There are at least three reasons to rotate crops.  

• To minimize pests
• To discourage Diseases
• To build soil fertility

To understand how to rotate, one must understand what families each plant belongs to.  Plants within families tend to have the same pests and use the same nutrients.  This is the best chart I have seen for identifying plant families at a glance.  It might not be easy to guess that peppers and potatoes are in the same family!

Family Name	Aliases	Members
		Crops	Ornamentals	Weeds
Solanaceae	solanaceous crops; potato, tomato or nightshade family	peppers (bell and chile), tomatoes, potatoes, eggplant, tobacco, tomatillo	petunia, million bells	nightshade, jimsonweed, henbane, groundcherry, buffalobur, horsenettle
Brassicaceae	Cruciferae; brassicas; cole crops; cruciferous crops; mustard family	horseradish, cabbage, cauliflower, broccoli, kohlrabi, kale, Brussels sprouts, turnips, Chinese cabbage, radish, rapeseed, mustard, collards, watercress, pak choi, bok choi, rutabaga	stock, alyssum, candytuft	shepherd's-purse, field pennycress, yellow rocket
Cucurbitaceae	cucurbits; cucumber family; squash family	cucumber, melons, watermelon, summer squash, pumpkin, gourds, winter squash
Rosaceae	rose family, rosaceous plants	apples, peaches, apricots, nectarines, plums, strawberries, blackberries, raspberries, pears, cherries		multiflora rose
Fabaceae	Leguminosae; leguminous crops; legumes; bean, pea or legume family	beans, peas, lentils, peanut, soybean, edamame, garbanzo bean, fava beans, hairy vetch, vetches, alfalfa, clovers, cowpea, birdsfoot trefoil, black medic		various vetches, clovers, black medic
Poaceae	Gramineae; grass family	corn, wheat, barley, oats, sorghum, rice, millet, rye, ryegrass, sorghum-sudangrass, fescue, timothy	ornamental grasses	brome, wild oats, crabgrass, orchardgrass, barnyardgrass, quackgrass, fall panicum, foxtail, Johnsongrass
Polygonaceae	Knotweed family	buckwheat, rhubarb		knotweed, smartweed
Liliaceae	lily family; alliums (for members of the Allium genera)	asparagus, onions, leeks, chives, garlic, shallot	tulips, daffodils, hosta, hyacinth, daylily	wild garlic and onions
Lamiaceae	Labiatae; mint family	lavender, basil, marjoram, oregano, rosemary, sage, thyme, mints, catnip	salvia, Molucella (bells-of-Ireland)	mints, catnip, henbit
Ericaceae	heather or blueberry family	blueberries, cranberries	rhododendrons, azalea, heather
Chenopodiaceae	goosefoot family	spinach, beets, chard, sugar beets		kochia, lambsquarters
Apiaceae	Umbelliferae; carrot family	carrots, parsnips, celery, dill, chervil, cilantro, parsley, caraway, fennel	Trachymeme, Buplerum	poison-hemlock, wild carrot
Asteraceae	sunflower family; aster family, Compositae	sunflowers, lettuce, endive, escarole, radicchio, dandelion, Jerusalem artichoke, artichoke, safflower, chicory, tarragon, chamomile, echinacea, sunflowers	marigold, mums, zinnia, aster, Calendula, cosmos, Rudbeckia, Tithonia, Centaurea, Helichrysum, yarrow, Leucanthemum, echinacea, sunflowers	dandelion, Jerusalem artichoke, chicory, echinacea, thistles, knapweeds, cocklebur, yarrow, ragweeds, goldenrod, groundsel, galinsoga, sunflowers

Originally printed in Virginia Vegetable, Small Fruit and Specialty Crops – May-June 2005.

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Virginia Cooperative Extension materials are available for public use, re-print, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.

Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Alan L. Grant, Dean, College of Agriculture and Life Sciences; Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.

August 17, 2009

http://pubs.ext.vt.edu/2906/2906-1393/2906-1393.html