Kale leaves turning yellow in hydroponic system

Question

I planted kale using hydroponic method. I planted them in September 7th, 2019 and now they're 41 days old. I put them in hydroponic system when they're 21 days old and have two leaves, with nutrition concentration at 0.5 mS/cm and pH 6.8. After three days, the leaves edge turned gray, it looked like burnt. I thought it was because the electrical conductivity quite high for the kale (around 2-3 mS/cm) and I didn't give any treatment, including trying to lower the nutrition concentration. But three days later the leaves turned green and looked normal.

When the kale turned 34 days old, I noticed some of the kale leaves turned light green. From the leaves turned green until that moment, the electrical conductivity was around 1.68 - 3 mS/cm and pH 6.0 - 6.5. I didn't give any treatment because I thought the nutrition concentration was quite high and there's no sign of pests. The pipes inside quite dirty with moss. By the way I alternate the hydroponic holes with romaine lettuce at that time. I thought they would grow fine and not lack of sun lights because the kale plants not tall enough.

Today (October 17th, 2019) I noticed some of the kale leaves have turned yellow.

The older leaves turned yellow. Then I found kale plant which leaves turn yellow, and have brown spots like this picture below.

My questions are:

1. What cause the kale leaves turn yellow like the last picture? I can't find the similar problems online and I'm still confused wether it's lack of nutrition in general or lack of specific nutrient constituent, or they compete with romaine lettuces in absorbing nutrition, or there are other aspects I miss.

2. Do moss inside the pipes affect the kale's nutrient absorption?

3. For those who have grown kale with hydroponic method, what's the optimum nutrition concentration (ppm) or electrical conductivity (mS/cm) and pH for kale growth and development?

Answer 1

In the last image it looks like your plants are running to seed due to heat. In temperate countries kale is known for its suitability for cold conditions and in fact improves after frosts, so you need to be very careful about choice of variety. There are Tuscany types available which can tolerate warmer conditions, but still 30 degrees would be very warm for this plant. It is not overly fussy about high light since it does well in low light winter conditions, so a semi-shady place or filtered light will be preferred to direct sun.

If algae are growing in your lines and boxes this means light is getting in, perhaps due to translucent lines or ill fitting grow pots. The algae will seize some of the nutrient so make sure that you are testing for pH and EC at the inlet and the outlet to ensure that you know the overall situation.

You might want to contact your local horticultural research institute and find out if they have done research into suitable micro greens for the local market.