Based on my prior knowledge, the number of grains per plant (NGL) can have a significant impact on the grain yield of soybean cultivars. Generally, a higher number of grains per plant is associated with a higher grain yield, while a lower number of grains per plant is associated with a lower grain yield.

To generate the dictionary with specific details, including typical NGL values for each target class ('low' and 'high'), I will rely on my experience and knowledge in soybean cultivation.

```json
{
	"low": [250.0, 260.0, 275.0, 280.0, 290.0],
	"high": [320.0, 330.0, 345.0, 360.0, 370.0]
}
```

Please note that the values provided above are just examples and should be adjusted based on the specific context and dataset being analyzed.