Macronutrients are those required in relatively high concentrations for the purposes of plant growth. This list includes: Carbon Hydrogen Oxygen Nitrogen Phosphorus Potassium Depending upon what mood the author is in, he or she might also include calcium and magnesium. It is also worth noting that of the micronutrients, iron is required in substantially higher concentrations than the others and may be subject to interpretation, i.e.: that iron may be considered a macronutrient rather than a micronutrient. Fortunately, nobody is really keeping score in this game, so it's a lose-lose proposition as to which side of the net it ends up on. Que sera.