I have a lucky bamboo plant, that I have had for probably close to 4 years. Now one of its stalks is turning yellow, which i have never had happen. It has been in its current location for over two years, and has been pretty robust. I changed its soil in the midsummer, and since then, it has grown exponentially, from the plant food contained in this potting soil blend. It should be close to exhausting its supply of nutrients, but this has probably happened within a couple of days. Any thoughts?