Based on my prior knowledge and experience, I will analyze the relationship between the number of legumes per plant (NGP) and the grain yield of the soybean cultivar. A higher NGP generally indicates a higher likelihood of a high grain yield. However, the specific range of NGP values corresponding to low or high grain yield may vary depending on various factors such as plant genetics, growing conditions, and farming practices.

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

```json
{
	"low": [2.5, 3.0, 3.5, 4.0, 4.5],
	"high": [5.0, 5.5, 6.0, 6.5, 7.0]
}
```

Please note that these values are just examples and may not represent the actual ranges of NGP values for low and high grain yield. It is important to consider domain knowledge and conduct further analysis or experiments to determine the specific relationship between NGP and grain yield in the given soybean cultivar.