Certified Professional Horticulturist (CPH) Practice Exam

What amendment should comprise ½ to 2/3 of the potting medium for containerized plants?

1. Perlite

2. Peat moss

3. Sand

4. Expanded shale

The correct answer is: Peat moss

Peat moss is an excellent amendment for potting mediums in containerized plants due to its ability to retain moisture while also providing good aeration and drainage. It is rich in organic matter, which not only helps to retain nutrients but also promotes healthy root development. The structure of peat moss allows it to hold water and nutrients, making them more available to the plant roots without becoming overly saturated. Using peat moss in the range of ½ to 2/3 of the total potting medium is common practice because it balances moisture retention with aeration. This ratio creates an ideal environment for many plants, promoting optimal growth and reducing the risk of root rot often associated with overly moist conditions. Other amendments, while beneficial in certain contexts, may not provide the same overall benefits in this specific role. For example, perlite primarily improves drainage but does not retain moisture as well as peat moss. Sand can help with drainage too, but it can lead to compaction when mixed improperly and lacks the nutritional benefits. Expanded shale is used to improve aeration and drainage but does not offer the same moisture retention and nutrient-holding capacity that peat moss does. Thus, peat moss stands out as the preferable choice for a successful potting medium composition for containerized plants