Cacti going sick yellow- help!

[nz_flyboy]

I have a few groups of San Pedro cacti. 3 years ago some of the groups next to the path started turning sickly yellow. The ones away from the path remained green & healthy. This happened around the time I was away and some friends were looking after the house. I suspect some people may have been urinating in the garden around the cactus's. However my neighbor who has healthy San Pedros reckons it may be due to 'nitrogen deficiency' and to heap on fertilizer (like blood and bone)- I wouldn't know. In the last year- I have seen a very few of the yellow cactus's growing new green growth at the top, but others have remained yellow with some of the most sick yellow ones rotting and collapsing. One small new green cactus I planted around the yellow ones has also gone yellow- indicating something wrong in the soil.
Any suggestions on how I can restore my cacti would be greatly appreciated.