The pie chart below shows the composition of a sample of soil. Use it to answer questions 1(a),(b) and (c).
- Name the component represented by X. [ 1 mark]
- Describe an experiment to show the presence of organic matter in a fresh sample of garden soil. [7 marks]
- From the experiment in 1(b), use notations only to calculate the weight of organic matter in the soil. [4 marks]
- State three problems of soils with a pH value of 4.0. [3 marks]
| This question was poorly attempted by the candidates. In 1(a), majority of the candidates could name the component represented by X. However, in 1 (b), many candidates were unable to describe an experiment to show the presence of organic matter in a fresh sample of garden soil.
Further to this, in 1(c), majority of the candidates could not use notations to calculate the weight of organic matter in the soil sample. Also, in 1(d), many candidates were unable to state problems of soils with a pH value of 4.0.
The expected answers include:
Description of the experiment to show the presence of organic matter in a fresh garden soil
- Weigh the sample of fresh garden soil (xg)
- Weigh an empty silica dish (yg) put soil into dish and
- Place dish and soil in an oven at about 105oc for many hours
- Cool in a desiccator and weigh (ng)
- Repeat several times until weight is constant
- Weight of dry soil = (x + y) –n)g = mg
- Then place dry soil and dish over a Bunsen burner and heat strongly for some time
- Stir as you heat and observe the colour change
- Cool in a desiccators and weigh
- Continue heating until there is not more smoke
- Repeat the heating, cooling and weighing until a constant weight is achieved (Zg)
- Difference in weight between final weight and that before heating over Bunsen burner gives the
organic matter content = Z - ng
Correct sequence to be followed. Any skipped step results in a lost mark.
Calculating the weight of the organic matter in the soil using notations
- Weight of dry soil + dish = ng
- Weight of soil + dish after ignition = z
- Weight of organic matter = n – z
Problems of soils with pH value of 4.0
- Devoid of many soil organisms/e.g. earthworms etc.
- Reduced microbial activities
- Increased availability of toxic elements e.g. AI, Fe
- Reduced availability of Ca and P
- Nitrogen fixation is hindered/reduced
- Reduced Cation Exchange Capacity (CEC)
- Stunted plant growth
- Increased leaching of some important plant nutrients.