TDS vs. EC.- pure water does not conduct electricity. Even a small amount of salts allows electricity to be conducted through water. Both TDS and EC sensors measure how easily electricity flows through water, which tells us how much total salts are in the water. Unfortunately these sensors cannot tell us what those salts are- like how much potassium is in the water. Also, many organic fertilizers like urea don’t show up because they do not effect the salt level in the water.