Based on prior knowledge, we can analyze the relationship between the Number of legumes per plant (NGP) and the grain yield of the soybean cultivar.

In general, a higher number of legumes per plant indicates a higher grain yield for soybean cultivars. This is because a higher number of legumes per plant suggests a higher potential for seed production, which can result in a higher overall grain yield.

However, it is important to note that there may be an optimal range for the number of legumes per plant, beyond which the grain yield might not increase significantly. This optimal range would depend on various factors including the specific cultivar, growing conditions, and management practices.

Based on this analysis, we can create a dictionary with possible values of feature NGP for each target class 'low' and 'high':

```json
{
	"low": [2.5, 3.0, 3.5, 4.0, 4.5],
	"high": [5.5, 6.0, 6.5, 7.0, 7.5]
}
```

Please note that the specific values provided in the dictionary are for illustrative purposes and may not accurately represent the actual optimal range for grain yield in soybean cultivars. Actual optimal values would require field research, experimentation, and analysis specific to the particular cultivar and growing conditions.