Answer: Sometimes new plants take a while to acclimate to new conditions. Often, the oldest leaves (at the base of the plant) are the first to go, since the plant directs its energy to its new growth. Dieffenbachia likes medium light. A good place would be in front of an east window, where it would get a little morning sun, then light but no direct sun for the rest of the day. The plant can adapt to lower light levels, like in a north window or in a bright room but away from the windows. It will also tolerate brighter light, though full sun in a south window might be too much for it.
I can think of two things causing your "crispy edges". One is low humidity. The plant likely came from a greenhouse, where it was used to fairly bright light and high humidity so it may take some time to adjust. Be sure the plant is not near heat ducts.
Often, overwatering can cause brown leaf margins. The best way to water is to let the plant dry out so the top inch or two of soil is dry. Then water thoroughly (until water just begins to drain out the bottom). Allow it to drain well, (don't let it sit in water) and don't water again until the top inch or two is dry again. How much you water really depends on where the plant is. The brighter the light, warmer the room, and lower the humidity, the more you will need to water. What you don't want to do is water with just a little water every few days. You want to water thoroughly, then wait a while to water again. You might need to water once a week, once every other week, or even once a month!
I would not bring the plant in and out of the house. First of all, the plant needs to adapt to its growing environment. If you bring a plant adapted to the low light indoors out into direct sunlight, it will suffer sunburn! Plants acclimated to outdoor growing develop tougher leaves to withstand harsh sun and wind. If you would like to put it outdoors for the summer, choose a somewhat shaded, protected spot, and move it outdoors gradually, a few hours a day at first, to give it time to adjust. When you return it to the indoors in the fall, you'll want to inspect it carefully for insect pests first.
Hope this helps!
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