Nitrogen deficiency first shows up as yellowing of the bottom leaves. Nitrogen is a mobile element. In other words, if the plant isn’t supplied with enough nitrogen, the plant will break down the stored nitrogen from the lower leaves and transport it to the new growth where it is needed the most. The older leaves start to turn pale yellow and eventually turn completely yellow and fall off. Sometimes its difficult to distinguish between a nitrogen deficiency and a magnesium deficiency since both are mobile elements and cause yellow leaves. But typically, nitrogen deficiency appears as a general yellowing of the lower to middle leaves, while magnesium deficiency shows up as a more distinctive interveinal yellowing. If you want to know for sure, perform a leaf analysis. Leaf tissue test kits are available that can test for nitrogen; indicating high, adequate or deficient levels.