Based on my prior knowledge, the Thousand seed weight (MHG) is an important feature for soybean cultivars. A higher MHG value generally indicates a higher grain yield, as it represents the weight of a thousand seeds. Therefore, we can expect that for the task of predicting the grain yield of this soybean cultivar, a higher MHG value would correspond to a high yield, and a lower MHG value would correspond to a low yield.

To create the requested dictionary, I will assume the following ranges for the MHG values:
- Low MHG values: 100-200 grams
- High MHG values: 300-400 grams

Based on these ranges, here is the dictionary:

```json
{
  "low": [120.0, 150.0, 180.0, 190.0, 195.0],
  "high": [310.0, 325.0, 345.0, 380.0, 395.0]
}
```

Please note that these values are just examples and may not necessarily reflect the actual distribution of MHG values for low and high grain yield. They are meant to provide a general understanding of how the feature relates to the target variable.