How to build a floating farm?

Question

This is one of a series of questions also discussing mosquito prevention and city building.

There is a civilization indigenous to an area like the whitewater flooded forests of the southern half of the Amazon basin. This civilization is in the Bronze age.

The forests are in a six month wet-dry cycle, with water level in forest going up and down by about 10m. In the 'dry' season, the people plant maize, beans, and sweet potatoes on terra firma on the edge of the floodplain as soon as the water levels drop. In the 'wet' season, these people will make floating gardens in which they plant arrowroot, leren, and runner beans (as well as vegetables, like tomato and chilis).

The selected of plants will need about 1 foot of soil to grow, preferably 2 feet. The floating farm would be filled with soil each year as the water levels rise, and then emptied during the dry season. A re-usable structure would be best. These floating farms are kept in the flooded forest area outside of the main streams, 'moored' to various tall trees; Acai or Kapok are often planted for this purpose. Stream flow rates are negligible for most of the season, its more like a lake, so the structure does not have to be that strong.

My question is: how would a floating farm be built, and how large would these farms be?

Answer 1

The Mayan created "floating" farms called Chinampa. But you want real floating farms to handle flooding, the good news is they already exist and have for centuries. Actual floating farms to deal with massive flooding has been used extensively in Bangladesh for centuries.

First you make a bed of reeds and bamboo and a thin almost sprinkling of soil followed by another layer of reeds and farm similar to hydroponics. You can read a more detailed account of how they are made and used here. http://www.irinnews.org/report/90002/bangladesh-spreading-floating-farms%E2%80%99-tradition

http://www.fao.org/climatechange/17849-0e277b46b31f98942e6bc81bb22319243.pdf

and a great cross section here http://practicalaction.org/floating-gardens

Note that besides vegetables floating platforms also make for very good fish farms.

Answer 2

UPDATED

For Bronze Age (as per OP):

Mostly a suspended platform from a great height for growing the runners/crops can be an option, for example using a suspension rope/bronze cables between a valley. Floating is out of the option for BRONZE age unless you are writing a fictional historic novel that contains some sort of magic/voodoo or if someone from the future is travelling back to B Age and giving you advanced tech.

Also i feel answer by @John is completely valid and should be an accepted answer

ALSO : JUST IN CASE ANYONE NEEDS IT FOR FUTURE AGE OTHER THAN BRONZE AGE

You could make a MAGLEV farm field, but before that you need to drain proof your field that would need a fail safe plan.

By drain proof i meant to make the field effectively transfer liquids all over the field without leaving an inch of it partially wet/flooded. To make the farm environment friendly you could power it by using renewable power resources.

According to the query :

these people will make floating gardens in which they plant arrowroot, leren, and runner beans (as well as vegetables, like tomato and chilis).

Seems perfect to me because runners and crops are light weight and there would only be small payload on levitating gardens/farms unlike big trees.

A small example is given below, for now its a show off/ display material but for future earth/mars this could be a viable solution :)