Based on prior knowledge, the number of legumes per plant (NGP) can be an important factor affecting the grain yield of soybean cultivar. Generally, a higher number of legumes per plant is associated with a higher grain yield, while a lower number of legumes per plant is associated with a lower grain yield.

To generate the dictionary, we need to provide 5 typical NGP values for each target class, 'low' and 'high'.

```json
{
	"low": [2.5, 3.1, 2.2, 2.9, 3.5],
	"high": [5.9, 6.2, 5.8, 6.5, 6.1]
}
```

Please note that these values are examples and may vary depending on the specific soybean cultivar and other factors.