I can almost guarantee its not S, that would be extremely rare. Looks like Nitrogen deficiency to me. What is going to be tricky here is that you wont see an immediate improvement. Slightly increase your N dosing and take note of where the yellowing occurs, if the yellowing continues up the stem over the next 2 weeks increase N a little more. If your test kits show a consistent presence of NO3 or NH4 in the water then obviously its not N deficiency and you should look into K.
Could be N problems, but new growth tends to be smaller than old growth and a little lighter/yellowish in color as well. Also the leaves that tend to die are usually the very oldest leaves at the bottom of the stem, not the leaves half way up the stem. Looks a bit odd, but I agree with ShaneSmith: up the nitrate levels.
I guess the pictures you have taken are just after topping/pruning them. If so the defficiency looking of the plant is caused by shadowing not particularly from a nutrient defficiency. And also when you replant the top you will see most of the leaves that will stay below will develop the same look as they were previously adapted to much more light.
Well i know that they were recently pruned, because i have the cuttings! that might me true but it couldn't hurt to up the N for a little bit to see an improvement, go back down if the problem truly is shading.
i did just prune and replant(cough*shane leave me feedback* cough ) the oldest leaves fell off, theres about 1 inch of the plant thats bare, probably from shading. the middle leaves are all turning clearish yellow grey, and the new leaves exhibit a faint transparency. i'll up my N and K for a few weeks to see how they respond. ive been dosing half EI as to not get any algae outbreaks since this tank is only 2 months old, but i guess its time to goo full EI. thanks guys