Based on prior knowledge, we can analyze the relationship between the PH feature and the task of predicting the grain yield of a soybean cultivar. 

It is important to note that without specific domain knowledge or available data, it is difficult to determine the exact ranges for the PH feature that correspond to low or high grain yield. Therefore, we will make an assumption that higher PH values are associated with high grain yield, while lower PH values are associated with low grain yield.

Assuming this assumption, we can create a dictionary with the following information:

```json
{
	"low": [1.0, 1.2, 1.4, 1.6, 1.8],  
	"high": [3.8, 4.0, 4.2, 4.4, 4.6]  
}
```

In this dictionary, we provide 5 typical PH values for each target class. The PH values for the "low" class include 1.0, 1.2, 1.4, 1.6, and 1.8. The PH values for the "high" class include 3.8, 4.0, 4.2, 4.4, and 4.6. Please note that this is a hypothetical example and actual ranges may vary depending on the specific context and available data.