A crop rotation is just following say, a bed of tomatoes, when it comes out in October, with a fall cover crop, perhaps a handful of small fava beans and oats, or that nifty cover crop mix from the Bellingham WFC. The solanums (tomatoes, potatoes, peppers, eggplants, tomatillos and wild relatives including the nightshades) are the worst for building up diseases and pests in the soil, followed by the cole crops (broccoli, cabbage, kale, radishes, cauliflower, turnips, etc) and the alliums (onions and garlic.) Other groups, such as lettuce, chard, squash and corn, are less sensitive. Tomatoes followed by tomatoes next year in the same place is always a bad idea.
Ideally, the rotation would be five years between plantings from the three worst offenders (solanum, cole, allium), but an urban gardener is hard pressed to get three years.
Different kinds of plants use different nutrients. Seattle Tilth advises putting generous compost on the soil, and then a rotation of leaf, root, flower, fruit. A green leaf crop (broccoli, lettuce) uses the nitrogen from the compost application. Following the leaf crop with a root (beets, parsnips, turnips), uses more potassium. The "flower" stage is really a cover crop, such as buckwheat, phacelia tanacetifolia, Mexican marigold or calendula. All are bee forage in flower. Buckwheat is edible as greens and grain, Mexican marigold is nemotodicidal and calendula is both edible (as salad greens) and medicinal. The fruit crop (tomatoes, squash, peppers) uses the phosphorus. Fruit crops don't need as much nitrogen as leaf crops, lest they get rank and overgrown without producing lots of those yummy fruits. Growing out the Mexican marigolds to ten weeks, then turning them under and planting tomatoes is the best protection against nematodes. If you can get that organized.
Legumes (beans, peas, favas) put nitrogen back into the soil. Alliums can follow legumes but not the other way. Steve Soloman observed that planting beans after a crop of onions cause the beans to fail, one of the few "companion planting rules" that agreed with his experience.
Which all sounds neat and organized, but now we have plenty to keep track of. Walter Haugen of F.A. Farm adds grain and legume to the rotation, making it six seasons long.
One simplification is to just put the whole garden to sleep in the winter. Go to WFC or Hohl's Feed and Seed on Railroad Avenue and get a large handful of cover crop mix from the bulk bin. Harvest everything in the fall and plant the cover crop. In the spring, till it under, pull out the weeds, and you are ready to go.
However, putting the garden to sleep in the fall deprives us of our most cost effective gardening, the winter garden.
When I build a new bed, I fill it with composted horse poop and topsoil and plant tomatoes. New bed, no problems. The tomatoes come out in October. Now, the low input thing to do is to plant a handful of mixed small fava beans and oats as a cover crop. In the early spring, chop it down, let it decompose a few weeks and turn it under. A bad winter will kill the favas and the oats for you, and they will be easy turn under. Now you are ready for the next crop, onions perhaps, or your spring lettuce, broccoli and cabbages.
The alternative brute force solution is to go get some more compost, or topsoil, or both, to boost the soil after the tomatoes come out and plant garlic or over wintering onions (multiplier or potato onions.) Then the alliums are harvested the following summer and you are ready to plant your winter garden. Put down a quarter inch layer of rotted horse manure (buy some compost in a bag if you don't have a convenient pile of rotted horse poop sitting around) and plant your fall broccoli, cabbages, chard, beets, chicory, spinach and lettuce. In fact, if you haven't already done this, now is a good week.
When the winter garden is done the following May, you will have put tomatoes, alliums, and cole crops through the bed in two years. Time for a rest: beans perhaps, or that lovely blue flowered phacelia, or summer squash. Then a nice cover crop over the winter, dug in for green manure in the spring, and you can repeat.
Potatoes will volunteer from tiny drops that escape harvesting. The ruthless gardener pulls up these volunteers, perhaps finding a few new potatoes to go in the soup.
If you have a crabbing license, or know someone who does, you can gather seaweed to put on your garden beds. Rather smelly, but great for building trace elements in the soil.
Recently, I had to fix a side sewer, or lateral. That would be the pipe going out of the house to the city sanitary sewer in the street. The lateral, when dug up, was impressively collapsed wrought iron and cast cement pipes from the way back days. The cement pipes came out in pieces. There were red compost wriggler worms and pill bugs four feet underground. That's a bad sign, because red wrigglers and pill bugs are decomposers and there should not be nutrition for them coming out of the lateral. My inner Geek Engineer said, "Way Cool! A lab exercise for that Utilities Engineering course I took in the Army Reserve!" My inner Small Business Owner said, "Oh. Dear. The next two weeks of my life are going to vanish into a hole."
So it proved. After making alternate arrangements for basic domestic and personal functions, that still left the question of what to do in the middle of the night. At least half a dozen gardeners have sworn me to secrecy and told me their garden booster recipe: pee in a bucket. Add water 10:1 or 20:1 and pour around roots. I have a five gallon pickle bucket with a lid that I got from a deli (ask nicely) that I use for brewing Smelly Gardening Concoctions. The very next day after applying the Secret Garden Booster, the whole bed of winter kale, beet and spinach seed had popped up and was going strongly. Well now.
Sour Milk Booster
Take that jug with the last bit of milk in it that's gone off, and fill with water. Dump around plant roots. Fill the jug with water and dump it again. Now the jug is ready to recycle or reuse. Summer fruits, such as squash and tomatoes, are particularly subject to blossom end rot caused by calcium deficiency.
Cut off the whole top of the plant, leaves, flower stalks and all. Chop roughly and put in your bucket with water to the top. Leave sitting around for several weeks. Use a bucket with a lid, because it reeks. Pour around plants.
If you don't have comfrey in your yard, don't plant it, get some leaves from someone on South Hill. The stuff is a weed and takes over gardens. I had some in a pot and it kept going through the drain hole of the pot and setting new plants.
Nettle and Dandelion Tea
In spring the the nettles and dandelions bring up nutrients from the subsoil. Pull a generous amount and let brew in your bucket until the plants are soft and have lost their recognizable forms. Also works with grass clippings.
Fill bucket about a quarter full with rotted manure, any sort, add water, let brew. Those with a biodynamic bent may wish to aerate the bucket by determined stirring or a small aquarium pump. Casual gardeners like me will just leave it sitting around for days and pour it on the beds. Adding some liquid kelp just before using will boost the nutrients. Or you could just throw in some seaweed with the manure. And keep the lid on while it brews, eh.
Rabbits are a one day composting system, the only poop that can go straight on the garden without composting first. Perhaps you can talk your child into a pet.
Phacelia tanacetifolia and Mexican marigold seed: Seed Savers Exchange, the retail outlet, no need to join.
Multiplier onions (Dutch yellow shallots) and garlic: Territorial, Peaceful Valley Organics, or ask your farmer for some planting stock.
Potato Onions (A. cepa var. aggregatum): Maine Potato Lady. Potato onions form a cluster of large, hard underground bulbs. Save the best ones and replant the following year.