Underplanting to maintain and increase fertility

I was under the impression that the nitrogen collected by beans/peas/etc was in "white nodules" attached to the roots, and was released the following year as they decomposed? Hence why brassicas follow legumes?

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But then you'd have brassica following brassica?

I have sandy soil and drainage ditches under each bed

Not knocking you, was just inquiring.

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Ok, I'm on track now, I missed the squashes part

Be interesting to see if it works.

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On countrywatch yesterday they visited an organic farm and their crops all were undersown with clover. It improves yield and the soil structure, when it is ploughed in, plus it stops unwanted weeds from growing.

How easy it would be to manage on an allotment, is another matter. Can just imagine the old boys' faces 

If its sweetcorn you're thinking of underplanting, you're not too early to get some peas or mange toute going. These  will find their own way up sticks you put in for them and also cling onto the sweetcorn for lateral support.

Start them off in toilet rolls and slot them in with a narrow trowel when they are tough enough to start going out alone.

I think green manures come into their own on beds containing squashes and the like, where other plants just wouldn't compete, or apparently most brassicas (not swede or turnip as they don't like root competition) benefit from a nitrogen fixer underplanting.

Fascinating stuff and as mumofstig says - just watch out for the old boys saying your beds are choked with weeds! Makes it worthwhile just thinking about that alone!

How long before it grows - mine have always shown through in a week or two. If they get too thick or tall and start competing you can just shear them down and they'll grow again (most legumes), making a rick carpet holding nutrients and moisture in the soil.

When do you dig it in? Well i hoe it off and let it rot down before the next crops. if it is an overwintering crop and potatoes are coming up behind, I just cover the manure crop with newspaper and deep layer of compost and the green manure just rots underneath making a great bed to plant your seed potatoes in. New potatoes then grow in the compost and not the soil; coming out clean and pest free. Wonderful.

I have trefoil growing under my brassicas. I aim to do the same for the sweetcorn which are in pots ready to replace the broad beans.

I have courgettes underplanted under my Borlotti Beans so somewhat reversing the process but the effect is the same.

I'm on sandy soil so the aim is to get as much organic matter into the soil as possible.