Based on prior knowledge, the thousand seed weight (MHG) is an important factor in determining the grain yield of soybean cultivars. Generally, a higher thousand seed weight tends to result in a higher grain yield.

To analyze the relationship between MHG and the grain yield task, let's assume the following approximate ranges for the MHG values:

- Low grain yield: MHG values less than or equal to 20g
- High grain yield: MHG values greater than 20g

With this in mind, we can create the dictionary with the requested format:

```json
{
	"low": [18.5, 19.1, 20.0, 19.8, 18.2],  
	"high": [21.4, 23.6, 22.9, 24.8, 26.2]
}
```

Please note that these MHG values are just examples and are not based on specific data. The purpose of providing these values is to demonstrate the structure of the output dictionary.