Question

Suppose roots, straw, and grain of a wheat crop producing dry matter of 2.5 t/ha, 6 t/ha, and 4 t/ha contain 0.2%, 0.5%, and 1.2% N, respectively. How much total N uptake by wheat?

• A. 65 kg/ha
• B. 75 kg/ha
• C. 85 kg/ha
• D. 95 kg/ha

Hint:

To solve nutrient uptake calculations quickly, remember that $1\%$ of $1000\text{ kg}$ is $10\text{ kg}$. So, $1.2\%$ of $4\text{ t}$ grain is $1.2 \times 4 \times 10 = 48\text{ kg}$, $0.5\%$ of $6\text{ t}$ straw is $0.5 \times 6 \times 10 = 30\text{ kg}$, and $0.2\%$ of $2.5\text{ t}$ roots is $0.2 \times 2.5 \times 10 = 5\text{ kg}$. Summing them gives $48 + 30 + 5 = 83\text{ kg/ha}$.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The total nutrient uptake by a crop is calculated by summing the nutrient accumulated in each of its physical parts (roots, straw, grain).
For each component, the nutrient uptake is:
\[ \text{Nutrient Uptake} = \text{Dry Matter Yield} \times \text{Nutrient Concentration} \]
Key Formula or Approach:
Convert dry matter yields from tonnes per hectare ($\text{t/ha}$) to kilograms per hectare ($\text{kg/ha}$) using:
\[ 1\text{ t/ha} = 1000\text{ kg/ha} \]
Calculate N uptake for each component, then sum them to find the total N uptake:
\[ \text{Total N Uptake} = N_{\text{roots}} + N_{\text{straw}} + N_{\text{grain}} \]

Step 2: Detailed Explanation:
Let us perform the calculations step-by-step:
1. N Uptake by Roots:
Dry matter yield of roots = $2.5\text{ t/ha} = 2500\text{ kg/ha}$
N concentration in roots = $0.2\% = 0.002$
\[ N_{\text{roots}} = 2500\text{ kg/ha} \times 0.002 = 5\text{ kg/ha} \]
2. N Uptake by Straw:
Dry matter yield of straw = $6\text{ t/ha} = 6000\text{ kg/ha}$
N concentration in straw = $0.5\% = 0.005$
\[ N_{\text{straw}} = 6000\text{ kg/ha} \times 0.005 = 30\text{ kg/ha} \]
3. N Uptake by Grain:
Dry matter yield of grain = $4\text{ t/ha} = 4000\text{ kg/ha}$
N concentration in grain = $1.2\% = 0.012$
\[ N_{\text{grain}} = 4000\text{ kg/ha} \times 0.012 = 48\text{ kg/ha} \]
4. Total N Uptake:
\[ \text{Total N Uptake} = 5 + 30 + 48 = 83\text{ kg/ha} \]
The calculated value of $83\text{ kg/ha}$ is closest to the given option of $85\text{ kg/ha}$.

Step 3: Final Answer:
The total Nitrogen uptake by the wheat crop is $83\text{ kg/ha}$ (reported as $85\text{ kg/ha}$), corresponding to option (C).