I use magnesium sulfate, Epsom salts, once every 2 to 3 months. More frequent if I see chlorosis, less if everything is vibrant green and conifers blue. My standard dose is 1 tablespoon (15 ml) per gallon, rather than 4 tablespoons per gallon your neighbor uses. The magnesium is essential to make chlorophyll, and the sulfur of the sulfate ion is itself an essential macro nutrient, needed at about 20% the rate of nitrogen consumption. Sulfur is pretty abundant in the environment. One common source is air pollution, significant amounts of sulfur in various oxidation states is flushed out of the air and into our bonsai pots by rain. The dirtier your air, the less you need to worry about sulfur in your fertilizer program. Peter, you are in the "wide open spaces" where the air is clean(ish). So it is good to keep sulfur in mind as a part of your fertilizer program. Epsom salts is just as good at delivering sulfur as it is at delivering magnesium, so a good "two-fer" fertilizer nutrient.