Fungus can cause rusts which look a bit like a cal. deficency, ie brown spots and can also cause blights which produce yellowing viens. The compost may be to rich in humus which in excess can burn as it has not yet broken down into humnic acid. Untill the microbial life breakdown the humus and ammonium nitrogen into nitrate it is unusable by the plant and can build up. If feeding with chemicals that can kill the microbes in the soil and the humus will not break down with out them. Nitrogen is washed away easily if that is the case and flushing is a good idea. Could I ask what you are feeding them?