Based on prior knowledge, the pH value of the soil can have an impact on the grain yield of soybean cultivars. To analyze the relationship between pH (PH) and the task of predicting the grain yield as low or high, we can consider the following:

1. Conduct a literature review or consult expert knowledge to determine the appropriate ranges of pH values that have been observed to affect grain yield.
2. Examine the data or available information to observe the distribution of pH values and how they align with low and high grain yield classifications.
3. Identify typical pH values associated with both low and high grain yield classifications.

Let's assume that based on prior knowledge and observation, the pH values would fall within a range of 1 to 14.

Based on this analysis, the dictionary representing the relationship between pH values (PH) and the grain yield task would look like:

```json
{
	"low": [3.5, 4.8, 5.2, 5.7, 6.1],
	"high": [6.5, 7.2, 7.8, 8.3, 8.9]
}
```

Note: The values presented in the dictionary are just examples and the actual values may differ based on the specific data or domain knowledge.