Based on prior knowledge, it is expected that the PH feature can affect the grain yield of soybean cultivars. However, without specific information about the ranges of PH values and their relationship with the target variable, it is not possible to determine the appropriate ranges for this analysis.

To create the dictionary, we will assume that a PH value between 1 and 2 represents low grain yield, while a PH value between 3 and 4 represents high grain yield. Please note that this assumption is arbitrary and may not reflect the actual relationship. 

With these assumptions in mind, here's the dictionary with 5 typical PH values for each target class:

```json
{
	"low": [1.1, 1.5, 1.8, 1.9, 2.0],
	"high": [3.1, 3.4, 3.6, 3.8, 3.9]
}
```

Please note that this analysis is based on assumptions and may not accurately reflect the relationship between the PH feature and the grain yield of the soybean cultivar. It is highly recommended to consult domain experts or conduct further analysis to determine the actual relationship.