Crop Rotation

Please could you give me some advice on the following...    I am rather confused with the rotation system, as some books suggest a 4 year cycle, others a 3 year cycle! 

Added to the confusion is the fact that I won't be growing lot of the vegetables that are used in the system to fix nitrogen, etc. For example, I grow my potatoes in  separate grow bags, to save space in the raised bed.
I don't grow any of the onion family, (as I am allergic to them), or parsnips, runner beans,turnips, swedes,squash, (we don't like them).   If I grow only the vegetables I like, it would not fit in with the 4 groups needed to use the rotation system, and so the soil would eventually become depleted of the nutrients needed.   I am thinking of  sugar snap peas, beetroot, dwarf French beans, broccoli, kale, broad beans, salad leaves, radish, and maybe courgettes,(although they could go in pots). 
Carrots I have put in pots before, they grew leaves, but no roots, as did the beetroots!  Is this mix feasible? Any other ideas and suggestions gratefully received.
Also, in a relatively small raised bed, (as opposed to a larger allotment area), is it really feasible to use the rotation system, because if it is followed to the letter, then  you are limited to a specific  group each year in the one bed, or am I misunderstanding how it works??
 
Regards,
Jane

Many thanks for your enquiry and let's have a go at de-mystifying crop rotation...   Crop rotation has been practiced in both agriculture and horticulture for centuries and plays an essential part in keeping your vegetable garden soil free from pests and diseases, ensuring the ground remains well cultivated and maintains as much of the natural soil fertility as possible.   Plant groups – usually root vegetables (with salads added), legumes (along with fruiting vegetables) and brassicas - are grown in different beds or locations in the garden over a 3 (preferably 4) year cycle to help reduce the threat of soil borne pests and diseases, arguably the most notorious of which is club-root which afflicts brassicas. If you’re unfortunate to get this in your garden through regularly planting brassicas in the same soil year after year, then you can say goodbye to growing cabbages, sprouts and the rest of the family for as long as 20 years!   Don’t be fooled into thinking rotating crops around in this fashion is a failsafe way of preventing these pests and diseases; you’ll still need to practice good garden husbandry but it certainly helps, especially if you follow a cycle in the plant group order listed above (eg. follow root veg with legumes etc).   Each group of crops will benefit from the previous; beans in the legume group will fix nitrogen into the soil ready for the hungry brassicas, which prefer a relatively firm, slightly alkaline and are followed by root crops (not to keen on nitrogen as it’s the root development we’re interested in, not leafy foliage) which require plenty of ground work – especially potatoes – and you start breaking up and mixing the soil without realising it!

Although crop rotation is still an integral part of raised bed gardening, it doesn't hold the same importance as in open soil beds, allotments and vegetable plots. That's because the soil in a raised bed is regularly topped up, replaced and generally has plenty of new material added each growing season, giving soil-borne diseases such as club-root less chance of taking hold. By all means practice crop rotation in your beds if possible, but don't get too hung up on it!   If you regularly top up your beds with organic matter (in the shape of home-made compost of well-rotted farmyard manure), prepare the ground properly for the forthcoming crop and generally feed and care for your plants well then you shouldn’t have any problems growing a real mix of plant groups in your raised beds.

Finally, with regard to your carrots; the leafy foliage and very little root growth is probably down too well-fertilized soil. Root crops don’t really thrive in what we would call fertile soil and any digging in of compost and organic matter should be done at least 6 months in advance of sowing. Try preparing a light, sandy, relatively poor soil for your carrots and if you are going to add fertiliser, make sure it’s not nitrogen heavy – our Organic Root Vegetable Fertiliser has an NPK ratio of 6:6:12 which is ideal.

I do hope this crop rotation information proves to be of some help but if there’s anything else you’d like to know, please don’t hesitate to ask me. You might also be interested in a Stephanie’s Kitchen Garden update I wrote back in February 2006 which includes details of our planting plan for the coming year; in the meantime, many thanks once again for your enquiry and the best of luck with your growing this year!

Martin