What types of nutrients do hydroponic melons require?

Q:

I have been experimenting using recirculating flood and drain, maintaining water temperature at 26°C, pH of 5.5-6, and using the Douglas Peckenpaugh formulation, which is N=215, 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, and with EC at about 2.0-2.5.

My water went through reverse osmosis, with very low to no amounts of minerals present. 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?

A:

Fruiting plants such as muskmelons being grown on this scale really do need to be provided with different 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 potassium levels, particularly in a recirculating system.

It would be recommended to start the young plants, right from seedling stage, on a balanced vegetative 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” formulation as soon as the crop is in the rapid fruit expansion 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 K 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).

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 recommended for each stage of growth: Seedling and early vegetative: N=227ppm, P=67, K=200, Ca=174, Mg=78, moving on to flowering/early fruit set: N=209ppm, P=85, K=276, Mg=89, Ca=174, then onto heavy fruit loading: N=183ppm, P=120, K=448 (or higher), Mg=112, Ca=174.

Since the RO tends to run a lower pH then 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 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!

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Written by Lynette Morgan
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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.  Full Bio