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

To determine the appropriate ranges of values for the feature "PH," we should consider the typical pH levels for soil and their effects on soybean growth. Soybeans generally tend to perform well in slightly acidic to neutral soil conditions, with a preferred pH range of around 6.0 to 7.5. Therefore, soybean cultivars are expected to have optimal grain yield within this pH range.

Based on this analysis, we can create a dictionary with the appropriate ranges for the feature "PH" for the target classes 'low' and 'high':

```json
{
	"low": [4.5, 4.7, 5.0, 5.2, 5.4], 
	"high": [6.5, 6.7, 7.0, 7.2, 7.4]
}
```

Please note that these pH values are provided as examples and can vary depending on the specific characteristics and requirements of the soybean cultivar.