Question

In saline soils boron concentration is very important from plant growth point of view. What is the safe limit of water soluble boron concentration in a saline soil.

• A. <0.7 mg/kg
• B. 0.7-1.5 mg/kg
• C. 1.5 - 2.0 mg/kg
• D. >2.0 mg/kg

Hint:

Boron toxicity symptoms typically manifest as characteristic marginal chlorosis (yellowing) and necrosis (browning/burning) of the older leaves, as boron is immobile in plants and accumulates at the ends of the transpiration stream (leaf margins).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Boron is an essential micronutrient required by plants for cell wall synthesis, sugar translocation, and reproductive development.
However, the margin between boron deficiency and toxicity is extremely narrow.
In saline and alkaline soils, boron often accumulates to toxic levels, making it critical to establish safe limits for irrigation and soil solutions.

Step 2: Detailed Explanation:
The safe limit of water-soluble boron in soil and irrigation water is determined by the sensitivity of the crops being grown.
Crops are generally classified into three categories based on their boron tolerance:
1. Sensitive crops (e.g., citrus, peach, grape, and some beans):
These crops are highly prone to boron toxicity.
The safe limit of water-soluble boron in the soil for these crops is strictly less than 0.7 mg/kg (or ppm).
Concentrations above this threshold lead to leaf tip burning, chlorosis, and reduced yields.
2. Semi-tolerant crops (e.g., wheat, barley, maize, and oat):
These crops can tolerate boron concentrations in the range of 0.7 to 1.5 mg/kg.
3. Tolerant crops (e.g., sugar beet, alfalfa, and turnip):
These can withstand concentrations from 1.5 to 2.0 mg/kg, or even slightly higher.
To ensure safe cultivation across all crop types, especially sensitive ones in saline environments, a water-soluble boron concentration of less than 0.7 mg/kg is considered the universal safe limit.

Step 3: Final Answer:
The safe limit of water-soluble boron concentration in a saline soil is <0.7 mg/kg (Option A).