Is rotation of crops necessary?

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 these plants are dug into the soil after the peas and beans have been cropped. 

I use 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 and this bed is devoted mainly to sweet peas.  I get a good crop of cut flowers during the year and then can dig in the plants as green manure in the autumn.  

Crop rotation helps to improve and maintain the friability of the soil and in improving and maintaining the organic content of the soil. When  potatoes are grown some people put mulches of comfrey leaves or manure along the rows. When the potatoes are lifted this gets incorporated into the soil improving it 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. I think that it 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:
All the soft fruit - blackcurrent, blackberry,raspberry, gooseberry, etc.  However, I would not plant them in the same place if I were replacing them because they could be affected by soil sickness.  Raspberries Rubus idaeus is particularly prone to soil sickness.  In order to avoid this I  completely change the soil but if you cannot do this it would probably be best to replant in a different area of the garden completely.
Any of the perennial herbs like thyme,mint,bay and sage.
Rhubarb

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.)

Bed 2: Peas (Douce Provence, 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.  

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

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

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

Bed 6 Potatoes.

This is my final crop rotation plan.  I doubt if I will change it again because everything fits in well.

I have kept the runner beans in the same place for many years but now it is 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
This is the way that I do it.