Based on my prior knowledge, the feature "PH" (potential hydrogen) can be an indicator of soil acidity or alkalinity. Soil pH levels can greatly affect the growth and yield of soybean crops. 

Analyzing the relationship between the feature "PH" and the target variable "grain yield" can help determine how high or low the yield of the soybean cultivar will be.

Here is the dictionary with the possible values of feature PH for each target class:

```json
{
	"low": [4.8, 5.1, 5.4, 5.7, 6.0],  
	"high": [6.6, 6.9, 7.2, 7.5, 7.8] 
}
```
Please note that these values are hypothetical and may vary depending on the specific conditions and context of the soybean cultivar being analyzed.