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Back to exam commentaries - Level 3 Plant Growth
by Janet Prescott - Monday, 7 January 2019, 1:20 PM

Another commentary to get us back into revision mode: this one from 3102 in February 2018:

Q5 a) State what is meant by ‘cation exchange capacity’.                              2 marks

b) State what is meant by the term ‘buffering capacity’.                                2 marks

c) Describe the characteristics of a clay soil under EACH of the following headings:

i) cation exchange capacity and nutrient availability;                                       4 marks

ii) buffering capacity and liming requirements.                                                  2 marks


This question relates to the following part of the syllabus:

1.2 Review the properties of soil organic matter, colloids and mineral components. 

Define the terms anion, cation and buffering capacity, and explain the significance of cation exchange in soils and growing media. 

Organic matter: Describe the process of humification. Describe the properties of humus. 

Colloids: Particle size, origins of colloids, properties to include:  cation exchange capacity and buffering capacity.

 Mineral components: Particle size (Soil survey of England & Wales classification). 

Describe the properties of sand, silt & clay, to include: surface charge, water holding ability, cation exchange capacity and buffering capacity. 

Describe how properties of these soil components influence soil temperature, nutrients, water holding ability and pH.  

4.2 Explain how various factors affect the availability of plant nutrients in soils and growing media. 

Describe how EACH of the following factors affect the availability of plant nutrients, to include pH, soil texture, soil organisms, water content and leaching.


Parts a) and b) are statements for 2 marks each.  The examiners’ comments give an indication of the amount of detail required for this.

For ‘cation exchange capacity’ a suitable answer was stated as needing to include the following: ‘cation exchange capacity (CEC) is the number of exchangeable cations a soil can hold and also more detailed information such as, at a given pH value, or high CEC prevents leaching of specific soil nutrients’.

For buffering capacity the examiners’ comments state a suitable answer as ‘the soils ability to resist changes in pH’. The comments indicate that this would just gain one mark, as they continue to state ‘However, the second mark was for a more developed answer including additional information such as, where the soil is acted upon by an alkaline or acidic agent’.  

It is useful to look at the marks allocated to work out how much information to include in your answer.

For c) it is important to remember that a clay soil is asked for when answering parts i) and ii).

The examiners’ comments suggest that the answer for part i) needed to include the negative charge on the surface of clay particles in a clay soil, which attract the positively charged cations. Examples of cations which are important plant nutrients were useful to include, such as calcium, C++, magnesium Mg++ and potassium K+, which are held in an exchangeable manner.   

The examiners’ comments for part ii) state that ‘Candidates who identified that clay soils have a high buffering capacity leading to a large amount of lime being required to change the pH, i.e. a high liming requirement, scored full marks’.