Based on prior knowledge and understanding of soybean cultivation, the feature "Number of legumes per plant (NGP)" can be analyzed to determine its relationship with the target variable, which is the grain yield of the soybean cultivar.

Higher values of NGP can generally indicate better grain yield potential in soybean cultivars. However, it is important to note that specific ranges of NGP values for low and high grain yield can vary depending on various factors such as cultivar, growing conditions, and management practices.

Given this information, the dictionary with NGP values for each target class ('low' and 'high') can be created as follows:

```json
{
	"low": [2.0, 3.0, 3.5, 4.0, 4.5], 
	"high": [5.0, 5.5, 6.0, 6.5, 7.0]
}
```

Please note that the values provided in the dictionary are just examples and should be adjusted based on the specific criteria used to classify "low" and "high" grain yield.