One thing I see a lot on here people see yellowing leaves and automatically assume nitrogen deficiency but there are a lot of things to consider other than deficiencies things like toxicity and natural causes toxicity is a good chance of some deficiencies depending on what it is as for natural causes leaves that yellow at the bottom even at a high rate of speed is sometimes due to the leaves being old new leaves being produced at the top they no longer need those large leaves that aren't producing enough energy to support their own cell as they start to release these