@Matthew Rudling - welcome to the forum. We have a few members that are from South Africa. Perhaps they can help you with local sourcing of bonsai supplies.
Fertilizer is a complex subject, and one loaded with "hangovers" from bad gardening practices promoted by the Royal Botanic Gardens at Kew, UK. For 2 centuries RBG was considered the ultimate source of information for gardening. Never mind modern agriculture and horticulture techniques coming out of major Universities and from the farm industries themselves. For some reason, if you called it gardening, "junk science" published by RBG Kew in the 1880's was viewed to trump modern science. To sort out what I mean, in 1880's RBG would say a 10-10-10 fertilizer was BALANCED. This is very wrong.
From modern agricultural science, for the vast majority of vascular plants a "Balanced to the needs of the plant" fertilizer would be 12% nitrogen (preferred as amino acids), 0.9% or slightly less Phosphorous, 4% Potash as K2O or 8% Potassium as the monovalent ion, Calcium would be at least 9%, Magnesium would be around 3%, Sulfur would be about 2%, Boron 0.025%, Copper 0.038%, Iron 0.155%, Manganese 0.078%, Molybdenum 0.015% and Zinc 0.077%. This 12-1-4 plus "the list of macro & micro nutrients" does not appear in "Gardening Books" because they are filled with outdated RBG Kew information. So if you are reading somewhere, and the proposed fertilizer does not scale to a 12-1-4, for example a 6-0.5-2 is essentially a 12-1-4.
Now it the nitrogen source is NITRATE rather than ammonia or amino acids, then the Potassium (K) needs to be equal to the Nitrate, for example with Nitrate as a source, 12-1-12 is appropriate. Potassium is used in the uptake of Nitrate. Potassium is not needed in the uptake of ammonia, ammonium, and amino acids.
So your question about a 5-1-5 is that the 5-1-5 is probably not too far off from a 12-1-12 assuming the N comes from nitrate. So not a bad choice.
Sulfur is a needed macronutrient, however, most soils, most potting media have a fair amount of naturally occurring sulfur. Also, rain, as in "acid rain" where air pollution is present, contains a fair amount of various sulfur oxides. So between acid rain, naturally occurring sulfur in ground water, and sulfur occurring in potting media, it is rare that bonsai would experience a sulfur deficiency. It doesn't hurt to supplement sulfur in your fertilizer, a moderate excess is well tolerated by most bonsai species.
Many people raise excellent bonsai using all manner of weird fertilizer formulations. One of the reasons the old 10-10-10 formulation still gets repeated is that plants tolerate all manner of bad fertilizer formulations IF YOU FLUSH WITH PLAIN WATER between fertilizer applications. A little rain, goes a long way to flushing out, or leaching out fertilizer imbalances. The tree hangs on to what it needs and allows the excess ions wash away. Where you will see the results of bad fertilizer formulations is if you go to a "continuous feeding" program in a greenhouse, where rain, or watering with clear water does not "wash away" imbalances. It will take a months to a year or two and the shortcomings of your fertilizer will show up in the growth.
So while helpful to really study plant nutrition, you can get away with just about anything if you flush with clear water in between fertilizer applications. There is nothing magical about organic fertilizers. The benefit of organic fertilizers is that the nitrogen released will be released as amino acids or ammonium. The disadvantage is that often organic fertilizers do not deliver as much nitrogen as one would like. It is possible to overdose organics, it is more likely that you will be under dosing organics. An error to the too little fertilizer side does have fewer bad consequences.
And about calcium sulfate - gypsum. In general, gypsum is only slightly soluble in water. When present in large amounts it can be lethal to plants. Only a small number of specialized species of plants thrive in gypsum soils. Gypsum in small amounts is a nutrient that is slowly released, in slightly larger amounts it interferes with iron, and manganese uptake. It is best to rely on other sources of sulfur than gypsum.
Calcium, here is a needed nutrient, it occurs widely in nature, limestone is calcium carbonate, most North American soils east of the Rocky Mountains are limestone derived soils. Similarly our drinking and irrigation water often comes from limestone aquifers. We rarely have to worry about a calcium deficiency. You indicate your water has 130ppm calcium carbonate, likely you are getting near sufficient calcium from your irrigation water. The amount in your fertilizer is more than enough to meet most tree's needs, without being an excess.
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@Matthew Rudling 's original question. is this fertilizer good. My answer - read the label. As long as the sulfur source is not gypsum, the fertilizer is probably quite good. Safe to use. If you apply clear water, meaning no added fertilizer, the next time the trees need watering after applying fertilizer, everything will be fine.
but mine are in large containers!
