Based on my prior knowledge, the number of legumes per plant (NGP) can have an impact on the grain yield of soybean cultivars. A higher number of legumes per plant generally indicates a higher grain yield while a lower number of legumes per plant indicates a lower grain yield.

Here is the dictionary with the possible values of feature NGP for each target class:

```json
{
	"low": [2.5, 2.8, 3.0, 3.2, 3.5],
	"high": [4.5, 4.8, 5.0, 5.2, 5.5]
}
```

Please note that these values are just examples and should be adjusted based on the specific cultivation conditions and historical data of soybean cultivars.