Duckweed Culture

Duckweed culture
The duckweed tank has an approximative surface of 31,6 m2. The tank has been roughly divided
in 10 sections of approximately 3 m2 each with the help of bamboos (see fig.1 and 2).
Fig.1 tank is divided in equal surface sections
of about 3m2 with bamboo sticks.
Fig.2 schematic division of duckweed tank
The optimal density for duckweed is about 600g/m2. The density should be checked at least once
a week with a calibrated mesh of 0.25 m2.
If the duckweed is at its optimal density, it should be harvest at about 100 g/m2, which corresponds
to 2 calibrated mesh per section of 3m2.
Fertilization
The required fertilization is given in the table 1 bellow (from Skillicorn et al., Duckweed
aquaculture- a new farming system for developping countries):
Table 2- nutrient content of manure from different animals
At the farm, the most promising fertilizer seems to be the feces from chicken. The composition in
terms of nitrogen and phosphorous is given bellow.
Apparently, in case of plants, chicken manure should be dried in the sun before being used in order
to kill potential pathogenic bacteria.
However, it might not be needed in case of fertilizing water pond.
Chicken manure is usually directly poured into fishpond to fertilize the water and create a
microalgae as well as zooplankton. Zooplankton are indeed eating both the microalgae and the
bacteria ( ftp://ftp.fao.org/fi/cdrom/fao_training/FAO_Training/General/x6709e/x6709e06.htm ). In
case of duckweed however, there is no microalgae as the duckweed prevents the microalgae to
make photosynthesis. However, there are still zooplankton in the water that can presumably eat
bacteria contained in chicken manure.
The requirements given in table 1 are not completely satisfactory as they disclose a fertilizer
content per unit of surface whereas, the requirements should be rather measured per liter of water.
Another book from the FAO ( http://www.fao.org/Ag/againfo/resources/documents/DW/Dw2.htm )
gives an approximate nitrogen requirement of 20-60 mg/l, i.e. 20-60 ppm. A level of total nitrogen
above 60 mg/l is however toxic for the plants.
Given the facts that the tank is about 0.23m ( 9,5 inches ) deep with a surface of 31.6 m2, it has a
volume of 7.268 m3, i.e. 7268 liters.
The total content of manure from different animals is given in table 2 above. The value of 4,6 %
nitrogen of oven dried manure has been arbitrary chosen for the calculations. Therefore an
adjustment may be required.
Tables 3-5 have been drafted based on table 1 from Skillicorn et al.
Nitrogen
Dried chicken manure (g per kg) 46
Requirements (ppm) 60
Total amount of required nitrogen in the
duckweed tank (grams)
0,06×7268=436
Max amount of chicken manure to be added in
an non fertilized tank (grams)
100×436/4,6=9480
Required fresh
daily production in
31,6 m2 (grams)
Required fresh
Daily production
kg/ha
2821,5 892,879746835443
Table 4 – daily maximum required production based on replacing 10% of 59 oz feed
Table 3- requires chicken manure to fertilize the duckweed pond at 60 ppm
FAO ( http://www.fao.org/Ag/againfo/resources/documents/DW/Dw2.htm ) indicates a simple way
to check if the duckweed plant requires more nitrogen in the water. Indeed, there is a reverse
correlation between root length and Crude protein content in duckweed.
Therefore, if the length of the roots exceed 10 mm, it means that the duckweed requires more
nitrogen.
Fig. 3 – Relationship between root length and
protein content in duckweed (Lemna minor)
(Le Ha Chau 1998)
Fertilizer Daily requirement
(kg/ha)
Daily requirement
kg (31,6 m2 pond)
Daily requirement
oz
Daily requirement
lb
Urea 18,0 0,1 2,0
Nitrogen
equivalent (kg/ha)
with nitrogen
content of urea
45% (Skillicorn et
al. )
8,1 0,0 0,9 0,1
Chicken manure
equivalent (4,6%
N oven dried)
176,1 0,6 19,5 1,2
Fig.4 – differences in root length between
duckweed from poorly fertilized duckweed tank
and duckweed from aquaponics with few plants
(01-11-2015)
Table 5 – daily chicken manure requirements (Skillicorn et al)
Collecting duckweed
When a system is properly fertilized, with optimal conditions duckweed can reach up to 600g/m2
(Skillicorn et al.). In such a system, about 100g/m2 can be harvested everyday, i.e. about 1/6 of the
surface every day.
Therefore, in order to know the amount that can be harvested, one need to know the density. I
prepared a grid at the farm that is about 0.25 m2, i.e 1/4 of a 1m2. Therefore, once in a while,
density should be measured by weighing the duckweed collected in the grid and multiply by 4 to
know the density.
Based on this result, and also later on based on experience of the skilled person, we can choose to
harvest either one or two grids per section each day.
Given the facts that every section is about 3m2, it means that at most, two grids per day per
section can be harvested every day.
Feeding duckweed to the fishes
As discussed above, about 10% of the daily feed can be replaced by duckweed without any impact
on the growth of tilapia.
For the commercial tank, the idea is to use on a daily basis duckweed as a replacement for 10%
feed.
For that, one can used a standardized container. Each tank is supplemented with either 1 or 2
tapas of feed per tank twice a day.
Therefore, for each duckweed added, the operator can remove from the tapa the equivalent of the
feed, i.e. 10% of a tapa.
Fig.5 – 0,25 m2 grid to collect duckweed
Nicolas Granet, 04-12-2015
Fig.6 – standardized container for
replacing feed by duckweed
Fig. 8- duckweed equivalent for
one “cucharita” in the nursery
Fig. 9- Duckweed equivalent of
one “cuchara” of feed
Fig. 7 – comparison between duckweed
volume left and its equivalent in feed (right)
Método para utilizar duckweed
Cada día:
Controlar los raíz
Mesurar el pH.
Encender la bomba
Si el pH está arribar abajo, checkar el amoniaco
Si hay ammonia dejar el sistema para un día, y checkar el día próximo si el duckweed utilizó el
ammonia. Si no, utilizar ácido o básico para cambiar el pH.
Si no hay ammonia, utilizar ácido o básico perro no fertilizar el Tank
Si el pH está entre 6 y 8:
Poner como una mitad de cubeta de caca de pollo seco (ver foto abajo) y poner la otra mitad en el
tank de la mosca negra.
Si los reís están siempre demasiado grande, poner más de caca de pollo el próximos día
Tomar entre una tela y dos telas en cada parte del tank (ver foto abajo)
Mesurar la cantidad que está recoleto y dar a los pescados con la tapa a duckweed (ver foto
abajo) y volver la cantidad equivalente de la comida
Información: si la botella azul es perdida aquí está el equivalente in onces (oz):
100 oz de duckweed mochada son como 6 oz de comida
Cada semana:
Pesar ( en gramos) la cantidad recoleta en la tela y multiplicar para 4 par saber la densidad para
metro cuadrado. Si está entre 400-600 g/m2, se puede volver dos telas para cada parte. Si está
menos que 400g/m2, volver sola una tela para parte.
Mesurar el ammonia, nitrito y nitrato
Nicolas Granet, 04-12-2015