Based on prior knowledge and understanding of soybean cultivation, the PH (acidity/alkalinity) of the soil can have an impact on the grain yield of soybean cultivars. 

Analysis:

- Low grain yield (target class: 'low'): In conditions where the soil pH is low, generally acidic (below the optimal range for soybean), the grain yield of the soybean cultivar is expected to be low. The lower end of the pH range for 'low' grain yield can be identified as pH values below 5.5.
- High grain yield (target class: 'high'): In conditions where the soil pH is within the optimal range for soybean cultivation, the grain yield of the soybean cultivar is expected to be high. The higher end of the pH range for 'high' grain yield can be identified as pH values above 5.5.

Therefore, based on this analysis, the dictionary for the given feature and task can be generated as follows:

```json
{
	"low": [4.5, 5.0, 5.3, 5.4, 5.5],  
	"high": [5.6, 5.8, 6.0, 6.2, 6.5]  
}
```

Please note that the suggested values in the dictionary are just examples and may vary based on specific soil and cultivar characteristics.