Last year I grew my runner beans in a large plastic container and they were very successful. I was once told that where runners have grown the soil is very good and can be used again as apparently the beans add something to it. Can someone tell me if this is indeed true as if so I could plant this years beans in it. I am a bit dubious, Thank you.
"In a four year crop rotation plan runner beans are included with others of the bean (legume) family. In reality however they cause very little build up of problems in the soil and can be grown anywhere. For convenience sake many gardeners erect permanent supports for runner beans and grow them in the same position year after year with no ill affects."
http://www.gardenfocused.co.uk/vegetable/runner-beans/grow.php
I on the other hand like to rotate crops and grow the beans in new soil each year. But I have a veggie patch and room to do this. You are likely to have success with your tub and if I were you I would top it up with some new soil and go for it.
Not just yet though, the weather needs to be warmer.
When the beans are grown in the ground the roots penetrate quite a way down and help to keep the soil nitrogen rich.
But if you have grown them in pots, the best thing you can do with the soil is throw it on your compost heap. The soil or compost you have used will have had all it's nutrients drawn out of it and will be spent.
If you want a good result this year change your soil or compost for some fresh stuff.
Not long now to start sowing your beans ... about a weeks time for me !