Based on prior knowledge and experience, the Number of grains per plant (NGL) is a crucial feature that is positively correlated with grain yield in soybean cultivars. Higher values of NGL typically indicate higher grain yields, while lower values are associated with lower grain yields. 

To create the dictionary, I will assume the following ranges for NGL values:

- Low NGL: 0-100 grains per plant
- High NGL: 500-1000 grains per plant

Based on these assumptions, here is the dictionary:

```json
{
	"low": [10.0, 25.0, 40.0, 60.0, 80.0],
	"high": [550.0, 700.0, 800.0, 900.0, 950.0]
}
```

Note: These values are just examples and should be tailored to the specific dataset and context.