This subtopic revisits concepts about ionic substances introduced in Stage 1 subtopics 2.2 and 4.2, and concepts of equilibrium introduced in subtopic 2.2.

Plants require nutrients, which they obtain from the soil.

• Explain why plants need soil nutrients in soluble form.

Video: Soil Nutrients and Fertilisers (Flippin' Science)

Soil productivity is related to the availability of plant nutrients, which need to be replenished naturally or by the addition of fertilisers.

Nitrogen, phosphorus, and potassium are the major nutrients that plants require from the soil.

• Explain how natural processes (including lightning, nitrogen-fixing bacteria, and decay) replenish soil nitrogen.
• Explain why fertilisers are required to improve the productivity of some soils.

Animated Notes

Excess nitrogen and phosphorus can be leached from soils and can cause eutrophication in water bodies.

• Explain the process and consequences of eutrophication.

Animated Notes

Silicon dioxide, silicates, and aluminosilicates are important components of rocks and soils.

• Write the formula of the anion given the formula of a silicate or aluminosilicate.

Animated Notes

Cations adsorbed on the surface of soil silicates and aluminosilicates are in equilibrium, and can be exchanged with, the cations in soil water, which are available as sources of plant nutrients.

Soil silicates and aluminosilicates are able to adsorb H⁺ and release cations.

• Explain how cations on the surface of soil silicates and aluminosilicates become available to plants.

Animated Notes

Nutrient cations on the surface of soil silicates and aluminosilicates are replaced if the concentrations of H⁺ or Na⁺ in soil water become too high.

• Explain how acidic or saline conditions (i.e. high concentrations of H⁺ or Na⁺) deplete the nutrient value of soils.

Animated Notes

Video: Soil Ion Exchange (Flippin' Science)