Tutorial, Calculations

Question 1

How would you calculate the quantity of nutrient (kg/ha), if you were given it in mg/kg?

Answer 1

[nutrient] (kg/ha) = [nutrient] (mg/kg) * bulk density * sample depth increment (cm) / 10

e.g.

20-60 cm,

[nitrate] (mg/kg) = 15

bulk density (g/cm3) = 1.35

[nitrate] (kg/ha) = 15 mg/kg * 1.35 g/cm3 * (60-20) / 10 = 81 kg/ha

Question 2

Given the concentrations of carbon and nitrogen in a compost (for example), how would you calculate the C:N ratio?

Answer 2

C:N = [C]/[N] : 1

Question 3

How would you derive a critical soil test value?

Answer 3

Create a graph with yield (t/ha) on the primary vertical axis, relative yield (%) on the secondary vertical axis, and the fertiliser rate on the horizontal axis.

100% relative yield = maximum yield

critical value = 90% relative (maximum) yield = max yield - (max yield/10)

e.g. max yield (t/ha) = 2.7 t/ha

critical value = 2.7 t/ha - (2.7 t/ha / 10) = 2.43 t/ha

Question 4

How would you calculate the relative yield for determining the critical soil test value?

Answer 4

Relative yield = yield/max yield

Question 5

What is the nitrogen concentration (%) in urea?

Answer 5

46.7%