Question
Deficiencis of Iron, Magnesium and Nitrogen all cause chlorosis. Iron chlorosis
develops between the veins of young leaves while magnesium and nitrogen
chlorosis develops in the older leaves. What are these differences... why? Why
does each deficiency lead to chlorosis and why are the patterns different?

Answer
'Chlorosis' means 'lack of Chlorophyll.' Plants need Nitrogen, Magnesium and (in SMALL amounts) Iron (THE most abundant mineral by weight on the planet -- only the most worthless Soils are missing it).

Let's start with Iron.  Bacteria, people, plants, vertebrates -- most living things need Fe++ ('Ferrous Iron') to survive.  As the Green Sea Venture website explains, 'Iron is a micronutrient essential for the synthesis of enzymes required for photosynthesis.'  Other authorities maintain it is used for DNA production in plants and animals.

Take away a plant's Iron, and suddenly you can't make Chlorophyll.  You rely on the Chlorophyll you have right now.

If you're an old leaf, you've got reserves.  If you're a young, new, growing sprout off the top of a shoot, you're in trouble.

You wake up one morning, and you're white as a sheet -- so you run downstairs, turn on the switch in the kitchen -- and no Chlorophyll.  You check the wires, you look at the circuitbreaker, and you realize you forgot to pay the Chlorophyll bill!

And it's SUNDAY!

There's no box anywhere in the house that can HOLD Iron,, and you can't just make it.  So you're down.  After all, you're a new, young sprout, with no way to get Iron for making Chlorophyll.  It's WAY TOO DARK in the kitchen without it.  You just have to hope you can hold out long enough until Help arrives to turn on the Chlorphyll and get the wires working again.

But let's say you're not a new sprout.  You're an OLD sprout.

You've been around the block.  And we find one morning that we are very low on Magnesium.

This is a problem.  Because you can't make Chlorophyll for breakfast without a few cups of Phillips Magnesium, and there's none of that in the fridge.

Ever see a Chlorophyll molecule?  C55H70MgN4O6.

You MUST have Carbon, Hydrogen, Magnesium, Nitrogen and Oxygen to make a Chlorophyll molecule.  Without Magnesium, a Chlorophyll molecule would be ... something else.

Chlorophyll has a VERY short lifespan.  It must be made constantly to replace the expired molecules -- note how quickly the Green disappears from cut Grass in the compost pile.

So if you're not eating Chlorophyll every morning for breakfast, even if you're an OLD leaf, you are going to have a VERY BAD DAY.  And you'll LOOK it.  Because you'll miss that Chlorophyll.  Skip a day without Magnesium and you're going to pay for it.

Ditto, Nitrogen.

Only ONE Molecule of Mg is in that huge Chlorophyll molecule.  But it's right there in the middle, and without it, there is no Chlorophyll.

Plant leaves must constantly make Chlorophyll for Photosynthesis, right?  Yesterday's Chlorophyll is not going to cut it.  But if you can't make more, you can't use the stuff from yesterday.

On the other hand, you've got plenty of Iron.  You need just a little, and as long as it's coming out of the tap, you'll be able to make

Why do young leaves suffer first from an Iron shortage?

Older leaves have a little Iron left over from other projects yesterday.  They have their Iron.  Newer leaves are new -- there IS no yesterday to them.  No Iron reserves.  Even with all the Mg, N, O, H and C in the world, there is no way to mix them together without Fe.  And they look it.

But the older leaves need Chlorophyll re-fills every day, just like the younger leaves.  Chlorophyll spoils way too easy.  They look awful.

Very simple, and a common pattern in many plant diseases.