Estimating Leaching Requirement in Soil

The question asks us to estimate the leaching requirement (LR) for a soil when we know the electrical conductivity (EC) of the irrigation water and the EC of the saturated soil extract that causes a specific reduction in crop yield.

Leaching Requirement is the fraction of irrigation water that must pass through the root zone to control salinity at a level that does not cause unacceptable yield loss. It is often expressed as a percentage.

Given Information

• Electrical conductivity of the saturated extract of soil ($EC_{se}$) at 25% reduction in crop yield = 10 m mho/cm.
• Electrical conductivity of irrigation water ($EC_{iw}$) = 1.5 m mho/cm.

Leaching Requirement Formula

The basic formula for Leaching Requirement (LR) is given by:

$$ LR = \frac{EC_{iw}}{EC_{dw}} $$

Where:

• $EC_{iw}$ is the electrical conductivity of the irrigation water.
• $EC_{dw}$ is the electrical conductivity of the drainage water leaving the root zone. This $EC_{dw}$ is ideally kept at a level that ensures the soil salinity in the root zone does not exceed the crop's tolerance limit.

Relating Soil Extract EC ($EC_{se}$) to Drainage Water EC ($EC_{dw}$)

The given $EC_{se}$ value (10 m mho/cm at 25% yield reduction) represents the salinity of the saturated soil extract, which is a standard measure of soil salinity. The drainage water leaving the root zone ($EC_{dw}$) typically has a higher salt concentration than the saturated extract ($EC_{se}$) from the same soil layer, especially at lower water contents found in the field compared to saturated conditions.

In some applications and simplified approaches, the maximum allowable $EC_{dw}$ is related to the critical $EC_{se}$ value that affects crop yield. A common approximation used in some contexts assumes that the drainage water concentration is about twice the saturated extract concentration that defines the crop tolerance limit or a specific yield reduction level.

Let's assume, for the purpose of this calculation based on common practices or implied by the options, that the required $EC_{dw}$ for managing salinity is approximately twice the given $EC_{se}$ value at 25% yield reduction. While this is a simplification, it allows us to proceed with the calculation.

So, we assume:

$$ EC_{dw} \approx 2 \times EC_{se} (\text{at specified yield reduction}) $$

Using the given value $EC_{se} = 10$ m mho/cm:

$$ EC_{dw} = 2 \times 10 \text{ m mho/cm} = 20 \text{ m mho/cm} $$

Calculating Leaching Requirement

Now, we can plug the values into the LR formula:

$$ LR = \frac{EC_{iw}}{EC_{dw}} $$

Substituting the given $EC_{iw} = 1.5$ m mho/cm and the calculated $EC_{dw} = 20$ m mho/cm:

$$ LR = \frac{1.5 \text{ m mho/cm}}{20 \text{ m mho/cm}} $$

$$ LR = 0.075 $$

To express the Leaching Requirement as a percentage, we multiply the fraction by 100:

$$ LR (\text{percentage}) = 0.075 \times 100\% $$

$$ LR (\text{percentage}) = 7.5\% $$

Conclusion

Based on the given information and the assumption that the required drainage water salinity ($EC_{dw}$) is approximately double the saturated extract salinity ($EC_{se}$) at 25% yield reduction, the estimated leaching requirement is 7.5%.

This means that for every 100 units of irrigation water applied, 7.5 units must drain through the soil profile to maintain the soil salinity at or below the level corresponding to 25% yield reduction.