When we first started working in an allotment garden oh, 20 years ago (aauugghh!) I read a lot of gardening books and magazines. The one thing that has really stuck from that reading is our crop rotation plan.
Our plan is adapted from one put forth by Sylvia Thompson, and which is described in her book The Kitchen Garden, although it only gets 3 pages there. I think I first found it as an article in The Kitchen Gardener magazine, an excellent but somewhat short-lived venture, where the plan was detailed in a longer article. No sign of it online, alas.
What I am about to describe is more-or-less her plan, much adapted to our own personal uses. That's one of the beauties of this scheme - it is a simple, even crude, four-part division, and as long as you remember the logic of the plan quite adaptable. On this blog I've previously discussed this plan briefly and often presented vegetables grouped together in the categories we use, but I don't think I have really delved into it in great detail.
There are 2 general reasons to rotate crops, even in a fairly small garden. The first is the one people tend to think of: to avoid the build-up of diseases and pests in the area. Unfortunately, rotation does not do much to alleviate this problem in a small garden. You are staving things off for perhaps 15 minutes in my experience. Still, we rotate crops partially because hope is as hard to kill as cucumber beetles, and partially because we do get the second benefit: we can plan and target soil amendments to keep our plants at the level of fertility best suited for them. Essentially, this plan groups vegetables according to their nutrient needs.
I'm being a bit facetious about the effectiveness of a rotation plan in avoiding diseases and pests, by the way. In reality while it doesn't do much to evade pests, I suspect it's quite useful in keeping the diseases down to a dull roar. Be sure however, that you are not saving diseased seeds and simply moving the problem along every year. (See our struggle with bean anthracnose.) And if you do get diseases don't put the season-end debris into the compost heap. If you have municipal compost pick up use that; if like us you have a large property, have a compost dump some good distance from the garden, from which you do not actually use the compost.
The groups that vegetables are divided into for this plan are as follows:
LEAFS are the vegetables with high nitrogen requirements. Many of these vegetables are the ones for which we use the leaves, such as most of the brassica family, spinach, chard, lettuce and most salady greens, mustards, etc. Also included is corn, which has a very large hungry plant, even if what is eaten is the seed or fruit of the plant. We grow all the brassicas here, even if they are used as roots (rutabaga, turnips, radishes) or stems (kohlrabi). Celery would be here, and we include celeriac as well. Leeks do better here than in with the roots, and other onions can be included here if the root section is more crowded than the leaf section - as you see we are making pragmatic exceptions already. We tend to have a lot of room left over in the leaf bed since we don't grow corn and brassicas are limited due to their incompatibility with our soil.
Some of the leafs go into and out of the garden fairly quickly; the smaller salady things as well as many oriental vegetables and things like rapini can produce usable material in as little as a month. That allows you to possibly plant some green manures in this section or to use part of it as a targeted compost site (i.e. just pile up your compost in a chosen spot over the summer and dig it in during the autumn.) Of course, you can succession plant instead.
We meet the need for extra nitrogen amongst this group of plants by amending each 8' x 5' section with one wheelbarrow full of either our own compost or composted manure acquired elsewhere, before planting. Our own compost is the cheapest, but maintaining a compost pile sufficient to produce the amount required is a challenge and takes quite a lot of work. When we were first gardening at this site and struggling to bring our soil fertility up from dire to acceptable, we put in very large amounts of composted manure brought in literally by the dump-truck load. Now we can maintain it with our own compost supplemented with bags of manure from the nursery when that runs out.
We target the brassica beds in particular with other nutrients. We add a little dolomitic lime to those beds as well as borax. The first helps raise the pH of our soil (lower the acidity) and the second helps with a suspected boron deficiency. Also as the season progresses, the leaf section is most likely to be treated with foliar spray fertilizers. We prefer organic ones based on fish, seaweeds, and molasses. (As you may suppose, such spraying sessions are immediately followed by laundry and bathing sessions... yeeeeeah. And we don't do it right before harvests - there would be no point since they wouldn't have time to do anything anyway.) Periodic dustings with food-grade diatomacious earth are of some use against leaf-munching insect pests.
FRUITS: are the vegetables where the seed bearing parts of the plant are eaten, with the exception of legumes which will form their own group, and corn which, as already mentioned, goes in with the leafs. Squash, melons, cucumbers, tomatoes, peppers, and eggplants are the main vegetables in this category. These will be planted where the leaf plants were in the previous season.
They need some nitrogen as most of them form fairly substantial plants, but too much will develop green leafy growth at the expense of the fruit. They also vary in their needs; squash and melons can have fairly substantial nitrogen requirements but peppers may not need very much at all. They all do need a certain amount of phosphorus. This should be remaining in the soil to an adequate degree from the amendment with compost made last year when leafs occupied these beds, but we generally add a little bone meal to make sure.
Since the fruit plants that need the most nitrogen are generally the largest, and are well spaced apart, it is easy to supply a little compost into the planting hole when they go into the ground. Just a little though! A handful to small shovelful will do, scaled to the plants' expected final size.
We used to put a little bone meal in the planting hole with the extra compost, but bone meal is attractive to certain animals (skunks? raccoons? we haven't caught them in the act so are not quite sure) but we may be sure that they are very inclined to pull up any plants that have bonemeal at the roots. It's better to sprinkle bone meal over the bed to be planted a week to a few days before planting and rake it in well to foil these pests. They will likely still come and root around for it but if it is well mixed in and the plants not yet in the ground, little damage will be done.
Epsom salts are often also added to the planting hole for tomatoes, peppers, and eggplants. Just a small spoonful should be sufficient, and mix it in a bit. Don't be put off by the name. It isn't a salt; it's crystallized magnesium and sulfur, which are useful in assisting the plants to produce blossoms and fruit. However, if your soil is not deficient in magnesium there is really no point in adding more. Our soil is sandy and acidic, which is often deficient in magnesium, but I can't say we really noticed any difference in productivity the few times we've added epsom salts. Now we don't bother.
Blossom end rot is a common problem in fruit crops. It is directly caused by a lack of calcium, however the problem is rarely an actual lack of calcium in the soil but is more related to too much water, too little water, or wildly fluctuating amounts of water. Keep an eye on the watering; most fruit crops in my opinion do best with steady but ever so slightly scant quantities of water, especially once they are on to the fruiting stage. Of course if it is raining non-stop there isn't much to be done. Although, in my experience, a tendency to blossom end rot is definitely something that can be selected against in seed saving, the same with a tendency to split. Keep that in mind during particularly rainy seasons as a silver lining. Organic fertilizers as described in the leaf section may help with blossom end rot, because they encourage the fungi which regulate mineral uptake in plant roots. It doesn't hurt to add crushed eggshells for the calcium content, but I don't think it particularly helps all that much either. If they are dried well before being crushed and scattered about they may deter snails and slugs to some degree though - which means you may actually prefer to save them for the short-assed leaf plants.
ROOTS: are the plants where the edible part grows underground, or at least at ground level. Carrots, onions and garlic, sweet potatoes, potatoes, parsnips, and beets are the main crops here - as mentioned above brassica root crops go with the rest of the brassicas in the leaf section.
Root crops in general do not like too much nitrogen. This is why they are saved for the crop which is 2 years away from a general application of compost. Too much nitrogen may cause skinny, hairy, split roots - a thing most likely to be really apparent in carrots. However when we first started in this garden we struggled with carrots. Now they do well. My conclusion is that they do like compost - provided it was applied 2 years earlier. Carrots go into a bed previously planted with peppers and eggplants, which are the lightest users of nitrogen and which were given the least in the way of planting-time compost. Parsnips and beets too, for preference.
Potash or potassium is the main nutritional requirement of the root crops. Since the previous rotations are light users of this nutrient, hopefully it is still present from the original application of compost. Wood ashes and kelp meal are recommended to boost potash if you feel it is necessary; we never have added either and our root crops seem to be doing fine. Your soil may vary of course, and if you happen to have some nice clean wood ashes a light sprinkling worked in a few days to a week before planting may be a good idea. I wouldn't go chasing after them, though.
Potatoes produce tubers in proportion to a good healthy top growth and are probably more interested in a nitrogen boost than any other root crop. For this reason, and also the fact that they are solanaceous crops, Sylvia Thompson recommended putting them in with fruit rotation. I don't do that because the fruit rotation is already the one that is bursting at the seams for us. They get their own bed and I don't mind adding a little compost for them. Do it with a light hand though; compost in contact with the forming tubers can cause fungal problems on them. Planting comfrey leaves with potatoes is often recommended and it is something I always mean to try since the neighbouring ditches are full of feral comfrey, but so far I always forget. Again, if you add it, it should not come into direct contact with the roots.
Potatoes, like tomatoes, eggplants and peppers (and tomatillos and ground cherries) are members of the solanacea family and should not follow any of those. We have 5 beds in each section; tomatoes, peppers, eggplants, tomatillos and ground cherries are all planted in the 3 central beds of the fruit section only, then in this rotation potatoes are only ever planted in the 2 outer beds. This is a rule we follow more assiduously than most.
Sweet potatoes have such lush growth they always look like they should require a lot of nitrogen, but they really don't. They come from tropical conditions where the soil is often somewhere between poor and completely lousy, and they contend with alternating torrential downpours and droughts. Consequently they are pretty tolerant to everything except cool weather.
Onions are another "root" vegetable that like a bit of nitrogen, or so I am told. Mine have always done just fine without. A foliar feed early in the season may be a good idea though. In fact onions consist of a swollen ball of basal leaves above the actual roots, and rarely grow deep in the soil unless you planted them too deeply. Leeks are more green and leafy, and I put them in with the leaf rotation. Shallots tend to go wherever I can find them some room, to be honest.
Finally, garlic is a bit of an oddball vegetable in that it is planted somewhere between late summer and fall, and harvested the next July. We put it in one of the beds that had tomatoes in it. As soon as frost touches the tomatoes and/or they have stopped producing, we remove them at once and plant garlic in their place. Then the garlic is in the right spot for the next summer. When the garlic comes out I replant the space with beets intended specifically for pickling - I don't mind if they end up a little on the small side. There are beets elsewhere in the root beds for summer eating and winter storage. Radishes could follow the garlic as well. Winter radishes in particular would work well and can be stored for a time. Yes, I usually want to put radishes in with the leafs but oh well. They are flexible enough for this. Watch your brassica positioning in relation to the radishes though, if you put them here.
LEGUMES: consist of all the peas and beans; peanuts, chick peas, lentils, and lima beans included, as well as some cover crops/green manures such as hairy vetch and red clover.
Legumes have the ability to fix nitrogen in their roots, supplying their need for nitrogen themselves and leaving some behind when they come out. Thus they are either the last crop of the rotation or the first, depending on how you want to look at it.
They will grow in quite poor soil, provided it has the fungus they interact with for nitrogen fixation. Our first bean and pea crops were very poor because our soil didn't have it. So, for the second, third, and fourth years we used a "bean inoculant" (which is actually a fungus) with our legumes when we planted them. Many soils already have the requisite fungus present, and bean inoculant can be hard to find and a bit pricey. However, our experience was that one application to each section of beds was sufficient but required and now all our peas and beans are growing well. If you are starting a new garden and struggling with peas and beans, consider using it.
Legumes are really the only set of vegetables that leave the soil with more nutrients than they found, as a result of this ability to hoard nitrogen. However, to benefit from this directly don't remove the finished plants by pulling them out - clip them off just above the ground. Otherwise their accumulated nitrogen simply moves on to the compost heap, assuming that is where you put them. That's not a disaster if you apply the finished compost back to the beds in the fullness of time, but since these beds will hold the nitrogen-hungry leaf crops next season it is better to leave it right in place for them.
With this plan, you need to allot equal amounts of space for each section, since each grouping will rotate through the whole garden in time. This can be a little hard to achieve, and a little moving of things around is tolerable to achieve the quantities and varieties you want. Just keep in mind the nutritional needs being catered to in each section, and move the plants most appropriate for the alternate section.
We have started growing and eating a lot more dry beans, and it was to some degree in response to the need to fill in space in the legume section. A happy accident, as it happens!
As you may suspect from all of this is that you really need to make a garden plan before you plant, and amend it as you go. If you have a garden of any size at all it will really help you maximize your productivity, keep track of where each plant is (what is this little green sprout, anyhow?) and if you keep your plans from year to year (I recommend it!) you can refer back to them to note the last time you grew something, and review how your garden has evolved over time.