Broad bean day

Sunday was a teaching day on the plot for me, and rather delightfully, it turned out to be balmy and sunny so students got to spend time outdoors in very clement weather, planting our broad beans!

One of the things we talked about was crop rotation and raised bed versus no bed systems of growing. I’ve noticed that people often don’t get the hang of raised bed crop rotation systems because it doesn’t get covered very much in books, and so there’s very little explanation of the downside of raised beds. Apart from the cost, there really isn’t much of an argument against raised beds, but what argument there is, can be based on crop rotation.

I’ve found from time to time that people just don’t understand that you can’t grow everything in the same kind of soil. Our great-grandparents knew this: they knew Kent was good for apple trees and Lincolnshire for potatoes, Yorkshire for rhubarb and Scotland for raspberries. But we want to grow everything well, regardless of climate and soil, and to a certain extent, we can. However, it’s important to understand the soil conditions that different plants require. You can grow fantastic sweetcorn in rich, deep soil, well-manured and high in nutrients, but plant Brussels sprouts in the same soil and they will produce flowery cabbages on the vertical stem instead of neat little sprouts, and parsnips planted in those conditions will probably fang, producing any number of roots like a wisdom tooth instead of one long, deep, straight root.

Brussels, you see, like a limed soil which is heavily compacted around their roots as both too much richness and too much wind can cause the sprouts to ‘blow’ into mini-cabbages while parsnips like free-draining soil, preferably sandy, and not too rich, or they too get carried away and try producing five roots when one would do. They’ll also fang (or fork) if they hit a pebble or rock, so you need to take all those impediments to straight growth out of a parsnip bed before you sow seed.

Most of us growing in open beds opt for a fairly neutral soil, liming a bit for brassicas, using potatoes to break up the ground where the brassicas were the previous year, and so on. But often raised beds get filled up with beautiful rich compost from day one, so they produce wonderful leafing and fruiting crops, but awful rooting ones and brassicas just refuse to play along at all in such potent soil And then the allotment holder or grower doesn’t know how to rectify the problem.

We work our raised beds on a rotation system with the top four inches of soil being taken off the bed every fifth year and replaced with fresh compost, after which they are used grow something insanely luxurious like melons for the first year. In the fifth year we use them to grow parsnips, adding quite a lot of sand to the less than rich soil, but also feeding the roots with granulated fertiliser and with a foliar feed. This means we get the maximum productivity from each bed before we have to scrape a top layer off (which is used in pots and planters) to replenish it with fresh soil. This also removes any build up of bacterial or fungal threats to our plants.

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Posted by The Allotment Blogger on Monday, November 14, 2011


Blogger Mal's Allotment said...

What a wonderful post. My plot is traditional open beds but I now have raised beds at home one of which I use as a brassica nursery bed.

Please confirm - You add granulated fertiliser and a foliar feed when growing parsnips and they really don't fork?? When I did that to celeriac it came out like a Medusa. My parsnips thrive on abject neglect, but I might be kinder to them in future.

November 14, 2011 at 1:16 PM  
Blogger The Allotment Blogger said...

Mal, they don't fork with a foliar feed and if you have 50% sand in the raised bed, they don't fork with granular feed either, because the soil fertility is so low. I wouldn't give them either if they were in open ground - I'd treat them with contempt because that's what they'd thrive on.

November 17, 2011 at 9:37 AM  

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