Discussion
Which of the following nutrient is immobile in soil but mobile in plants ?
Answer: A
The nutrient is Phosphorus (P).
In soil, phosphate is quickly fixed: it reacts with calcium in alkaline soils and with iron and aluminium in acid soils to form compounds of low solubility, and it moves only a few millimetres, mostly by diffusion. That is why phosphatic fertiliser is placed near the seed. Inside the plant, however, phosphorus is highly mobile and is moved from old leaves to young tissues, so its deficiency shows first on older leaves.
Nitrogen as nitrate is mobile in soil, which is why it leaches. Calcium is fairly mobile in soil but immobile in the plant, so its deficiency appears in young tissues. Potassium is more mobile in soil than phosphorus and is also mobile in the plant, so it does not fit the 'immobile in soil' half of the description as well.
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