As
a

general


guide,


flow


rates


for


each

gully

should


be

1

liter

per


minute.


At


planting,


rates


may


be

half


this
and
the


upper

limit


of


2L/min


appear

about


the


maximum.


Flow

rates


beyond


these


extremes

are


often
associated

with


nutritional


problems.


Depressed


growth


rates


of


many


crops


have

been


observed

when
channels

exceed


12

meters


in


length.


On


rapidly


growing


crops,


tests

have

indicated


that,


while


oxygen


levels
remain
adequate,


nitrogen

may


be

depleted


over


the


length


of


the


gully.


Consequently,


channel


length


should
not

exceed


10–15


meters

.


In


situations


where


this

is


not


possible,


the


reductions

in


growth


can

be

eliminated
by
placing

another


effluent


water


feed


half


way


along


the


gully

and

reducing


flow


rates


to


1L/min


through


each
outlet.
Passive

sub

-

irrigation
Passive

sub-

irrigation,


also


known


as

passive


Aquaponics,


is


a

method


where


plants

are


grown


in


an

inert
porous
medium

that


transports


effluent


water


to


the


roots


by

capillary

action

from

the


fish

tank


as

necessary,
reducing

labor


and

providing

a

constant


supply


of


water


to


the


roots.


In


the


simplest


method,


the


pot


sits

in


a
shallow

solution


of


effluent


water


or


on

a

capillary

mat


saturated

with


effluent


water.


The


various


Aquaponic
media

available,


such

as

expanded


clay


and

coconut


husk,


contain


more


air

space

than


more


traditional
potting

mixes,


delivering


increased


oxygen


to


the


roots,


which

is


important


in


epiphytic


plants

such

as

orchids
and
Bromeliads,


whose


roots


are


exposed


to


the


air

in


nature.


Additional


advantages

of


passive


Aquaponics
are

the


reduction

of


root


rot


and

the


additional


ambient

humidity


provided


through


evaporation.
Ebb

and


flow

/


Flood


and


drain


sub

-

irrigation
In

its

simplest


form,


there


is


a

tray

above


a

reservoir


of


effluent


water


solution.


The


tray

is


either


filled

with
growing

medium

(clay

granules


being


the


most


common)


and

planted


directly;


or


pots


of


medium

stand


in


the
tray.

At


regular

intervals,


a

simple


timer


causes

a

pump


to


fill


the


upper

tray

with


effluent


water,


after


which

it
drains

back

down


into

the


reservoir.


This


keeps

the


medium

regularly


flushed


with


effluent


water


and

air.


Once
the

upper

tray

fills


past


the


drain


stop


it

begins


recirculating


the


water


until


the


pump


is


turned


off


and

the
water

in


the


upper

tray

drains


back

into

the


reservoirs.
Media
One

of


the


most


obvious


decisions

Aquaponic


farmers


have

to


make


is


which

medium

they


should


use.
Different

media


are


appropriate

for


different


growing


techniques.
Pozzolanic
Lassenite
Sedimentary
rock


medium

that


consists

of


the


fossilized

remains

of


diatoms.


Pozzolanic


lassenite


is


extremely
high

in


Silica


(87

-

94%),


an

essential


component


for


the


growth


of


plants

and

strengthening

of


cell


walls.
Pozzolanic

lassenite


products


include

Diahydro


and

Higromite.
Expanded

clay


(Ex-

clay)
Baked

clay


pellets,


also


known


under

the


trademarks


'Hydroton'


or


'Hydrokorrels'
or

'LECA'

(lightweight

expanded


clay


aggregate),

are


suitable


for


Aquaponic
systems
in


which

all


effluent


water


are


carefully


controlled.


The


clay


pellets


are
inert,

pH


neutral

and

do

not


contain


any

nutrient


value.
The

clay


is


formed

into

round

pellets


and

fired


in


rotary

kilns


at


2192


F°.


This