Answer: There are several things that could cause older leaves to yellow. Too much water, even for a few days, can cause root death and yellowing of some leaves. If your soil has 12" of well drained, well prepared mix, waterlogging shouldn't be a problem. However, you might try digging down 6" or so and see what you think.
Another cause of yellow leaves could be a lack of nitrogen or a nutrient imbalance caused by high or low levels of various nutrients or a pH way out of range. Contact your county Extension office for information on having your soil tested.
Since this is a problem on many crops, I think the likely culprit is one of these cultural problems. I doubt the New Guinea impatiens are very happy with the hot afternoon sun. Is there another location you could choose?
One other possible cause is the presence of nematodes or some other root damaging pest. Dig up a plant and examine the roots for swollen areas, decay (brown rather than cream-colored roots), or severed roots (from soil insect feeding).
If these investigations produce more information on symptoms, submit another question through the website and we will be happy to try to be of more assistance.
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