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ask the experts
Q
I have been experimenting using recirculating flood and drain, maintained water temperature at 26°C,
pH of 5.5-6, and using the Douglas Peckenpaugh formulation, which is:
N=215 ppm, P=86, K=343, Mg=85, Ca=175, S=113, Fe=6.8, Zn=0.25, B=0.7, Mn=1.97, Cu=0.07, Mo=0.05
EC is about 2.0-2.5. My water went through reverse osmosis, with very low to no amounts of mineral. I grow
50-100 muskmelons. During the season, I only use this particular formula recipe from transplantation to fruiting
until harvest, but I see that once the fruit started to set, it seems the plants showed signs of potassium deficiency,
and the pH started to get on the lower side. It also looks like the plant is taking up a lot of water because the EC in
the nutrient reservoir went up at the end of the day. My question is: Do you have a suitable recipe for muskmelons
in different stages of development?
Ryan L. 
A
Hello Ryan,
Fruiting plants such as muskmelons being grown on
this scale really do need to be provided with differ-
ent vegetative and fruiting nutritional formulations
as they move through the different growth stages.
Melons take up very high levels of potassium once
they have set fruit and this can rapidly deplete potas-
sium levels, particularly in a recirculating system.
It would be recommended to start the young plants,
right from seedling stage, on a balanced vegeta-
tive formulation, switch to a flowering/early fruit set
formulation as soon as the first small fruitlets have
set, then change again to a heavy fruit set formula-
tion as soon as the crop is in the rapid fruit expan-
sion stage. From then on, the nutrient recirculating
in the system should either be regularly analyzed
for all macro elements (N, P, K, Ca, Mg) to determine
if the fruiting formulation is providing sufficient
potassium for the current level of fruit development
in the crop. Alternatively, some growers prefer to do
complete or partial replacements of the recirculating
nutrient solution during this stage to ensure these
stay in balance. With a recirculating solution and a
heavy feeding crop, the nutrient solution can become
imbalanced or deficient in certain elements relatively
quickly, even with a good fruiting formulation (there
is no one perfect formulation that suits a particular
crop) as the mineral uptake of individual crops varies
from grower to grower, in different environments,
with different cultivars, and with different levels of
fruit set—so monitoring of nutrient levels is vital for
crop performance. As a starting basis, the
following nutrient levels would be recom-
mended for each stage of growth:
Seedling and early vegetative:
N=227 ppm, P=67, K=200, Ca=174, Mg=78
Flowering/early fruit set:
N=209 ppm, P=85, K=276, Mg=89, Ca=174
Heavy fruit loading:
N=183 ppm, P=120, K=448 (or higher), Mg=112, Ca=174
Since the RO tends to run a lower pH than other
water sources, this is best increased with use of a
10 per cent solution of potassium hydroxide as that
will add additional potassium into the solution and,
since this is required in large amounts, poses the
least risk to creating imbalances with pH adjustment
chemicals. If your plants do start to show symptoms of
any nutrient deficiency, or even suspected deficiency,
the first immediate step would be to completely
replace the nutrient solution as that then gives the
plants a newly balanced nutrient, foliar, and solution
analysis that can then be used to fine-tune the
formulations in use and prevent future issues.
Melon plants do have a large leaf area and under good
growing conditions will take up a significant volume
of water each day, which can increase the EC rapidly.
Under these conditions, it’s best not to let the EC climb,
but keep it stable with regular water top-ups throughout
the day (an automated water top-up system may be
required). Melons are sensitive to high EC and keeping
this stable, particularly under warm growing conditions,
is essential for both water (transpiration) and nutrient
uptake. Good luck with your future melon crops!
Kind Regards,
Dr. Lynette Morgan
Suntec International Hydroponic Consultants
Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD
in hydroponic greenhouse production from Massey University,
New Zealand. Lynette is a partner with Suntec International
Hydroponic Consultants and has authored several hydroponic
technical books. Visit suntec.co.nz for more information.
ELECTRICAL CONDUCTIVITY (EC)
In hydroponics, all nutrients are made of mineral
salts, which are dissolved into water, and the
strength of the nutrient solutions can be detected
by an electrical conductivity (EC) meter. The higher
the ionic (salt) concentration, the higher the level
of EC. The EC of a nutrient solution is therefore a
measurement of its “strength” as indicated by the
actual amount of salts dissolved.
– growthtechnology.com
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