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Another exam question commentary - level 3 Plant Growth
by Janet Prescott - Tuesday, 23 June 2020, 3:37 PM

R3102 June ‘19

Q1. Describe how low organic matter content of a soil can influence the following:

i)                 Moisture holding capacity;   3 marks

ii)                Soil temperature;                     4 marks

iii)              Nutrient holding capacity.    3 marks


This question relates to the following part of the syllabus:

Outcome 1. Understand formation and properties of soils and other growing media.

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.

This is an example of a question where it would be very easy to miss an important part just reading through once. Looking at it quickly it appears that the question is just about organic matter influencing the moisture, temperature and nutrients in a soil. However, it is important that you relate all of this specifically to a soil with low amounts of organic matter. It is well worth taking a little time to think about the question before rushing in to an answer.

The examiners comments suggest the following for moisture holding capacity:

Low organic matter in the soil leads to reduced moisture holding capacity because soil organic matter itself will hold moisture in the soil. Low organic matter also reduces aggregation and so reduces mesopores with a subsequent reduction in water holding capacity.

It is useful to include technical terms such as mesopore, which shows you have a greater understanding of the subject. Water drains through the larger pores, or macropores; it is held too tightly to be available to plants in the smaller pores, or micropores. The mesopores, or middle sized pores can hold water that is available to plants.

The examiners’ comments suggest the following for part ii) on soil temperature;

The low organic matter content of the soil, with lower water content will lead to a more rapid change in temperature with soils warming faster in the spring and cooling more quickly in the autumn.

A reduction in organic matter leads to a lighter coloured soil which results in reduced solar gain.

It was also pointed out that some candidates gave the answer of reduced levels of soil microbes leading to a cooler soil. Whereas this would be relevant in a compost heap the effects on temperature are insignificant in the soil.

Part iii) on nutrient holding capacity also needs some thinking time. First of all consider how nutrients are held in the soil and what prevents them from being leached out. This brings you to colloids, and in organic matter this will be humus.

The examiners’ comments suggest the following:

In general, low organic matter content of the soil reduces nutrient holding capacity through reduced levels of colloidal humus and thus reduced cation exchange sites, reducing nutrient availability and allowing increasing leaching of nutrients.