Question

Suppose roots, straw, and grains of a wheat crop produce dry matter of 2.5 t/ha, 6 t/ha, and 4 t/ha containing 0.2%, 0.5%, and 1.5% N, respectively. How much total N uptake by the above-ground portion ?

• A. 30 kg/ha
• B. 60 kg/ha
• C. 90 kg/ha
• D. 120 kg/ha

Hint:

Always read the system boundaries in calculation questions carefully. Keywords like 'above-ground' or 'vegetative' specify which plant parts should be included or excluded from the final sum.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Nutrient uptake by a crop is calculated by multiplying the dry matter yield of each plant component by its respective nutrient concentration.
To find the total nitrogen (N) uptake of the above-ground portion, we must sum the nitrogen accumulated in the straw and grain, while excluding the underground root system.

Step 2: Key Formula or Approach:
The formula used to calculate nutrient uptake for each component is:
\[ \text{Nutrient Uptake (kg/ha)} = \text{Dry Matter Yield (kg/ha)} \times \frac{\text{Nutrient Concentration (\%)}}{100} \]
First, convert the yields from tonnes per hectare ($\text{t/ha}$) to kilograms per hectare ($\text{kg/ha}$), where $1 \text{ t/ha} = 1000 \text{ kg/ha}$.

Step 3: Detailed Explanation:
Let us identify the components and perform the calculations:
- Straw Yield = $6 \text{ t/ha} = 6000 \text{ kg/ha}$
Straw Nitrogen Content = $0.5\%$
\[ \text{N uptake by straw} = 6000 \times \frac{0.5}{100} = 30 \text{ kg/ha} \]
- Grain Yield = $4 \text{ t/ha} = 4000 \text{ kg/ha}$
Grain Nitrogen Content = $1.5\%$
\[ \text{N uptake by grain} = 4000 \times \frac{1.5}{100} = 60 \text{ kg/ha} \]
- Root Yield = $2.5 \text{ t/ha} = 2500 \text{ kg/ha}$
Root Nitrogen Content = $0.2\%$
\[ \text{N accumulation in roots} = 2500 \times \frac{0.2}{100} = 5 \text{ kg/ha} \]
Since the question asks specifically for the above-ground portion, we exclude the roots and sum the straw and grain values:
\[ \text{Total Above-Ground N Uptake} = \text{N in straw} + \text{N in grain} \]
\[ \text{Total Above-Ground N Uptake} = 30 \text{ kg/ha} + 60 \text{ kg/ha} = 90 \text{ kg/ha} \]

Step 4: Final Answer:
The total nitrogen uptake by the above-ground portion is 90 kg/ha, corresponding to option (C).