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THECOMMERCIAL GREENHOUSEGROWER • JULY 2017
GROWINGMEDIA •
to test its commercial effectiveness by usingbespoke
blends, created from readily availablematerials, in a range
of crop sectors includingmoduleproduction for field
salads, soft fruit production, protectedornamentals and
nursery stock.
Now entering its third year, Project CP138: Transition
to responsibly sourcedgrowingmedia usewithinUK
horticulture, has already demonstratedwhy coir is such
an effectivepeat alternative:When its physical properties
areplottedon a three-dimensional graph, it is not too far
away frompeat in terms of itsmake-up. “Overall we have
around twenty industry partners, includingour grower
trial sites,” explainsDr BarryMulhollandof ADASwho is
leading theproject.
He explained that the first stepwas todevelop a series
of standard testswhich couldbe applied to all growing
media, frompeat togreenwaste composts andbark,
inorder toprovide a commonway of analysing and
comparingdifferentmaterials. From this amodel to
predict howdifferentmaterials perform has been created
and this, in turn, is being used todevelop newblends
which should have specificproperties for different uses.
“Thephysical characteristicswere the key todefining
the growingmedia,” he explained. “The theory is that our
[first] prototypeblendswill workquitewell as they are
close topeat in terms of their properties.We’re trying to
find the sweet spot for the growingmedia components
themselves.”
Having startedout looking for particular attributes, the
project is also looking to seewhat effect othermixes
of different ingredients couldhave indifferent growing
sectors. “We’re going to have a higher number ofmixes
with low replication to see if we’vemissed anything,”
commentsBarry, describing the next phase of theproject.
“We are having togobackwards togo forwards.”
and nutritionprogrammes,” stressesDeanStafford,
ICL technical area salesmanager for theNorthWest.
“Commonly, peat-free growingmediamixes contain
proportions of coir, bark andwood fibre. As such they are
freer draining necessitating a ‘little andoften’ watering
regime andgreater vigilance. Peat-free growingmedia
cangive the appearance of beingdry on the surface,
while remainingmoist below and a failure toobserve this
can lead toover watering, which in turn causes nutrient
deficiencies due to leaching and fungal pathogens.”
Peat-freemixes also tend to have higher pH andEC
levels, aswell as higher rates of degradability. These can
be addressed and compensatedby changes in the levels
and compositionof thebase fertilizer and ICLproduce a
number of products for this purpose.
At the endof theday, growers and thewider industry
need to remember that theplant’s requirements should
drive the choice of growingmedia, rather than vice
versa. “The rawmaterial used ingrowingmedia is there
to support theplant, providewater and nutrients, and
provide air to the roots, stressesDr JimSmith. “Peat
is still themost adaptable and cheapest rawmaterial,
but bark is the next best andwithproper screening and
nitrogen adaptation canbe used as an alternative topeat.
Coir is goodproviding the sodium chloride is removed,
but cannot provide the structure neededwheremore
openmixes are required.Wood fibre is popular, but care
needs tobe taken tomix it evenly and it canonly be used
up to 30per cent. Greenwaste canpose a risk in terms
of its conductivity and [potential] traces of herbicides.
If greenwaste canbe obtained from a consistent single
source then it could have somepotential, but as it is
heavy the source needs tobe close to a growingmedia
manufacturing site - this is generally not the case.
Roadside collectionof greenwaste is too risky for
professional use.”
Toprovide a scientificway forward, Defra, the industry
andAHDBHorticulture have funded a five-year project
todevelop a newmodelling tool for growingmedia and
Digitalis albagrowing inMelcourt’s conventional Sylvamix
Pottingmediaon the left and theorganicSylvamixNatural
on the right.
Dr BarryMulhollandof ADAS.
