Based on prior knowledge of soybean cultivation, the PH level of the soil can have an impact on the grain yield of soybean cultivars. Generally, a PH value between 6 and 7 is considered suitable for soybean growth, while values below 6 or above 7 can have a negative impact on yield.

Given this information, we can analyze the relationship between the PH feature and the task of predicting grain yield.

Analyzing the Relationship:
- Low Grain Yield: It is expected that soybean cultivars with low grain yield will have PH values that deviate significantly from the suitable range of 6-7. Therefore, we can consider PH values below 6 or above 7 as indicative of low grain yield.

- High Grain Yield: In contrast, soybean cultivars with high grain yield are likely to have PH values within the suitable range of 6-7. So, PH values between 6 and 7 can be considered as indicative of high grain yield.

Based on this analysis, we can generate the dictionary:

```json
{
	"low": [5.5, 5.8, 5.9, 7.3, 5.4],
	"high": [6.2, 6.5, 6.1, 7.1, 6.7]
}
```

Note that these are just example values and the actual values may vary depending on the specific data and context of the problem.