K-Mag is not normally recommended for use in fertigation or general application through an irrigation system. Although K-Mag is water soluble, its rate of solubility is slower than muriate of potash. Also, there are usually small traces of naturally-occurring phosphates in water. In many cases, phosphate fertilizers are added to irrigation water. Magnesium in water with even small amounts of phosphates present may react and form a gel that will clog nozzles of the irrigation equipment.
Most crops require more potash than sulfur.
Generally, K-Mag is not used as a source for the total potash requirement. For most field crops, K-Mag will be recommended at a rate to supply most, if not all, of the magnesium and sulfur fertilizer requirements. I am not aware of a sulfur need. All plants need sulfur. Experience is the best indicator for sulfur fertilizer need.