Question

Which one of the following essential element is immobile in plant and very mobile in soil:

• A. Magnesium
• B. Boron
• C. Copper
• D. Iron

Hint:

To remember Boron's behavior: It "flows" with the water in the soil but "sticks" to the tissue in the plant. High soil mobility often leads to leaching, which is why Boron deficiency is common in sandy, acidic soils.

Answer 1

The Correct Option is B

Concept: The mobility of nutrients determines where deficiency symptoms first appear in a plant and how easily they are leached or moved within the soil profile. Nutrients that are immobile in plants show deficiency symptoms in younger leaves, while those mobile in soil are easily available to roots but also prone to leaching.

Step 1: Analyze Plant Mobility.
Elements like Boron ($B$) and Calcium ($Ca$) are considered highly immobile within the plant's vascular system (phloem). Once they are incorporated into plant tissues (like cell walls), they cannot be translocated to new growing points. Consequently, deficiency symptoms for these elements appear first on the terminal buds and youngest leaves.

Step 2: Analyze Soil Mobility.
In the soil, Boron exists primarily as boric acid ($H_3BO_3$), which is uncharged at typical soil pH levels. Because it does not readily bind to negatively charged soil particles (clay or organic matter), it remains in the soil solution and is highly mobile. This makes it easily accessible via mass flow but also highly susceptible to leaching in high-rainfall areas.

Step 3: Compare with other options.
Magnesium is highly mobile in plants (deficiency appears in old leaves). Iron and Copper are generally immobile in plants but have low to intermediate mobility in soil depending on pH and chelation. Boron is the unique case that fits both criteria of being plant-immobile and soil-mobile.