Based on prior knowledge, the pH of soil can have a significant impact on the grain yield of soybean cultivars. Typically, a pH range between 5.5 and 7.0 is considered optimal for soybean growth. pH values below 5.5 or above 7.0 are generally associated with decreased grain yield.

Here's the dictionary with the relationship between pH (feature) and grain yield (target):

```json
{
	"low": [4.8, 5.0, 5.2, 5.4, 5.6],  
	"high": [6.8, 6.9, 7.1, 7.3, 7.5]  
}
```

Please note that the provided pH values are just examples. The actual range and values may vary depending on the specific experiment or study.