This is leafy vegetable, sorrel leaves. Is this potassium deficiency or magnesium or iron deficiency. Started appearing in other plants too.

It's looks like iron deficiency. Symptoms: interveinal chlorosis, green veins in younger leaves.

Does it cause burning of small seedlings leaves too .... Many seedlings have died like that

In some pics , I can see intervienal chlorosis and margin necrosis. That might be because of calcium deficiency.

As the mature leaves also shows symptoms, it may not be iron deficiency.

So , is it potassium or magnesium deficiency

Is it affected growing points of the plant?, Any distortion in leaves?

For Mg and K deficiencies, symptoms will appear in older leaves first.

There is no distortion. But all leaves are effected as in the pics...and many seedlings have died already when it has two leaves only. So I there are no older leaves..so which deficiency i need to decide

These are the pics of small seedlings

I believe there is potassium deficiency in the soil.