What factor primarily hinders seedling emergence?

Surface crusting is the primary factor that hinders seedling emergence because it forms a hardened layer on the soil surface after heavy rainfall or irrigation. This crust creates a physical barrier that makes it difficult for seedlings to break through and emerge from the soil. Even though the seeds may be planted at the proper depth and require the right moisture conditions, the crust can inhibit their upward growth, leading to poor plant establishment.

Other factors, such as root disease, soil acidity, and excessive moisture, can also impact seedlings, but they do not typically create the immediate physical barrier that surface crusting does. Root disease can affect the health of seedlings once they emerge, while soil acidity can influence nutrient availability and root growth over time. Excessive moisture can lead to other problems like poor aeration or root rot, but again, it doesn't create a physical obstacle at the surface level. Therefore, among these factors, surface crusting is particularly significant in directly obstructing the process of seedling emergence.