Based on prior knowledge, the feature "Number of grains per plant (NGL)" is likely to have a positive relationship with the grain yield of soybean cultivars. Generally, a higher number of grains per plant would be expected to result in a higher grain yield.

To create the dictionary with the requested format, here are five typical values for each target class:

```json
{
	"low": [150, 180, 160, 140, 170],
	"high": [220, 250, 230, 240, 210]
}
```

Please note that these values are just examples and may not reflect the actual range of values for the target classes.