Single leaf yellowing?

[spinrah]

Hi all,

One of the upper leaves on my phal orchid has started yellowing and I’m wondering why. It is planted in bark and top dressed with sphag (recently repotted 3 months ago) and I water it about every 4-5 days depending on how the roots look. I was watering initially by immersing the whole pot for 10 mins in water but lately I have stopped doing that as the bark is breaking down. My room temps are about 68-69F and 50% moisture. I have also been fertilizing with 1/16th a teaspoon of k-lite at every watering.

Could this be due to over fertilization or too much water? The visible roots look fine to me.

The leaf is second from the top (2 leaves below it).