Based on the given feature description, the Number of legumes per plant (NGP) can be analyzed to determine its relationship with the task of predicting the grain yield of a soybean cultivar as low or high.

Analysis:
Typically, a higher number of legumes per plant indicates a higher potential for grain yield. Therefore, it can be inferred that soybean cultivars with a high NGP value are more likely to have a high grain yield, while those with a low NGP value are more likely to have a low grain yield.

Based on this analysis, the following dictionary can be created:

```json
{
	"low": [1.0, 2.0, 3.0, 4.0, 5.0],
	"high": [10.0, 12.0, 15.0, 18.0, 20.0]
}
```

In the dictionary, the "low" key represents the target class of low grain yield, and the associated NGP values are [1.0, 2.0, 3.0, 4.0, 5.0]. Similarly, the "high" key represents the target class of high grain yield, with the associated NGP values being [10.0, 12.0, 15.0, 18.0, 20.0]. These values are just examples and can vary based on the specific dataset and domain knowledge.