Iron chlorosis is an off duck.  Soil is basically iron with some aluminum, silicon and oxygen added in for good measure, but plants can lack for iron because it is not available to be taken up.  This is often because the soil pH is high due to carbonates which tie up the iron, causing it to precipitate.  Another cause is asphyxiation of the roots from too much water.  Avocados are especially sensitive to asphyxiation and will show classic interveinal chlorosis and as time goes on, the leaf will turn almost white.   They will also show tip burn, like chloride damage in some cases, along with the yellowing.  The image of the tree below is of an avocado in a loamy soil with a pH near 7, but is irrigated every two weeks with about 400 gallons of water – kind of boom and bust.  The trees are drowning and showing classic iron chlorosis symptoms, but they could also be confused with root rot.