Based on prior knowledge, the number of grains per plant (NGL) can be a strong indicator of the grain yield of a soybean cultivar. Generally, a higher number of grains per plant would contribute to a higher grain yield, while a lower number of grains per plant would result in a lower grain yield.

To generate the dictionary with specific details, let's assume the following:

For the 'low' target class, typical NGL values could be [50, 60, 70, 80, 90].
For the 'high' target class, typical NGL values could be [150, 160, 170, 180, 190].

Based on this analysis, the dictionary would look as follows:

```json
{
	"low": [50.0, 60.0, 70.0, 80.0, 90.0],
	"high": [150.0, 160.0, 170.0, 180.0, 190.0]
}
```

Please note that these values are assumed and could vary depending on the specific characteristics of the soybean cultivar and the context of the analysis.