Certified Crop Advisor Practice Exam

Potassium fixation primarily occurs when which type of clay is present in soil?

1. Kaolinite

2. Montmorillonite

3. Illite

4. Sand

The correct answer is: Illite

Potassium fixation primarily occurs in soils containing illite clay. Illite has a layered structure similar to that of mica and has the ability to retain potassium ions within its crystal lattice. The layers in illite can hold potassium tightly, making it less available for plant uptake. This characteristic can limit the availability of potassium, especially in soils where illite is predominant. In contrast, other types of clays like kaolinite do not tend to have the same strong affinity for potassium, as kaolinite has a more stable structure that allows for less ion exchange. Montmorillonite can hold potassium but does so differently and often allows for more potassium availability compared to illite due to its expansive nature and greater cation exchange capacity. Sand, being coarse and lacking the fine particles necessary for ion retention, does not contribute to potassium fixation at all. Thus, illite's unique properties are what lead to the fixation of potassium in soil, making it the correct choice for this question.