How to diagnose what nutrient deficiency your lemon tree has

My daughter's lemon tree had an obvious problem of nutrient deficiency. But what nutrient was it deficient in? The new Google image search (or look here for a video) is a quick method of diagnosis. Upload the image of your tree. The images it found were not clear immediately so the search facility asks for a hint in words. I added "nutrient deficiency lemon" and got an image from this site which told me the lemon tree was suffering from a deficiency of magnesium.

On another website I also found: "When a magnesium deficiency is present, the young citrus leaves appear green and normal, but older leaves turn yellow and often have an inverted ‘V’ of green at the leaf base. In advanced cases, leaves fall prematurely and cropping declines." This described the leaves on my daughter's lemon tree exactly.

And then of course Peter Cundall came up trumps with a solution. A foliar feed with a solution of Epsom Salts and now I am waiting to see if the tree will look healthier.