Neco Model Questions Vol1 2018 Agricultural Science Question 2

A soil sample was collected from a cultivated field and analysed for organic carbon using the Walkley‑Black dichromate oxidation method. The procedure was as follows: 1. 2.5 g of air‑dry soil was weighed into a digestion flask. 2. 10 mL of 1 N potassium dichromate (K₂Cr₂O₇) and 20 mL of concentrated sulphuric acid (H₂SO₄) were added and the mixture was heated for 30 min. 3. After cooling, the excess dichromate was titrated with 0.5 N ferrous ammonium sulphate (FAS) solution using diphenylamine as indicator. The titration data for the three replicate determinations are: | Replicate | Volume of FAS used (mL) | |-----------|--------------------------| | 1 | 12.4 | | 2 | 13.0 | | 3 | 12.7 | The correction factor for incomplete oxidation is 1.3. The soil bulk density is 1.35 g cm⁻³ and the sampled depth is 15 cm. Using the data above, answer the following: a) Calculate the average % organic carbon (C) in the soil (expressed to one decimal place). b) Determine the stock of organic carbon (in tonnes per hectare) present in the top 15 cm of the soil. c) The exchangeable bases (Ca²⁺, Mg²⁺, K⁺, Na⁺) in the same soil were found to be 12, 5, 2 and 1 cmol kg⁻¹ respectively. Compute the Cation Exchange Capacity (CEC) in cmol kg⁻¹ and comment on the fertility implication of the obtained CEC value. d) Identify two sources of error that could affect the accuracy of the Walkley‑Black determination and suggest a way to minimise each error.