i don't now how can i save them

I have 2 plants in my office desk, from last 2 years. In these years these plants grew and i feel very good when i look at it. I take care of them as much as possible. Give water, dig mud, give tea leaf as fertilizer, and clean their leaves, keep outside every weekend to give sunlight. But don't know why, from last few days its new leaves becoming yellow from bottom part of it. i don’t know why this is happening. Today i put them in baranda to give them direct sunlight. Let’s see. Feeling bad for them.