverse after the nutrient is added (although not in senescing leaves). Pale green leaves will green up all over the plant after N is added. It usually takes 3 days after chemical fertilizer, so probably one week after blood meal.

Hi this could be a Magnesium deficiency. This also causes yellowing of leaves esp in tomatoes and cucumbers. It can be fixed with Epsom salt. Just add two tables spoons to a gallon of water and give your plants a drink. You can pick up a box at the grocery store for about $2.00. Also you can use it as a foliage spray. Hope this works and you get loads of cucs.

Will the leaves re-green? Depends on how far gone they are. Snip off that yellow leaf. It's done. Newer growth should look much better. Give them Epsom salt drink every three weeks or so. Your plants will reward you for it.

If your concern is the lower leaves of squash family, potato, tomato, ...then those are normal. Plants abort theirs lower leaves, and even branches, when they are no longer functional or needed.

Judging from the squash picture, it looks pretty normal. But a light doze of top dressing with an ALL PURPOSE fertilizer can help with color and vigor.

Thanks everyone. I'll give it a few more days with the blood meal then try some Epsom salts. The bed has dried out a little as well after my son "helped" water. I'll follow up with an all purpose fertilizer next time.