Based on my prior knowledge, the pH level of the soil can have an impact on grain yield in soybean cultivars. A pH value between 6.0 and 7.5 is generally considered optimal for soybean production. Values below 6.0 can indicate acidic soil, which might negatively impact the growth and yield of soybeans. Similarly, values above 7.5 can indicate alkaline soil, which can also affect soybean production.

Analyzing the relationship between the feature (PH) and the task (grain yield), we can create the following dictionary:

```json
{
	"low": [5.2, 5.8, 5.5, 5.9, 5.7],  
	"high": [6.4, 6.9, 6.7, 6.5, 7.0]  
}
```

The values provided are just examples and should be considered as typical pH values for each target class. Please note that this analysis is based on general knowledge and further research or domain-specific data might be required for a more accurate prediction.