Rotation

I have been religiously rotating crops for about 30 years now without really questioning it properly. 

There is some indication that crop rotation does in fact improve your harvest of most vegetable crops, particularly if it includes a legume element.  

If you are rotating with a green manure as one of the crops it will improve fertility by adding nitrogen and making other nutrients available for plants to use. I like to use a mixture of grazing rye grass and tares. Rotating using peas and beans as one of the crops also helps to add nitrogen to the soil if the whole of these plants are dug into the soil after the peas and beans have been cropped.  Many people will suggest that only the roots need to be dug in but these only contain about 30% of the available nitrogen.  The other 70% is locked into the tops.  So if there is no disease on the tops, I dig these in as well.  If you think about it, farmers do not cut off the tops of clover before they plough the plants into the soil.  They plough the whole plant into the soil to obtain the full benefit of the nutrients fixed by the clover.

I have a six year rotation with 6 fairly similarly sized beds.  This might seem to be a little excessive because most examples in books suggest a three year rotation.  I have one year when very few vegetables are grown on one of the beds. This bed is devoted mainly to sweet peas.  I get a good crop of cut flowers during the year and then, as these are legumes and have nitrogen fixing bacteria in nodules on their roots, I dig in the plants as green manure in the autumn.  

Crop rotation helps to improve and maintain the friability of the soil and improve and maintain the organic content of the soil. When you cultivate potatoes some people put mulches of comfrey leaves or manure along the rows. When the potatoes are lifted this gets incorporated into the soil and improves the soil for the following crop. 
 
Rotation improves the use of nutrients through the soil by varying the length of plant crop roots. Some plants like beetroot have relatively shallow roots while parsnips can have very long roots. They can get nutrients from different depths of soil. 
 
It enables you to use manure and fertilizers more efficiently, targeting crops that need high nutrients. If these are followed by crops that need fewer nutrients then no manure needs to be added the following year. Lime need only be added to the brassicae bed (It is a source of calcium for plants but also prevents club root Plasmodiophora brassicae ) as they rotate around the allotment. You do not have to add excessive amounts of lime to the soil because calcium stays in the soil for a relatively long time as does phosphorus. 
 
It does help you control some weeds, insect pests and plant diseases. I think that I have reduced the level of club root on the allotment significantly by a very strict rotation especially for brassicas. 
 
It is said to improve the diversity of micro organisms in soil. A monoculture of the same plant growing in the same area of soil must reduce the number of different organisms that can live in that area. Plants secrete substances from their roots that maintain a large population of soil organisms.  These organisms may be specific and highly adapted to particular plants excluding others that might be as or more valuable to the gardener.  Rotation will enable a variety of different organisms to survive and will favour the generalist organisms that will aid a wide variety of plants.    

I think that rotation maintains the health of the soil and this is what all the plant crops depend on. So I would suggest that rotation is the best method of managing the allotment.

Some things that I would not rotate are:

The soft fruit - blackcurrent, blackberry,raspberry, gooseberry, etc.
Any of the perennial herbs like thyme,mint,bay and sage.
Rhubarb
Perennial nitrogen fixers like laburnum, lupin and clover.  I rotate clover when I use it as a cover crop during the winter.

2012's  rotation is:
Bed 1: Roots (carrots, parsnips, scorzonera, salsify, Hamburg parsley and beetroot)  and leaves (salad burnet Sanguisorba minor, chamomile, spinach, lettuce, chard, celery, parcel, celeriac and maybe some of the other herbs.)  

Carrots under the enviromesh

Parsnips and lettuce with salsify in the
background

Beetroot, and fennel

More beetroot and salad burnet

Celery in the background; lettuce and tansy

Chamomile flowers ready for picking to make
chamomile flower tea

The second successive sowing of Webb's
Wonderful and fennel

Soil preparation:  The old pea and bean plants are dug in in the autumn with any annual weeds.  Grazing rye green manure dug in in the spring.  The roots get some comfrey liquid when they are being sown but nothing else.  The 'leaves' get comfrey, inoculated charcoal and chicken manure. Mychorrhizal fungi is placed in the planting holes of the leafy plants.   The leafy plants will be watered with comfrey liquid every two weeks.  

Bed 2: Peas (Douce Provence, Oskar, Early Onward and Hurst Green Shaft); climbing French Bean (Trail of Tears and Cobra); broad bean (my own saved seeds); asparagus beans; mange tout  and the strawberries.  I may have some dwarf French beans as well but they did not do very well last year so I may leave them out. 

Douce Provence early peas.  

Trail of Tears climbing beans

Soil preparation:  Ground is forked over when the brassicas are taken out and chicken manure and inoculated charcoal  are put along the rows of peas and beans.  Mychorrhizal fungi is put in some of the planting holes and the peas are watered in with comfrey liquid.   The pea and bean plants will be watered with dilute comfrey liquid every two weeks.  

Bed 3 Brassicas (purple sprouting broccoli,   Brussel sprout, red cabbage, cabbage, cauliflower, calabrese, kohlrabi, swede, turnip, American land cress and rocket.)

Trafalgar Brussel sprouts

Calabrese and others.

Soil Preparation: Sweet pea and runner bean plants dug in in the autumn.  Winter tares, clover and grazing rye green manure dug in in the spring. A liberal dose of lime is put on the whole area.  The brassica seedlings are watered in with comfrey liquid.  The cauliflowers and cabbages will be watered with comfrey liquid every two weeks.  Brassicas are non mychorrhizal so this fungus will not be used on this bed.  (This year I made a Hugelkultur bed for the cauliflowers.  I will probably not do this next year.)

Bed 4 Sweet peas, runner beans and this year some climbing French beans.

Sweet peas, runners and french beans

Soil Preparation: Horse manure and tree leaves were dug in during the winter.  The seedlings were watered in with comfrey liquid and mychorrhizal fungi put in some of the planting holes.  Inoculated charcoal was put into each of the planting holes.

Bed 5 Alliums (onions, garlic, shallots, leeks) and cucurbits. (pumpkin, courgettes, squashes, cucumbers, maize, tomatoes).

Alliums- some affected by onion miner fly

Squashes, marrow and courgettes

Soil Preparation: The soil was double dug with incorporation of compost, turf, weed turfs, tree leaves, shredded branches and horse manure.  Bonfire ashes were put on the Allium area.  Chicken manure will be used as a base fertiliser.  Mychorrhizal fungi was put into some of the Allium planting holes.  The tomatoes are on a raised bed of lawn mowings covered in a thick layer of cow manure.

Bed 6 Potatoes.

Potatoes

Soil Preparation:  The soil was double dug with the incorporation of horse and pigeon manure.  Compost was also dug in but not throughout the area because I ran out.  The potatoes were watered in with dilute comfrey liquid and mychorrhizal fungi was dusted over them.

This is my final crop rotation plan.  I doubt if I will change it again because everything fits in well. Next year (2013) the vegetable areas will move on one bed down the allotment.  The potatoes will return to the top of the allotment on bed 1.  This will give me a six year rotation.

There is some benefit in planting nitrogen requiring plants such as brassicas after nitrogen fixers such as peas and beans.  The benefit comes from digging in the pea and bean plants as a kind of green manure. If the tops of the beans and peas are removed and put onto a compost heap then there is much less benefit to a following crop.  If the soil is cultivated by addition of organic matter then it does not really matter what follows what.

The idea is not to grow the same vegetable in the same spot for more than one year.  This is primarily to avoid disease and pests.  There is also a reduction in vigour of vegetables grown in the same place year after year, which could be due to a reduction in particular nutrients.  However, there may also be a build up of substances excreted by the plant root that inhibits growth.   As most plants form an association with mychorrhizal fungi, there is some thought that these fungi might produce chemicals that depress vigour of similar plants. Regardless, this soil sickness that is similar to the rose and raspberry experience when planted on old rose and raspberry beds must be due to some characteristic of the soil and this can be ameliorated by rotation and growing in new soil.

I have kept the runner beans in the same place for many years but now they are part of the rotation. I have some tree posts that are fairly easy to move and use these to make a climbing frame for the beans. 

If your ground has been left fallow for some time, I think that it would be fine not to rotate for a couple of years. However,  why wait until you have disease and nutrient depletion before you begin to rotate?

There are many ways to rotate and this is just one of them:
http://www.thegardenersalmanac.co.uk/Data/Crop%20rotation/Crop%20rotation.htm