Kitchen Garden: Crop Rotation - Some details (why required, what happens, and so on)

The previous post talked about crop rotation in a brief way, without going into too much detail about why this is required, which are the types of crops for which this is needed, and so on. So, here are some more details.

The whole purpose of rotating crops is to:
1) Ensure that harmful pathogens (insects or disease) specific to the crop do not build up in the soil over a number of years, and;
2) To ensure that soil does not get depleted of specific nutrients due to some plants that are heavy feeding. It is an integral management tool for organic farmers, and homeowners with large vegetable gardens.

Protection against pests and disease: If you want to rotate your crops, and want to avail the best benefits in doing so, make sure you choose plants from different plant type over successive years, and so to be not closely related, else you lose the benefit. If you rotate broccoli and then cabbage, you are not likely to get away form insects and diseases that tend to zero in on the Cruciferae family (to which both cabbage and brocooli belong), and your crop rotation won't do much good, because they are both attacked by the same types of insects and disease.
Now onto the other major reason for crop rotation, being able to fix nutrients.

Adding to nutrients in the soil: There are an awful lot of people who have not done enough research to find out more about crop rotation, in terms of both crops that use up nutrients, and those crops that add nutrients to the soil. For example, if you consider the Fabaceae family of plants (legumes, beans, peas, lentils, alfalfa, clover, caraganas, honeylocusts, etc.), they have a very useful ability to add nitrogen to the soil, under certain conditions. This is a process whereby they are able to extract nitrogen from the air and turn it into the soil (actually in special nodules along the roots) in a form that's usable by plants.
The contra position for these plants is that one should not add nitrogen to the soil if these plants are being grown, since an excess of nitrogen would be harmful for these plants. They are very useful if you have soil that is deficient in nitrogen, and are used by knowledgable gardeners and enthusiasts, especially since these legumes remain in the soil even when the plants die. The idea is by rotating crops all around the garden, section by section, one can spread the ability to enrich nitrogen around the garden.
How does this actually happen ? This is also called "inoculation". Inoculation defines the process by which members of these specific plant families are able to fix atmospheric nitrogen, and this is done in collaboration with a set of bacteria that attach themselves to the roots of these plants. These bacteria are essential to do this fixing. Most decent garden soils already have these necessary bacteria in them, so usually it's not an issue. However, if you don't have some "inoculants" in your soil, you can buy them and put it in the soil. These are mixed into the soil when planting happens, to ensure that the plants will have the proper bacteria when they develop their roots so that they can fix their nitrogen and thrive.

When you read all about the plants that help in increasing nitrogen, what about the plants that suck up all the nutrients from the soil. One of the main families is called nightshades (Tomatoes, peppers, potatoes, and eggplants).
The plants belonging to the nightshades family are part of some of the biggest starvation incidents in history. They are equally famous for being the plants that encourage pests, soil infections, and disease. If you grow them in your garden, then don't grow them in the same soil until other crops have been grown that give the nutrients back, and not before a period of 3-4 years.