Based on prior knowledge, the feature "Number of grains per plant (NGL)" is likely to have a positive correlation with grain yield. Typically, a higher number of grains per plant would result in a higher grain yield.

To generate the dictionary, we need to provide a list of typical NGL values for each target class ('low' and 'high'). Since no specific ranges are given, we will assume that values below the median value are considered 'low' and values above the median are considered 'high'.

Here is the dictionary for the feature NGL:

```json
{
	"low": [100, 150, 200, 250, 300],
	"high": [400, 450, 500, 550, 600]
}
```

Please note that these values are just examples and can be adjusted based on domain expertise or any specific knowledge about soybean cultivars.