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 is the total N by the above-ground portion?

Correct option is C

Correct option: (c) 90 kg/ha
Step-wise explanation:
· The question asks for total nitrogen in the above-ground portion, which includes:
· Straw
· Grain
· Roots are below-ground, so they are not included in the calculation.
Nitrogen in straw:
· Dry matter = 6 t/ha = 6000 kg/ha
· N content = 0.5%
· Nitrogen = 6000 × 0.5 / 100 = 30 kg N/ha
Nitrogen in grain:
· Dry matter = 4 t/ha = 4000 kg/ha
· N content = 1.5%
· Nitrogen = 4000 × 1.5 / 100 = 60 kg N/ha
Total above-ground nitrogen:
· = 30 + 60
· = 90 kg N/ha
➡️ Hence, the correct answer is 90 kg/ha.
❌ Why other options are incorrect
Option (a) 30 kg/ha
· Includes only straw nitrogen.
· Grain nitrogen is ignored.
Option (b) 60 kg/ha
· Includes only grain nitrogen.
· Straw contribution is ignored.
Option (d) 120 kg/ha
· Incorrectly includes nitrogen from roots or involves overestimation.
· Roots are below-ground and excluded as per question.
�� Key Points to Remember
· Above-ground biomass = straw + grain
· Always convert tonnes to kg before percentage calculation.
· %N × dry matter gives nutrient uptake.