First off, different varieties of potatoes are better suited to baking. The russets, as grown in the United States and other areas typically do have a higher starch content or solids than do reds, and most of the yellow varieties, which are typically moist when boiled or baked. However, even within those russet varieties some potatoes may have more water content depending on where they are grown. For example, in Idaho, during the potato growing season we only receive 10-12 inches of rain. We have the advantage of being able to use irrigation from underground springs or nearby canals to add in moisture as needed rather than depending on Mother Nature to help us out. In my travels to other potato growing states, it is not unusual for the potatoes to get that much moisture in one or two rain storms. If the potato can’t shed that moisture it will absorb some of it and over a period of time will end up with lower solids. Soil or dirt has a lot to do with the solids content of the harvested potato. Potatoes grown in clay or very dense soil have a tendency to not be able to get rid of the moisture versus potatoes grown in sandy soil or volcanic soil; which is what Idaho soil is.
I spent some time in the San Francisco area in my youth… near earthquake faults. The earthquake scales go up in small numbers, but every step up can make a major difference in how much the earth shakes and the damage occurs. Think about potatoes using this scenario. At 16-18% solids, a potato is fairly moist and would be considered low solids for baking. At 21% you’ll get a beautiful baked potato, dry and fluffy inside. Above that, say 23-24% the potato may be too dry or make a fresh fry that breaks apart. So, the differences are not much in variation of the numbers to end up cooking up dry or moist. By the way, 21% solids means the potato still contains nearly 80% water. There’s a quick test for high solids… click on this link from a previous Dr. Potato posting: Idaho potatoes and “specific gravity”.
The last part of your question…watery potatoes are not really a result of storage. If a potato is stored without humidity it will dry up (shrivel) over a period of time because it is losing moisture. Hope this information is helpful to you.
